scala variable assignment

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Variables are nothing but reserved memory locations to store values. This means that when you create a variable, you reserve some space in memory.

Based on the data type of a variable, the compiler allocates memory and decides what can be stored in the reserved memory. Therefore, by assigning different data types to variables, you can store integers, decimals, or characters in these variables.

Variable Declaration

Scala has a different syntax for declaring variables. They can be defined as value, i.e., constant or a variable. Here, myVar is declared using the keyword var. It is a variable that can change value and this is called mutable variable . Following is the syntax to define a variable using var keyword −

Here, myVal is declared using the keyword val. This means that it is a variable that cannot be changed and this is called immutable variable . Following is the syntax to define a variable using val keyword −

Variable Data Types

The type of a variable is specified after the variable name and before equals sign. You can define any type of Scala variable by mentioning its data type as follows −

If you do not assign any initial value to a variable, then it is valid as follows −

Variable Type Inference

When you assign an initial value to a variable, the Scala compiler can figure out the type of the variable based on the value assigned to it. This is called variable type inference. Therefore, you could write these variable declarations like this −

Here, by default, myVar will be Int type and myVal will become String type variable.

Multiple assignments

Scala supports multiple assignments. If a code block or method returns a Tuple ( Tuple − Holds collection of Objects of different types), the Tuple can be assigned to a val variable. [ Note − We will study Tuples in subsequent chapters.]

And the type inference gets it right −

Example Program

The following is an example program that explains the process of variable declaration in Scala. This program declares four variables — two variables are defined with type declaration and remaining two are without type declaration.

Save the above program in Demo.scala . The following commands are used to compile and execute this program.

Variable Scope

Variables in Scala can have three different scopes depending on the place where they are being used. They can exist as fields, as method parameters and as local variables. Below are the details about each type of scope.

Fields are variables that belong to an object. The fields are accessible from inside every method in the object. Fields can also be accessible outside the object depending on what access modifiers the field is declared with. Object fields can be both mutable and immutable types and can be defined using either var or val .

Method Parameters

Method parameters are variables, which are used to pass the value inside a method, when the method is called. Method parameters are only accessible from inside the method but the objects passed in may be accessible from the outside, if you have a reference to the object from outside the method. Method parameters are always immutable which are defined by val keyword.

Local Variables

Local variables are variables declared inside a method. Local variables are only accessible from inside the method, but the objects you create may escape the method if you return them from the method. Local variables can be both mutable and immutable types and can be defined using either var or val .

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How to Declare Scala Variables – Scope & Examples

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What are Scala variables, and how do we declare them? What is type inference? How do we perform multiple assignments? What is the variable scope? These are the questions we’ll be answering today.

So, let’s start Scala Variables Tutorial.

Introduction to Variables in Scala

A variable is a reserved memory location to store values. The compiler allocates memory and decides what goes in based on a variable’s data type. We can assign different types like integers, characters, or decimals to variables. Let’s begin.

Declaring Scala Variables

We can declare a Scala variable as a constant or as a variable:

a. Constant

To declare a value that we cannot mutate/change, we use the keyword ‘val’. This is immutable.

b. Variable

A variable is mutable. We can change its value as many times as we like. Declare it using the keyword ‘var’.

Scala Variables Type Inference

When we assign an initial value to a variable, the compiler infers its type based on the types of the subexpressions and the literals. This means that we don’t always have to declare the type of a variable.

Scala Variable Data Types

We can specify a variable’s type after a colon after its name. Take an example:

Multiple Assignments

How do we fit multiple assignment statements into one statement? We assign a Tuple to a val variable.

Scala Variable Scope

We have three kinds of scopes for Scala variables. Let’s see them one by one.

If an object owns a variable, the variable is a field in it. We can access fields from inside every method in the object. Well, we can also access them outside the object if we declared them with the right access modifiers. A field may be mutable or immutable, and we can define them using ‘var’ or ‘val’.

b. Method Parameters

A method parameter is a variable that we can use to pass a value inside a method when we call it. We can only access it inside the method, but if we have a reference to that object from outside the method, we can access the objects from outside. A method parameter is always immutable, and we define it using the ‘val’ keyword.

c. Local Variables

A local variable is one we declare inside a method. While we can only access it from inside a method, if we return the objects, that we create, from the method, they may escape it. A local variable may be mutable or immutable, and we may define it using the keywords ‘var’ or ‘val’.

So, this was all about Scala Variables. Hope you like our explanation.

Hence, we discussed about how to declare Scala variables, type inference, data type, and scope. If you have any query regarding Scala Variables, please comment. See you again with another topic to learn with Scala. Have a good day.

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Please add examples in point 7

while declaring variables of method parameters in scala why we don’t use var or val

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Scala Variables – Syntax, Declaration, Use case, Examples

Introduction to scala variables.

After you have created your own Scala project, you can begin to write code. In this short article, we are going to learn how to use and set Scala variables.

How to set Scala Variables?

If you want to declare a variable, you must use the keyword var .

This line will declare a variable named variableName  of value 12 .

How to set Scala Values?

In Scala, we prefer to use immutable  variables. You can read why on StackOverflow .

This is how you declare a value:

This line will declare a value named valueName of value 12 .

