Rust currently provides three approaches to performing some kind of iterative activity. They are: loop
`loop,
`, while
`whileand
` and for
`for`. Each approach has its own set of uses.
The infinite loop
`loopis the simplest form of loop available in Rust. Using the keyword
loop, Rust provides a way to loop indefinitely until some terminating statement is reached. Rust's infinite
loop`s look like this:
loop { println!("Loop forever!"); }
Rust also has a while
`while` loop. It looks like this:
let mut x = 5; // mut x: i32 let mut done = false; // mut done: bool while !done { x += x - 3; println!("{}", x); if x % 5 == 0 { done = true; } }
while
`while` loops are the correct choice when you’re not sure how many times
you need to loop.
If you need an infinite loop, you may be tempted to write this:
fn main() { while true { }while true {
However, loop
`loop` is far better suited to handle this case:
loop {
Rust’s control-flow analysis treats this construct differently than a while true
, since we know that it will always loop. In general, the more information
we can give to the compiler, the better it can do with safety and code
generation, so you should always prefer loop
`loop` when you plan to loop
infinitely.
The for
`forloop is used to loop a particular number of times. Rust’s
forloops work a bit differently than in other systems languages, however. Rust’s
forloop doesn’t look like this “C-style”
for` loop:
for (x = 0; x < 10; x++) {
printf( "%d\n", x );
}
Instead, it looks like this:
fn main() { for x in 0..10 { println!("{}", x); // x: i32 } }for x in 0..10 { println!("{}", x); // x: i32 }
In slightly more abstract terms,
fn main() { for var in expression { code } }for var in expression { code }
The expression is an iterator. The iterator gives back a series of
elements. Each element is one iteration of the loop. That value is then bound
to the name var
`var, which is valid for the loop body. Once the body is over, the next value is fetched from the iterator, and we loop another time. When there are no more values, the
for` loop is over.
In our example, 0..10
`0..10is an expression that takes a start and an end position, and gives an iterator over those values. The upper bound is exclusive, though, so our loop will print
0through
` through 9
`9, not
`, not 10
`10`.
Rust does not have the “C-style” for
`for` loop on purpose. Manually controlling
each element of the loop is complicated and error prone, even for experienced C
developers.
When you need to keep track of how many times you already looped, you can use the .enumerate()
function.
for (i,j) in (5..10).enumerate() { println!("i = {} and j = {}", i, j); }
Outputs:
i = 0 and j = 5
i = 1 and j = 6
i = 2 and j = 7
i = 3 and j = 8
i = 4 and j = 9
Don't forget to add the parentheses around the range.
for (linenumber, line) in lines.enumerate() { println!("{}: {}", linenumber, line); }
Outputs:
0: Content of line one
1: Content of line two
2: Content of line tree
3: Content of line four
Let’s take a look at that while
`while` loop we had earlier:
let mut x = 5; let mut done = false; while !done { x += x - 3; println!("{}", x); if x % 5 == 0 { done = true; } }
We had to keep a dedicated mut
boolean variable binding, done
`done, to know when we should exit out of the loop. Rust has two keywords to help us with modifying iteration:
breakand
` and continue
`continue`.
In this case, we can write the loop in a better way with break
`break`:
let mut x = 5; loop { x += x - 3; println!("{}", x); if x % 5 == 0 { break; } }
We now loop forever with loop
`loopand use
` and use break
`breakto break out early. Issuing an explicit
return` statement will also serve to terminate the loop early.
continue
`continue` is similar, but instead of ending the loop, goes to the next
iteration. This will only print the odd numbers:
for x in 0..10 { if x % 2 == 0 { continue; } println!("{}", x); }
You may also encounter situations where you have nested loops and need to
specify which one your break
`breakor
` or continue
`continuestatement is for. Like most other languages, by default a
breakor
` or continue
`continuewill apply to innermost loop. In a sitation where you would like to a
breakor
` or continue
`continuefor one of the outer loops, you can use labels to specify which loop the
breakor
` or
continue
`continuestatement applies to. This will only print when both
xand
` and y
`y` are
odd:
'outer: for x in 0..10 { 'inner: for y in 0..10 { if x % 2 == 0 { continue 'outer; } // continues the loop over x if y % 2 == 0 { continue 'inner; } // continues the loop over y println!("x: {}, y: {}", x, y); } }