Loops, part 2: for

Having to manually increment a counter variable is somewhat tedious. The pattern is also extremely common!
To make this easier, Rust provides a more concise way to iterate over a range of values: the for loop.

The for loop

A for loop is a way to execute a block of code for each element in an iterator¹.

Here's the general syntax:

#![allow(unused)]
fn main() {
for <element> in <iterator> {
    // code to execute
}
}

Ranges

Rust's standard library provides range type that can be used to iterate over a sequence of numbers².

For example, if we want to sum the numbers from 1 to 5:

#![allow(unused)]
fn main() {
let mut sum = 0;
for i in 1..=5 {
    sum += i;
}
}

Every time the loop runs, i will be assigned the next value in the range before executing the block of code.

There are five kinds of ranges in Rust:

• 1..5: A (half-open) range. It includes all numbers from 1 to 4. It doesn't include the last value, 5.
• 1..=5: An inclusive range. It includes all numbers from 1 to 5. It includes the last value, 5.
• 1..: An open-ended range. It includes all numbers from 1 to infinity (well, until the maximum value of the integer type).
• ..5: A range that starts at the minimum value for the integer type and ends at 4. It doesn't include the last value, 5.
• ..=5: A range that starts at the minimum value for the integer type and ends at 5. It includes the last value, 5.

You can use a for loop with the first three kinds of ranges, where the starting point is explicitly specified. The last two range types are used in other contexts, that we'll cover later.

The extreme values of a range don't have to be integer literals—they can be variables or expressions too!

For example:

#![allow(unused)]
fn main() {
let end = 5;
let mut sum = 0;

for i in 1..(end + 1) {
    sum += i;
}
}

Further reading

• for loop documentation

Exercise

The exercise for this section is located in 02_basic_calculator/07_for