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Difference between let and var variables in Mojo with Example

Mojo is a programming language designed as a superset of Python, meaning it includes all the features and concepts of Python while also providing additional powerful features for systems programming.

Mojo is a programming language that builds upon Python, adding extra features for systems programming. If you're familiar with Python, you'll find Mojo easy to understand.

How to write a first program in Mojo?

Let's start with a simple example. In Mojo, you can write a "Hello World" program just like you would in Python:

print("Hello Mojo!")

What is the use of  let and var in Mojo?

In Mojo, let and var are keywords used for variable declarations, but they have different characteristics:

1. let declaration:

   • Variables declared with let are immutable, meaning their values cannot be changed once assigned.
   • let variables have block scope, which means they are only accessible within the block (e.g., function, loop, or conditional statement) in which they are declared.
   • If you try to reassign a value to a let variable, it will result in an error.
   • let variables are useful when you want to ensure that a value remains constant or immutable within a block.

2. var declaration:

   • Variables declared with var are mutable, meaning their values can be changed after assignment.
   • var variables have function scope, which means they are accessible within the entire function in which they are declared.
   • You can reassign a value to a var variable without encountering an error.
   • var variables are useful when you need to modify the value of a variable throughout a function.

Here's an example to illustrate the difference:

def example_function():
    let x = 5
    var y = 10

    if x == 5:
        let x = 15
        var y = 20
        print("Inside if block: x =", x, "y =", y)

    print("Outside if block: x =", x, "y =", y)

example_function()

 In this example, we have a function called example_function. Inside the function, we declare two variables, x and y, using let and var respectively.

Within the if block, we have a new let variable x with a value of 15, which shadows the outer x variable with a value of 5. We also have a new var variable y with a value of 20, which shadows the outer y variable with a value of 10. When we print the values inside the if block, we see the updated values.

Outside the if block, we print the values again, and we can observe that the let variable x retains its original value of 5, while the var variable y retains its updated value of 20.

When you run this program, it will output:

Inside if block: x = 15 y = 20
Outside if block: x = 5 y = 20

This example demonstrates how let and var declarations differ in terms of mutability and scope in Mojo.