How to declare the Type?

Variables will have a type inferred by the compiler. If you would like to add a type to your variable, you must use the :  syntax.

In Scala, everything can be assigned to a value:

• integers, strings, …
• lambda functions

We are going to talk more about the above in upcoming articles.

What is the keyword lazy  for?

You might have seen the keyword lazy .

In this instance, I am going to use complexMath  as an example of a value. We could imagine this value as being long and costly to calculate.

With the keyword lazy , complexMath ‘s value will only be computed once when used for the very first time.

The code used to generate complexMath ‘s value will not be executed until it used for the first time.

Also, the value generated, in this case complexMath , will be stored and the code used to generate this value will not be executed again a second time.

Practical example

Let’s go through a short example to practice what we have learned.

Feel free to play with it.

If the playground above does not load, you can  go directly to Scastie .

What to remember?

A few things to remember about Scala variables:

• var  for mutable
• val  for immutable
• Use immutable as much as possible and avoid using mutable
• Syntax is val <name>:<type> = <value>
• lazy  will delay the evaluation of the value to the first use

By understanding these basics, you should now be able to declare and use variables in Scala code.

Feel free to leave comments if you have any questions.

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Variables in Scala

In the following lesson, you will be introduced to variables and how they are declared in Scala.

Introduction

• Declaring a Variable
• Choosing the Right Keyword
• Multiple Variables

Imagine you have multiple boxes and in each box, you can store one item. Before you can store anything, you need to decide what type of item can be stored in each box. But now you have so many boxes, you don’t remember what is in each one. You decide to give each box a unique label. This will help you keep track of what each one contains. The labels act as identifiers, giving each box a unique identity.

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Scala Basic Tutorial - How To Declare Variables And Types

In this tutorial, we will go over declaring variables and learn about  the data  types that are supported in Scala.

1. Immutable variables As we described in the Scala Introduction tutorial , immutability is a first class citizen in the Scala programming language. To define immutable variable, we use the keyword val with the following syntax:

As an example, you can define an immutable variable named donutsToBuy of type Int and assign its value to  5 .

Now, if you try to change the value of  donutsToBuy to say 10 , the Scala compiler will not be happy :(

If you then hover your mouse on the red lines in IntelliJ, you will see the message " Reassignment to val ". The message tells us that once a value has been assigned, we can no longer change it!

Feel free to review the Scala Introduction where we discussed some of the benefits of having immutable variables.

2. Mutable variables OK, fair enough that Scala is a functional programming language which favours the immutable pattern . But, what if you are in a loop and need to increment some variable. That's a perfectly legitimate use case and to declare a variable as mutable, we use the keyword var as opposed to val .

• The Scala compiler no longer complains that we are reassigning the favoriteDonut value from "Glazed Donut" to "Vanilla Donut".
• The type of favoriteDonut is defined as String using syntax : String

3. Lazy initialization Sometimes you may wish to delay the initialization of some variable until at the point where it is consumed by your application. This is usually referred to as lazy initialization  and we need to make use of the lazy keyword

• We've not specified the type of donutService! However, in this case, the Scala compiler knew that it should be of type String. This is called  type inference which we will see in the upcoming tutorials.

4. Scala Supported Types

If you have used other programming languages such as Java or .NET , you would intuitively look for the supported  types similar to Java's primitives or .NET's built-in types. However, Scala does not have built-in types! Instead, it was designed from the ground up to have a set of classes for representing its supporting types as shown below:

• Scala also provides a type called Unit . For now, you can think of the Unit type similar to Java's or .NET's void keyword.

5. Declare a variable with no initialization Sometimes you may not know the value of your variable immediately. You can only assign your variable's value at some later point in time during the execution of your application.

Let's assume that you need to declare a variable called leastFavoriteDonut of type String, but you won't initialize it just yet.

• We've use the wildcard operator _ when defining our variable.
• Somewhere later in our code base, we can then assign a String value for our variable which in our case was "Plain Donut" String value.
• Now, this may be controversial as you should strive to use the immutable pattern. As a matter of fact, when you code review some Scala application, you should always look out for code smell such as the use of var ! There are better ways to avoid the use of var which we will see in upcoming tutorials.

This concludes our tutorial on Scala Basic Tutorial - How To Declare Variables and Types and I hope you've found it useful!

Stay in touch via Facebook and Twitter for upcoming tutorials!

Don't forget to like and share this page :)

In this article, we went over the following:

• How to declare immutable variables in Scala
• How to declare mutable variables in Scala
• How to declare the various data types that Scala provides
• How to declare a variable with no initialization.
• If you come from an Object Oriented language , it may be a paradigm shift for using the immutable pattern via the val keyword. But, it will come with lots of benefits especially as you start getting more familiar with the functional programming side of Scala.

Source Code

The source code is available on the allaboutscala GitHub repository .

What's Next

In the next tutorial , I will show you how to use String interpolation in Scala.

Stay tuned!

16 Assign multiple variables

Using Tuples, it is possible to assign multiple values to multiple variables (either var or val ).

