Inclusive for. This program uses the most common, and probably clearest, syntax for iteration in Rust. It starts at a number, and ends when a second number is reached.
Important The range looped over by for is exclusive, so the second number is never reached in the loop body.
fn main() {
// Loop over first 3 numbers.// ... The top bounds is exclusive (not included).
for i in 0..3 {
println!("Hello: {}", i)
}
}Hello: 0
Hello: 1
Hello: 2
Inclusive for. Sometimes, like when looping up the maximum integer value in a type, we want to include the second number. We can use the equals sign in the range for this.
Here We loop over the last 4 numbers in the i32 type. We use the i32 MAX constant to avoid having to type out the entire number.
fn main() {
// Loop over last 4 numbers in the i32 type.// ... Use "=" in range to include that number.// Use "i32::MAX."
for i in i32::MAX - 3..=i32::MAX {
println!("I: {}", i)
}
}I: 2147483644
I: 2147483645
I: 2147483646
I: 2147483647
For each loop. We can use a for-in loop over a vector or slice. Here we create a vector and push 3 integers to it. Then we use it in 2 for-loops, one after the other.
Detail If we want to use a for-in loop but do not care about the elements, we can use an underscore "_" to eliminate compiler warnings.
fn main() {
let mut values = vec![];
values.push(10);
values.push(20);
values.push(30);
// Use for-loop over values.// Borrow the vector so we can use it in the next loop.
for value in &values {
println!("VALUE: {}", value);
}
// We can ignore the iteration variable if we want.
for _ in &values {
println!("ITERATION");
}
}VALUE: 10
VALUE: 20
VALUE: 30
ITERATION
ITERATION
ITERATION
Loop keyword. For complex loops with unknown end points, an infinite loop with a condition that tests for the end point is often best. In Rust we can use the "loop" keyword for this.
Tip The loop keyword is like a "while true" loop. It is important to always test for the end condition in the loop body.
Also We should make sure to have a "mut" local variable for the index variable, and increment or decrement it on each iteration.
fn main() {
let mut test = 10;
loop {
// Test for modulo 5.
if test % 5 == 0 {
println!("Divisible by 5: {}", test);
}
// End at zero.
if test == 0 {
break;
}
println!("Loop current: {}", test);
test -= 1
}
}Divisible by 5: 10
Loop current: 10
Loop current: 9
Loop current: 8
Loop current: 7
Loop current: 6
Divisible by 5: 5
Loop current: 5
Loop current: 4
Loop current: 3
Loop current: 2
Loop current: 1
Divisible by 5: 0
Enumerate. Consider the for-in loop: we may often want to get both the element value at each index, and the index itself. We can use enumerate() to get these 2-value pairs.
Tip We call enumerate() on the result of iter(). We can get the 2 values from the tuple directly in the for-loop.
Here We access the 3 elements from the str array, alongside the 3 indexes (0, 1 and 2).
fn main() {
let items = ["chair", "computer", "screen"];
// Use enumerate to get an index and value for each element.
for (i, item) in items.iter().enumerate() {
println!("ITEM: {} = {}", i, item);
}
}ITEM: 0 = chair
ITEM: 1 = computer
ITEM: 2 = screen
While loop. Rust supports the while-keyword, and this is the classic while loop. It continues iterating while the specified condition is true.
Here We start the remaining counter at 10, and decrement it by 2 each pass through the loop. Once it goes negative, we end the loop.
fn main() {
let mut remaining = 10;
// Continue looping while a condition is true.
while remaining >= 0 {
println!("{}", remaining);
// Subtract 2.
remaining -= 2;
}
}10
8
6
4
2
0
Reverse. To loop in reverse, we can get the iterator with iter() and then call rev(). This starts at the last element, and proceeds to the first.
fn main() {
let items = vec![10, 20, 30, 40];
// Loop over items in reverse.
for item in items.iter().rev() {
println!("ITEM: {item}");
}
}ITEM: 40
ITEM: 30
ITEM: 20
ITEM: 10
For chars. In programs we often need to iterate over every character in a string. With for, we can do this in Rust—we can avoid some bugs easily this way.
A brief summary. For-loops are clear and easy-to-use in Rust. One important trick is the inclusive end value in range—the equals sign is part of the range.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Feb 10, 2023 (simplify).
