Compound Assignment Operators

Goal of This Episode

Learn to update a variable’s value using shorthand like += and -=.

Main Text

Last episode we learned let mut, which makes variables changeable. Today let’s learn a lazier way to write updates.

What Does x = x + 5 Mean?

First, a very important idea. Suppose you have a variable x that’s 10, and you want to add 5 to it:

fn main() {
    let mut x = 10;
    x = x + 5;
    println!("{}", x); // 15
}

Here x appears on both the left and right sides of =. This is not the mathematical statement “x equals x + 5” (which makes no sense in math, right?). In programming it means: first compute the right side, x + 5 (that is, 10 + 5 = 15), then store the result back into the x on the left. So x goes from 10 to 15.

The += Shorthand

That line actually has a shorter way of being written:

fn main() {
    let mut x = 10;
    x += 5;
    println!("{}", x); // 15
}

x += 5 means exactly x = x + 5, just more concise.

The Other Compound Assignment Operators

Subtraction, multiplication, division, and remainder all have corresponding shorthands:

fn main() {
    let mut a = 20;

    a -= 3;
    println!("20 - 3 = {}", a); // 17

    a *= 2;
    println!("17 * 2 = {}", a); // 34

    a /= 4;
    println!("34 / 4 = {}", a); // 8 (integer division)

    a %= 3;
    println!("8 % 3 = {}", a);  // 2
}

At a Glance

A Small Reminder

To use these operators, the variable must be declared with let mut, because you are changing its value.

Recap

• The compound assignment operators: +=, -=, *=, /=, %=.
• x += 5 is shorthand for x = x + 5 — compute the right side first, then store it back on the left.
• Requirement: the variable must be declared with let mut.