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Variables are simply storage locations. Every variable is known by its name and stores some known and unknown piece of information known as value. So one can define a variable by its data type and name, a data type is responsible for allocating memory for the variable. In Scala there are two types of variables: 

• Mutable Variables
• Immutable Variables

Let’s understand each one of these variables in detail. 

Mutable Variable: These variables are those variables that allow us to change a value after the declaration of a variable. Mutable variables are defined by using the var keyword. The first letter of data type should be in capital letter because in Scala data type is treated as objects. 

Syntax:   

Example:  

Here, name is the name of the variable, string is the data type of variable and geeksforgeeks is the value that store in the memory.  Another way of defining variable:  

Syntax:  

For Example:  

 Here, the value is the name of the variable. 

Immutable Variable:   These variables are those variables that do not allow you to change a value after the declaration of a variable. Immutable variables are defined by using the val keyword. The first letter of data type should be in capital letter because in Scala data type is treated as objects. 

Here, a name is the name of the variable, a string is the data type of variable and geeksforgeeks is the value that store in the memory. Another way of defining variable: 

 Here value is the name of the variable. 

Rules for naming variable in Scala  

• Variable name should be in lower case.
• Variable name can contain letter, digit and two special characters(Underscore(_) and Dollar($) sign)
• Variable name must not contain the keyword or reserved word.
• Starting letter of the variable name should be an alphabet.
• White space is not allowed in variable name.

Note: Scala supports multiple assignments but you can use multiple assignments only with immutable variables.

Variable Type Inference In Scala: Scala supports variable type inference. In variable type inference values are directly assigned to the variable without defining its data type, the Scala compiler automatically resolves which value belongs to which data type. 

Here, name1 is by default int type and name2 is by default string type. 

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A basic Scala if statement looks like this:

You can also write that statement like this:

The if / else construct looks like this:

The complete Scala if/else-if/else expression looks like this:

if expressions always return a result

A great thing about the Scala if construct is that it always returns a result. You can ignore the result as we did in the previous examples, but a more common approach — especially in functional programming — is to assign the result to a variable:

This is cool for several reasons, including the fact that it means that Scala doesn’t require a special “ternary” operator.

Aside: Expression-oriented programming

As a brief note about programming in general, when every expression you write returns a value, that style is referred to as expression-oriented programming , or EOP. This is an example of an expression :

Conversely, lines of code that don’t return values are called statements , and they are used for their side-effects . For example, these lines of code don’t return values, so they are used for their side effects:

The first example runs the doSomething method as a side effect when a is equal to b . The second example is used for the side effect of writing a string to STDOUT. As you learn more about Scala you’ll find yourself writing more expressions and fewer statements . The differences between expressions and statements will also become more apparent.

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Exploring The World Of Scala Variables

Explore the nuances of Scala variables in this concise guide. Delve into best practices, common pitfalls, and real-world applications, ensuring a solid grasp for developers at any level.

💡KEY INSIGHTS

• Scala's distinction between `val` (immutable) and `var` (mutable) variables is crucial for writing robust and maintainable code.
• Effective use of type inference in Scala simplifies variable declarations while maintaining type safety.
• Understanding the scope and visibility of variables in Scala is essential for structured and secure coding practices.
• Adherence to best practices , like using descriptive names and favoring immutability, enhances code readability and efficiency.

Scala variables play a pivotal role in the realm of programming, offering both mutable and immutable options. As you navigate the Scala ecosystem, understanding these variables is paramount. This knowledge not only enhances code efficiency but also ensures a smoother development experience.

Types Of Scala Variables

Declaring and initializing variables, mutable vs immutable variables, variable scope and visibility, best practices for using variables, real-world use cases, common mistakes and how to avoid them, frequently asked questions.

Val (Immutable) Variables: Once you assign a value to a val variable, you cannot change it. It's immutable, meaning its value remains constant throughout the program. This immutability ensures safety in concurrent programming.

Var (Mutable) Variables: On the other hand, var variables are mutable. You can reassign values to them after their initial assignment. While they offer flexibility, it's essential to use them judiciously to avoid unintended side effects.

Variable Types Based On Data

Scala is a statically-typed language, which means you need to declare a variable's type at compile time. However, due to type inference, Scala often determines the type for you. Here are some common data types:

• Int: Represents 32-bit signed integers.
• Double: Represents 64-bit double-precision floating-point numbers.
• Boolean: Represents true or false values.
• String: Represents sequences of characters.

Using the right variable type and understanding its properties can significantly impact the efficiency and clarity of your code.

In Scala, declaring and initializing variables is a straightforward process. Let's break it down step by step.

Step 1: Choose The Variable Type

Before declaring a variable, decide whether it should be mutable ( var ) or immutable ( val ). Remember, val variables cannot be reassigned after initialization, while var variables can.

Step 2: Declare The Variable

To declare a variable, use the val or var keyword followed by the variable name, a colon, the data type, and an equals sign.

Step 3: Initialize The Variable

For val variables, initialization happens at the time of declaration. However, for var variables, you can delay initialization. Later, you can assign a value using the equals sign.

Step 4: Use Type Inference (Optional)

Scala's type inference capability often determines the data type for you. If the compiler can infer the type, you can omit the data type during declaration.

