TopolearnLoops and branches are everyday tools, and Rust's versions are expressions with a few useful twists.
if conditions must be a bool; there is no truthiness, so if count { } on an integer does not compile. Write if count != 0. As an expression, if/else can produce a value as long as both arms have the same type.
Rust has three loops:
loop { }repeats forever untilbreak. It is an expression:break valuemakes the loop evaluate tovalue, which is a neat way to retry until something succeeds.while cond { }runs while a condition holds.for item in iterable { }walks anything iterable. It is the most common loop and avoids index bugs.
Ranges drive counting loops: 0..5 is 0 to 4, 0..=5 includes 5, and (0..5).rev() counts down. To loop over a collection without consuming it, borrow it: for x in &items. Use for (i, x) in items.iter().enumerate() when you also need the index, instead of indexing manually.
Nested loops can be labelled with a lifetime-like name, 'outer: for ..., so that break 'outer or continue 'outer targets the outer loop.
Since Rust 1.88 the 2024 edition also allows let chains in if and while: if let Some(x) = opt && x > 10 { }. You will meet if let with Option shortly.
There is no C-style for (i = 0; i < n; i++) loop and no ++ operator; use ranges and += 1.
Example
fn main() {
let items = ["ant", "bee", "cat"];
for (i, name) in items.iter().enumerate() {
println!("{i}: {name}");
}
let mut n = 27u64;
let steps = loop {
// loop is an expression; break returns a value
if n == 1 { break 0; }
let mut count = 0;
while n != 1 {
n = if n % 2 == 0 { n / 2 } else { 3 * n + 1 };
count += 1;
}
break count;
};
println!("steps = {steps}"); // steps = 111
'outer: for x in 1..=3 {
for y in 1..=3 {
if x * y == 4 { println!("found {x}*{y}"); break 'outer; }
}
}
}Remember
- Conditions must be
bool; there is no truthiness for x in &collectionand ranges like0..nreplace index loopsloopcan return a value withbreak value; labels target outer loops
Watch out for
- Writing
for x in vecand then being surprised the vector was moved; use&vecto borrow.