[−]Keyword for
The for keyword.
for is primarily used in for-in-loops, but it has a few other pieces of syntactic uses such as
impl Trait for Type (see impl for more info on that). for-in-loops, or to be more
precise, iterator loops, are a simple syntactic sugar over an exceedingly common practice
within Rust, which is to loop over an iterator until that iterator returns None (or break
is called).
for i in 0..5 { println!("{}", i * 2); } for i in std::iter::repeat(5) { println!("turns out {} never stops being 5", i); break; // would loop forever otherwise } 'outer: for x in 5..50 { for y in 0..10 { if x == y { break 'outer; } } }Run
As shown in the example above, for loops (along with all other loops) can be tagged, using
similar syntax to lifetimes (only visually similar, entirely distinct in practice). Giving the
same tag to break breaks the tagged loop, which is useful for inner loops. It is definitely
not a goto.
A for loop expands as shown:
for loop_variable in iterator { code() }Run
{
let mut _iter = std::iter::IntoIterator::into_iter(iterator);
loop {
match _iter.next() {
Some(loop_variable) => {
code()
},
None => break,
}
}
}RunMore details on the functionality shown can be seen at the IntoIterator docs.
For more information on for-loops, see the Rust book or the Reference.