Step 5: Reassigning Variables (For Var Only)

If you've chosen a var variable, you can reassign it to a new value of the same data type. Remember, reassignment is not possible with val .

By following these steps, you can effectively declare and initialize variables in Scala, ensuring your code remains clean and efficient.

In Scala, the distinction between mutable and immutable variables is fundamental. This distinction revolves around whether a variable's value can be changed after its initial assignment.

Mutable Variables ( var )

Immutable variables ( val ), pros and cons.

Mutable variables, declared with the var keyword, allow their values to be modified after initialization. They offer flexibility in situations where the value of a variable needs to change during the program's execution.

Immutable variables, declared with the val keyword, cannot have their values changed once they are initialized. They are ideal for ensuring data consistency and safety, especially in concurrent programming scenarios.

Choosing between mutable and immutable variables depends on the specific requirements of your program. It's essential to strike a balance to ensure both flexibility and data integrity.

In Scala, as in many programming languages, the scope and visibility of a variable determine where it can be accessed and modified within the code.

Local Variables

Member variables, global variables, access modifiers.

Local variables are declared inside a function or a block. Their scope is limited to that function or block, meaning they can't be accessed outside of it. Once the function or block execution is complete, the local variable is destroyed.

Member variables are declared inside a class but outside any method. They are accessible from any method within the class. Their visibility can be controlled using access modifiers like private , protected , and public .

Global variables, often termed as object-level variables , are declared inside an object. They remain accessible throughout the object's lifecycle and can be accessed from any location within the object.

Scala provides access modifiers to control the visibility of variables. These include:

• private : Restricts access to the containing class or object.
• protected : Allows access to subclasses.
• public : Grants access from any location (default if no modifier is specified).

Using the appropriate scope and visibility for variables is crucial for maintaining clean, organized, and secure code. It ensures that variables are only accessible where they're needed, reducing the risk of unintended modifications.

When working with variables in Scala, adhering to best practices ensures that your code is readable, maintainable, and efficient. Here are some guidelines to consider:

Use Descriptive Variable Names

Favor immutability, limit variable scope, avoid global variables, use type inference, initialize variables properly.

Choosing meaningful names for variables makes your code self-explanatory. Avoid generic names like temp or x . Instead, opt for names that describe the variable's purpose.

Whenever possible, prefer using val (immutable) over var (mutable). Immutable variables lead to safer and more predictable code, especially in concurrent scenarios.

Declare variables as close as possible to where they're used. This minimizes their scope, making the code easier to understand and reducing the risk of unintended access or modification.

Unless absolutely necessary, avoid using global variables. They can introduce side effects and make the code harder to debug. Instead, pass variables as parameters or use class members.

Scala's type inference is powerful. While it's good to specify types explicitly for clarity, in many cases, you can rely on Scala to infer the correct type.

Always ensure that variables are initialized before use. Uninitialized variables can lead to runtime errors or unexpected behavior.

When working with Scala, developers often encounter scenarios that require specific solutions or techniques. Here are some real-world questions and answers that shed light on common challenges and their solutions in Scala:

Accessing Environment Variables In Scala

Casting variables in scala, retrieving variable names at runtime, understanding variable initialization in scala, running shell commands with scala, listing bound variables in scala.

In Java, environment variables are accessed using System.getenv() . In Scala, starting from version 2.9, you can achieve the same using the sys.env method. For instance, to access the "HOME" environment variable:

When you have a variable of type Graphics and need to cast it to Graphics2D , the recommended approach in Scala is pattern matching. This method offers a graceful way to handle cases where the value might not be of the expected type:

To retrieve the name of a Scala variable at runtime, macros can be employed for AST (Abstract Syntax Tree) manipulation. Here's an example of how to achieve this:

In the provided code snippet, the program executes successfully even before the initialization of variables x1 and y1 . This behavior is due to the App trait extending DelayedInit , which means the code in the constructor executes as part of delayed initialization. It's essential to understand the difference between constructor variable declaration and method variable declaration in Scala to grasp this concept.

Executing shell commands in Scala can be achieved using the sys.process package. If you need to run a command that involves variables, string concatenation can be used. For example, to run an awk command with a dynamic regex pattern:

To get a list of bound variables in Scala, you can use the :power mode in versions 2.9, 2.10, and 2.11. The command intp.visibleTermNames.sorted.foreach(println) will display all variables, while intp.definedTerms.foreach(println) will show only the user-defined ones. In version 2.10 and above, $intp is always available, allowing you to access these methods without entering power mode.

Not Specifying Variable Types

Overusing mutable variables, ignoring return values, misusing option types, not handling exceptions, overcomplicating pattern matching, frequently modifying collections.

While Scala offers type inference, it's a good practice to specify variable types explicitly, especially for public APIs. This ensures clarity and avoids potential type-related issues.

Scala encourages functional programming and immutability. Overusing mutable variables can lead to unpredictable behavior. Instead, opt for immutable collections and variables.

When a function or method returns a value, ignoring it can lead to missed opportunities for error handling or optimization. Always check return values and handle them appropriately.

Option types are powerful in Scala, but misusing them can lead to null pointer exceptions. Always use methods like getOrElse or pattern matching to safely extract values from Option .

