..=
for inclusive ranges
Since well before Rust 1.0, you’ve been able to create exclusive ranges with .. like this:
for i in 1..3 {
println!("i: {}", i);
}
This will print i: 1
and then i: 2
. Today, you can now create an
inclusive range, like this:
# #![allow(unused_variables)] #fn main() { for i in 1..=3 { println!("i: {}", i); } #}
This will print i: 1
and then i: 2
like before, but also i: 3
; the
three is included in the range. Inclusive ranges are especially useful if you
want to iterate over every possible value in a range. For example, this is a
surprising Rust program:
fn takes_u8(x: u8) { // ... } fn main() { for i in 0..256 { println!("i: {}", i); takes_u8(i); } }
What does this program do? The answer: nothing. The warning we get when compiling has a hint:
warning: literal out of range for u8
--> src/main.rs:6:17
|
6 | for i in 0..256 {
| ^^^
|
= note: #[warn(overflowing_literals)] on by default
That’s right, since i
is a u8
, this overflows, and is the same as writing
for i in 0..0
, so the loop executes zero times.
We can do this with inclusive ranges, however:
fn takes_u8(x: u8) { // ... } fn main() { for i in 0..=255 { println!("i: {}", i); takes_u8(i); } }
This will produce those 256 lines of output you might have been expecting.