I need to repeat some code for special times, now I'm using following code:
for i in 0..n {
// some code
}
but it's a little annoying because i don't need i in my code and '0' means nothing to me.
is there a better way to do this? Someting like this
loop n {
}
// or
loop!(n, {})
for _ in 0..n { ... }
It's not that much more to type. If you really, desperately cannot stand typing four extra tokens, define a loop! macro.
is there any extra cost on boundary check using for?
Actually, what I mean is to repeat some code.
println!("some code");
println!("some code");
println!("some code");
// code above can fold as
loop 3 {
println!("come code");
};
And what more important is it's more clear than for .
@DanielKeep Can I achive this by define loop! macro? as I know macro cannot capture n then repeat special times.
Isn't that what all loops are for? How is your loop different?
Every kind of finite loop has boundary checks.
There's no static unrolling. That's an optimisation LLVM will do if it feels it's necessary.
If n is an actual literal, you could do it with a proc macro (which I don't believe are stabilised in function form yet). If it's not a literal, you can't do it.
Just use a for loop unless you can demonstrate that it's actually an issue.
I have already know how many times i need to repeat, so I don't want boundary check.
And, I think for i in 0..n is not clear enough to express repeat n times. For example for i in 1..n+1 do the same thing but express different meaning.
yes, I understand. thanks a lot.
This doesn't make any sense to me... I still don't understand how your loop is somehow special and not like any other loop.
In C, if I wanted to printf "jump" 10 times, I would code:
size_t i;
for(i = 0; i < 10; i++)
{
printf("Jump\n");
}
Here, I know that I want to print "Jump" 10 times.... I still have to specify 0-9 aka i < 10. That's just how programming works. Alternatively, I could specify:
size_t i;
while(i < 10)
{
printf("Jump\n");
i++;
}
Same thing, really. How else can I print jump 10 times aside from either writing a recursive solution or just copy/pasting printf("Jump\n"); into the source-code 10 times???
Sure, I could #define some macro which just pastes printf 10 times, but what's the point?
I'm not sure why this is an issue, and I'd just go with @DanielKeep's suggestions since it's pretty canonical and readable in rust. But just as a basic Rust exercise, here are a couple ways to not care about 0..n vs 1..n+1.
The loopn! (loop is a keyword) you asked for:
macro_rules! loopn {
($n:expr, $body:block) => {
for _ in 0..$n {
$body
}
}
}
Alternatively, a helper function supporting an iterator-based approach:
fn times(n: usize) -> impl Iterator {
std::iter::repeat(()).take(n)
}
fn main() {
for _ in times(5) {
println!("Hello world!");
}
}
Both can be seen in this playground.
I second the comment to check whether this is actually an issue, but you can unroll constant loops with crunchy.
I see 1..n+1, but no one has mentioned the inclusive-range operator as an alternative: 1..=n. Stable since Rust 1.26.
because it looks ugly. 
I strongly encourage getting used to ..'s half-openness -- it's far better in algorithms because it splits elegantly: if you divide low..high into low..mid and mid..high, you have all the same items with no overlap and no error-prone ±1 adjustments. This has been known since at least 1982: E.W. Dijkstra Archive: Why numbering should start at zero (EWD 831)
I was offering the inclusive range as an alternative to adding 1 on both sides. Personally, I wouldn't add 1 in the first place, and would stick to the exclusive range.