Scala provides Try , Success , and Failure for exception handling . Not using them can lead to unhandled exceptions and application crashes. Always wrap risky operations in a Try .

Pattern matching is a strong feature in Scala. However, overcomplicating patterns can make code unreadable. Stick to simple and clear patterns for better maintainability.

Instead of frequently modifying collections, consider building them in a way that they remain immutable. This approach aligns with functional programming principles and ensures thread safety.

How does Scala differentiate between mutable and immutable variables?

In Scala, variables declared using val are immutable, meaning their value cannot be changed once assigned. On the other hand, variables declared using var are mutable, allowing their values to be modified.

Can I use underscore _ as a variable name in Scala?

Yes, the underscore _ is a valid variable name in Scala. However, it's often used as a wildcard in pattern matching or to ignore specific values. When used as a variable name, it can make the code less readable, so it's best to use it judiciously.

How does Scala handle type inference in variable declarations?

Scala has a powerful type inference system. When you assign a value to a variable without specifying its type, Scala will automatically infer the type based on the assigned value. For example, val number = 42 will infer number as an Int.

What's the difference between lazy val and val in Scala?

While both val and lazy val declare immutable variables, the key difference lies in their initialization. A val is initialized when it's declared. In contrast, a lazy val delays its initialization until it's accessed for the first time, making it useful for expensive computations or when dealing with circular dependencies.

Let's see what you learned!

Which of the following is NOT a valid way to declare a variable in Scala?

Continue learning with these scala guides.

• Scala Currying: What It Is And How To Use It
• Concurrent Programming With Scala Multithreading
• Understanding Scala Spark And Its Practical Application
• Diving Deep Into Scala Collections And Their Uses
• Exploring Scala Generics: An In-Depth Study

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Home » Scala

Scala Variables

In this tutorial, we will learn about the different types of Scala variables, their usage, and examples. By Shivang Yadav Last updated : April 02, 2023

What are Scala Variables?

Scala variables are the named references to memory locations. The location stores the data that is used by the program. Based on the data type of the variable the memory size allocated is defined.

Type of Scala Variables

Scala allows you to define variables of two types:

• Mutable Variables
• Immutable Variables

1) Mutable Variables

Variables that allow us to change its value any time in the code. Scala variables are created using the var keyword . You can optionally give the data type of the variable with data type name with first letter capital.

Syntax to declare mutable variables

Scala example of mutable variables, 2) immutable variables.

Variables that are made immutable are read-only variables in Scala . This means their value remains unchanged throughout the program. Scala variables are created using the val keyword . You can optionally give the data type of the variable with data type name with first letter capital.

Syntax to declare immutable variables

Scala example of immutable variables, 3) lazy initialization of variables.

Lazy initialization of variables are those variables that are calculated when the first time they are accessed. In scala mutable variables cannot be lazy.

Only val i.e. immutable variable can make lazy. This means these variables are calculated only once.

Syntax for lazy initialization of variables

Scala code for lazy initialization of variables.

Related Tutorials

• Scala Basics and Syntaxes
• Scala Comments
• Scala Semicolons
• Scala Code Blocks
• Scala Data Types
• Scala Boolean Data Type
• Scala Char Data Type
• Scala Numeric Data Types
• Scala Unit Data Type
• Scala Nothing and Null Data Types
• Scala Literals
• Scala Options
• Scala Ranges
• Scala First Program
• How to Print a Line in Scala?
• Scala Type Hierarchy
• Scala Type Casting
• Scala Keywords
• Scala Identifiers
• Scala lazy val
• What does a Lazy val do in Scala?
• Scala Type Inference
• Scala Operators
• Scala Symbolic Operators
• Scala var Keyword
• Scala val Keyword
• Scala def Keyword
• Scala final Keyword
• Scala var vs. val keywords
• Scope of Scala Variables
• Replacements for ++ and -- in Scala
• Uses of Underscore (_) in Scala
• Scala Constructs

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what does it mean assign "_" to a field in scala?

I saw some scala code that assign "_" to a field of class, what does it mean ? Thanks

• stackoverflow.com/questions/9581444/first-steps-into-scala/… (I tried overview of ~ 10 usages of "_" in the language: –  Gene T Commented Apr 7, 2012 at 0:56

It means: assign default value . Default value is defined as null , 0 or false depending on the target type.

It is described in 4.2 Variable Declarations and Definitions of the The Scala Language Specification :

A variable definition var x : T = _ can appear only as a member of a template. It introduces a mutable field with type T and a default initial value. The default value depends on the type T as follows: 0 - if T is Int or one of its subrange types, 0L - if T is Long, 0.0f - if T is Float, 0.0d - if T is Double, false - if T is Boolean, () - if T is Unit, null - for all other types T .

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Scala Variable

Variables in Scala are used to store data that can be changed or mutated during the execution of a program. There are two main types of variables in Scala: Mutable variables (var) which can be modified after initialization, allowing us to change their values during program execution, and Immutable variables (val) which once initialized, cannot be changed. They promote functional programming practices and ensure data integrity.

What is a Variable in Scala?

In Scala, a variable is a named container that can hold a value. Variables allow us to store and manipulate data within our programs. These data values can be changed or mutated during the execution of a program in the case of mutable variables, or they can remain constant and immutable in the case of immutable variables.

