Rust loop

In this tutorial, you will learn about loop expressions in Rust with the help of examples.

In programming, a loop is used to execute a code block multiple times. For example, to print a number 100 times, we use a loop instead of writing the print statement repeatedly.

In Rust, you can use three different keywords to execute a code block multiple times:

  • loop
  • while
  • for

Loop Expression

In Rust, we use the loop expression to indefinitely execute a block of code. If we use a loop, the code execution inside of the loop code block doesn't stop and runs forever.

The syntax of the loop expression is:

loop {
    // code to execute
}

Let's see an example.

fn main() {
    //  loop expression
    loop {
        println!("Loop forever!");
    }
}

Output

Loop forever!
Loop forever!
Loop forever!
.
.
.

This example code will print "Loop forever!" indefinitely unless the user terminates the program. Since the loop runs forever, it is also known as an infinite loop.


Terminating Loop in Rust

We use the break keyword to terminate a loop. For example,

fn main() {
    // initiate an infinite loop
    loop {
        println!("Loop forever!");
        
        // stop infinite loop
        break;
    }
}

Output

Loop forever!

Here, the break keyword terminates the loop. That is why the println! macro is executed only once.

Note: In Rust, we often use a loop and break together.


Example: Print First 10 Natural Numbers using Loop

fn main() {
    let mut number = 0;
    
    // infinite loop starts here
    loop {
        number += 1;
        println!("{}", number);
        
        if number >= 10 {
            // exit the loop
            break;
        }
    }
}

Output

1
2
3
4
5
6
7
8
9
10

In the above example, we have used a loop expression to print the natural numbers. Here, the initial value of the number variable is 0.

Working of Loop

The below table shows the working of the loop in each iteration.

number Inside loop number >= 10
0 number is increased to 1
1 is printed
false
1 number is increased to 2
2 is printed
false
2 number is increased to 3
3 is printed
false
3 number is increased to 4
4 is printed
false
4 number is increased to 5
5 is printed
false
5 number is increased to 6
6 is printed
false
6 number is increased to 7
7 is printed
false
7 number is increased to 8
8 is printed
false
8 number is increased to 9
9 is printed
false
9 number is increased to 10
10 is printed
true (loop terminates)

Hence, we see numbers 1 to 10 printed on the screen.


Working of loop and break in Rust

Working of loop and break in Rust
Working of loop and break in Rust

To learn more about the break keyword, visit Rust break and continue.