Types of Variables in Scala

In Scala, there are two primary types of variables: mutable and immutable . These variables serve different purposes and have distinct characteristics.

• Variables declared with the var keyword can have their values changed or reassigned after their initial declaration. They are mutable, meaning their contents can be modified during the execution of a program.
• They offer flexibility but can lead to unexpected changes in state, especially in multi-threaded or concurrent programs.
• Mutable variables are declared using the var keyword followed by the variable name and an optional type annotation.
• Variables declared with the val keyword are immutable, meaning their values cannot be changed once assigned. They are constant and provide safety against accidental modifications.
• Immutable variables promote functional programming practices, making code easier to reason about and debug.
• Immutable variables are declared using the val keyword followed by the variable name and an optional type annotation.

Variable Mutability and Concurrency:

Variable mutability plays a significant role in concurrent programming, where multiple threads or processes may access and modify shared data simultaneously.

• Mutable variables can introduce race conditions and data inconsistencies in concurrent programs. When multiple threads or actors modify the same mutable variable without proper synchronization, it can lead to unpredictable behavior and bugs.
• Immutable variables ( val in Scala) are inherently thread-safe because their values cannot be changed after initialization. This makes them a preferred choice in concurrent programming.

Scala's Concurrency Constructs:

• Future: Scala's Future is a concurrency tool for non-blocking, asynchronous operations, commonly employed in concurrent programming for handling potentially time-consuming tasks.
• Actor Model: Scala's Actor model isolates state within actors and uses message passing for communication, resolving shared mutable state challenges in concurrency.

Rules for Naming Variables in Scala

In Scala, variable names must adhere to specific rules and conventions. Adhering to these rules ensures code clarity and maintainability. Here are the rules and conventions for naming variables in Scala:

Case Sensitivity:

• Scala is case-sensitive, so variable names like myVar and myvar are considered different.

Identifier Rules:

• Variable names must start with a letter (uppercase or lowercase), underscore ( _ ), or a dollar sign ( $ ).
• After the initial character, we can use letters, digits, underscores, and dollar signs.
• Variable names cannot start with a digit.

Reserved Words:

• We cannot use Scala's reserved keywords as variable names. For example, we cannot use class , if , else , while , or val as variable names.

Unicode Characters:

• Scala allows the use of Unicode characters in variable names. While this can be useful for internationalization, it's generally recommended to use English letters, digits, and underscores for the sake of readability and compatibility.

CamelCase vs. snake_case:

• Scala conventionally uses camelCase for variable names. In camelCase , the first word starts with a lowercase letter and subsequent words are capitalized, with no spaces or underscores. For example: myVariableName .

Meaningful and Descriptive:

• Variable names should be meaningful and descriptive, reflecting the purpose and content of the variable.
• Avoid using single-letter variable names (e.g., x , y ) unless they have a clear and universally understood meaning in the context.

Examples of valid variable names in Scala:

Examples of invalid variable names in Scala:

Variable Declaration

In Scala, variable declaration involves specifying the name of a variable and, optionally, its data type. There are two main types of variables in Scala: var for mutable variables and val for immutable variables. Here's how we declare variables in Scala:

1. Mutable Variable Declaration (var):

• In Scala, we use the var keyword to declare variables that can be modified.
• We can optionally specify the data type of the variable using a colon ( : ) followed by the type.
• Initialize the variable using the = operator.

2. Immutable Variable Declaration (val):

• Immutable variables are declared using the val keyword.
• We can also specify the data type if we want, but it's often optional because Scala can infer the type from the initial value.

3. Pattern Matching in Variable Declarations:

Pattern matching in variable declarations lets you efficiently destructure complex data structures like case classes, simplifying the process of extracting values and binding them to variables, resulting in concise and expressive code.

Destructuring Case Classes: Pattern matching can be used to extract and assign values from case classes directly to variables during variable declaration.

Here, the person case class is destructured in the case Person ( name , age ) pattern, and the values are assigned to the name and age variables.

Handling Optional Values: Pattern matching can also be used to handle optional values, like those returned from Option types.

Here, pattern matching helps handle both cases when maybeName contains a value ( Some ) or when it's empty ( None ).

Matching Nested Structures: Pattern matching can be nested to handle more complex structures, such as nested case classes or deeply nested data.

Here, the pattern matching extracts both the name and checks if the Address has " London " as the city.

Variable Data Types

In Scala, variable data types determine the data a variable can store, establish during compile-time, and enforce type compatibility, requiring explicit conversions for value changes.

Here are some common data types in Scala:

1. Integers:

• Int: Represents 32-bit signed integers.
• Long: Represents 64-bit signed integers.
• Short: Represents 16-bit signed integers.
• Byte: Represents 8-bit signed integers.
• It's commonly used for whole numbers within a specific range.

2. Floating-Point Numbers:

• Float: Represents 32-bit floating-point numbers.
• Double: Represents 64-bit floating-point numbers.

3. Characters and Strings:

• Char: Represents a single Unicode character. It's used to store individual characters like letters or symbols.
• String: Represents a sequence of characters. It's used to store and manipulate text data in Scala.

4. Booleans:

• Boolean: Represents a true or false value.
• It's used for logical conditions and control flow in programs.

5. Collections:

Scala provides a rich set of immutable collections, including List , Set , and Map , which are essential components of functional programming.

• List: stores ordered collections of elements.
• Set: stores unique elements without duplicates.
• Map: stores key-value pairs for efficient data retrieval.

Advantages of Immutable Collections in Functional Programming:

• Functional Purity: Functional programming promotes pure functions without side effects. Immutable collections support functional purity by preventing data alteration within functions.
• Thread Safety: Immutable collections are naturally thread-safe in multi-threaded environments as they cannot change after creation, removing the need for explicit synchronization like locks when handling shared data.
• Referential Transparency: Immutable collections ensure referential transparency, simplifying testing and reasoning by guaranteeing consistent outputs for the same inputs without hidden state changes.

6. Custom Types:

• We can create our custom data types using class or case class in Scala.
• Classes allow us to define custom data structures with methods and properties.
• Case classes are similar to classes but come with default implementations of methods like equals and toString.

7. Option Type:

• Option represents a value that can be either Some (value) when present or None when absent.
• It's used for the safe handling of potential null or missing values.

Variable Type Inference

Variable type inference is a feature in Scala that allows the compiler to automatically deduce the data type of a variable based on its assigned value, without requiring an explicit type declaration. This feature enhances code conciseness and reduces redundancy while maintaining strong type safety.

Here's how variable type inference works in Scala:

Automatic Type Deduction: When we declare a variable without specifying its data type, Scala's compiler analyzes the assigned value and infers the most specific data type that is compatible with that value.

Type Safety Preserved: Despite not explicitly specifying the data type, the compiler ensures that the inferred type is consistent with the assigned value. Type errors are identified during compilation, not during program execution.

Complex Type Inference: Scala's type inference can handle complex situations, such as inferring the types of elements within collections, function return types, and more.

Explicit Type Annotations (Optional): While type inference is powerful, we can still provide explicit type annotations if we want to specify the type explicitly or improve code readability.

Multiple Assignments

Multiple assignments , also known as tuple unpacking or pattern matching , are a feature in Scala that allows us to assign values from a data structure like a tuple , case class, or other iterable types to multiple variables in a single statement. This feature can make our code more concise and expressive when dealing with multiple values at once. Let's dive into the details:

Tuple Unpacking: Tuples are collections that can hold a fixed number of elements of different types. Scala allows us to easily extract and assign values from tuples using multiple assignments.

In this example, the values from the tuple are unpacked and assigned to the variables number and language .

Pattern Matching: Pattern matching is a powerful feature in Scala that allows us to destructure and match data structures based on specific patterns. We can use pattern matching to assign values based on conditions or specific patterns within a data structure.

In this example, the pair is pattern-matched to determine which case block is executed based on the values of x and y . Pattern matching can be used for complex data structures and provides a concise way to handle various cases.

Function Returns: Multiple assignments can be used to capture multiple return values from a function, which is particularly useful when a function returns a tuple or other compound value.

In this example, the divideAndRemainder function returns a tuple, and the values are directly assigned to the result and remainder variables using multiple assignments.

Variable Scope in Scala

In Scala, variable scope refers to the region of our code where a variable is visible and can be accessed. Scala supports various levels of variable scope, which determine where we can declare and use variables. Understanding variable scope is essential for writing clean and maintainable code. Here are the primary types of variable scope in Scala:

1. Local Scope:

• Variables declared within a block of code or a method have local scope.
• They are only accessible within the block or method where they are declared.
• Local variables can shadow (hide) variables with the same name in outer scopes.

2. Enclosing Scope (Outer Scope):

• Variables declared in an outer block or method are accessible in inner blocks or methods.
• Inner scopes can access and, if not shadowed, modify variables from the enclosing scope.

3. Class/Instance Scope:

• Variables declared within a class or object have class-level scope.
• They are accessible within the class or object methods and can be considered instance variables.
• Class-level variables can also be accessed using class or object instances.

4. Package Scope:

• Variables declared at the package level have package scope.
• They are accessible throughout the package but may not be accessible from outside the package unless explicitly imported.

5. Companion Object Scope:

• In Scala, classes and objects can have companion objects with the same name.
• Variables declared in a companion object are accessible both from the class and the object.
• A Scala variable is a named storage location that holds data and can be either mutable ( var ) or immutable ( val ).
• Scala's powerful type inference system often allows omitting explicit type annotations while maintaining strong type safety.
• Variables in Scala follow strict rules and naming conventions for clarity and consistency.
• Scala supports multiple assignments, enabling the simultaneous extraction and assignment of values from data structures.

Usages of Underscore (_) in Scala

Last updated: March 18, 2024

• Scala Syntax

1. Overview

The underscore (_) is one of the symbols we widely use in Scala. It’s sometimes called syntactic sugar since it makes the code pretty simple and shorter. But, this often results in a lot of confusion and increases the learning the curve.

In this tutorial, we’ll look at the different and most common usages of underscores in Scala.

2. Deprecation Warning

As of Scala 3, some uses of the underscore have been deprecated or removed (such as the syntax of wildcard arguments and vararg splices ), with new syntaxes introduced to replace these usages. The features discussed in this article work well in Scala 2, but not all of them are compatible with Scala 3.

3. Pattern Matching and Wildcards

We widely use the underscore as a wildcard and in matching unknown patterns. This, perhaps, is the first usage of underscore we come across when learning Scala. Let’s see some examples.

3.1. Module Import

We use underscore when importing packages to indicate that all or some members of the module should be imported:

3.2. Existential Types

Also, we use the underscore as a wildcard to match all types in type creators like List, Array, Seq, Option, or Vector :

With _, we allowed all types of elements in the inner list.

3.3. Matching

Using the match keyword, we can use the underscore to catch all possible cases not handled by any of the defined cases .

For example, given an item price, we want to either buy or sell the item based on certain special prices. If the price is 130, we want to buy, but if it’s 150, we want to sell. For any other price outside these, we need to obtain approval for the item:

We can also read our  pattern matching tutorial for more examples.

4. Ignoring Things

The underscore can be used to ignore variables and types that aren’t used anywhere in the code.

4.1. Ignored Parameter

For example, in function execution, we can use the underscore to hide not used parameters :

Using the underscore, we ignored whatever value we have in the map anonymous function; we just return Int for each element of the range. We may use the anonymized parameter as a placeholder in function. This makes the code clean; though less explicit:

Here, the mapping is equivalent to:

We can also use the underscore to access nested collections :

We already saw a hidden import in pattern matching; that can also be referenced as ignoring things. We can ignore a given module and import the rest:

With this, we’ve imported all members of the junit package except (ignoring) Before .

4.2. Ignored Variable

When we don’t care about variables for constructed entries, we can ignore them using the underscore.

For example, we want only the first element in a split string:

The same is applicable if we want only the second one:

We can extend this to more than two entries:

To ignore the rest of the entries after the first, we use the underscore together with *.

We can also ignore randomly using the underscore for any one entry we don’t want:

4.3. Variable Initialization to Its Default Value

When the initial value of a variable is not necessary, we can use the underscore as default:

This doesn’t work for local variables; local variables must be initialized.

5. Conversions

In several ways, we can use the underscore in conversions.

5.1. Function Reassignment (Eta expansion)

With the underscore, we can convert a method to a function:

5.2. Variable Argument Sequence

We can convert a sequence to variable arguments using seqName: _* (a special instance of type ascription).

For example, let’s define our own sum function that accepts varargs (of type Int ) and sum all:

5.3. Partially-Applied Function

By providing only some arguments in a function and leaving the rest to be passed, we can generate a partially-applied function.

The unprovided parameters are replaced by the underscore:

The use of underscores in a partially-applied function can also be grouped as ignoring things . We can also ignore parameter groups in functions with multiple parameter groups to generate a kind of partially-applied function:

5.4. Assignment Operators (Setters overriding)

Overriding the default setter can also be considered as a kind of conversion using the underscore:

6. Miscellaneous Usages

We have other usages of the underscore that might not fit into any of our groups.

6.1. Joining Letters to Operators/Punctuation

We can’t use punctuations in variable names like we do with alphanumerics. If using punctuations in a variable name makes it more explicit, we can do that by joining the letters to the punctuations with the underscore:

6.2. Numeric Literal Separator (Scala 2.13+)

Scala introduced a numeric literal separator using an underscore in Scala 2.13. Let’s see some examples:

6.3. Higher-Kinded Type

A Higher-Kinded type is a type that abstracts over some type that, in turn, abstracts over another type. In this way, Scala can generalize across type constructors. It’s quite similar to the existential type.

We can define higher-kinded types using the underscore:

7. Conclusion

In this tutorial, we’ve seen many different ways to use the Scala underscore.

As always, the full source code of the article is available  over on GitHub .

Scala if then else syntax (and returning a value from an if statement)

Table of contents, scala 2 if/then/else syntax, scala 3 if/then/else syntax, using if/then like a ternary operator, assigning ‘if’ statement results in a function.

Scala FAQ: Can you share some examples of the Scala if/then/else syntax? Also, can you show a function that returns a value from an if/then/else statement?

In its most basic use, the if / then / else syntax in Scala 2 is similar to Java:

In Scala 3 the if / then / else syntax is a little different:

Note that in Scala 3, the parentheses and curly braces are not required. You use the then keyword at the end of each statement, and then indent your code in each block.

Also note that in these examples I am a “four space indent” kind of guy. Most other people prefer two spaces, so if you prefer that, go ahead and use it.

A nice improvement on the Java if/then/else syntax is that Scala if/then statements return a value. As a result, there’s no need for a ternary operator in Scala ., which means that you can write Scala 2 if/then statements like this:

where, as shown, you assign the result of your if/then expression to a variable.

In Scala 3 that now looks like this:

Because the Scala if / then syntax can be used as a ternary operator — an expression , really — it can be used as the body of a Scala function.

You can also assign the results from an if expression in a simple function, like this absolute value function using Scala 2:

As shown, the Scala if/then/else syntax is similar to Java, but because Scala is also a functional programming language, you can do a few extra things with the syntax, as shown in the last two examples.

Here’s what that looks like in Scala 3:

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