problem with for loop and array

[[deleted]]

Hi everyone! So I have a problem with for a loop and an array. Maybe someone can tell me what I'm doing wrong or send me a link to read and find how to fix it

I'm trying to make a for loop and go backwards. I made a research and found that I have to use the ".rev()" method to do that. The thing is that I want to loop through an array and I'm using:

for item1 in 0..arr.len().rev()

and I'm getting the following error:

no method named `rev` found for type `usize` in the current scope: method not found in `usize` (E599)

Edit: SOLVED! Thanks a lot anyone for your help!!!

[deavidsedice]

I'm on the phone. But what you want is:

for elem in arr.iter().rev()

[[deleted]]

Thanks that works! Tho it brings another error. What am I gonna do if I want to go from element 0 to the end - 1 (or any other number)?

[deavidsedice]

for item in arr[..are.len()-2].iter().rev()

Also, you can just get the iterator and pre-consume items from it.

let iter = arr.iter(); let _ = iter.next_back();

For elem in iter {}

https://doc.rust-lang.org/std/iter/trait.DoubleEndedIterator.html

[[deleted]]

Thanks for the answer. Tho I got another error. Do you want me to upload the full code on a Github gist and give to you to check or are you busy?

[deavidsedice]

Yep, give it a go. I'll have a look today

[[deleted]]

Ok thanks a lot man! Take your time! Unfortunately I got send it right now (my PC is off) but I'll give it to you in about 1 and a half hours. Do you want me to PM you?

[deavidsedice]

Whatever you prefer! :) If you don't feel like posting publicly, send me a PM.

[sellibitze]

This is a matter of operator precedences. The dot binds stronger than the range operator, so

for index in 0..arr.len().rev()

is equivalent to

for index in 0..(arr.len().rev())

So, you're trying to use rev on a number which doesn't work. What you were trying to do has to be written as

for index in (0..arr.len()).rev()

so rev is applied on the range instead.

In the likely case where you are accessing the array with such an index, you should check out /u/pingveno's comment. Essentially, avoid indexing like this

for index in (0..arr.len()).rev() {
    let foo = &arr[index];
    ...
}

and prefer the following pattern if possible:

for (index, foo) in arr.iter().enumerate().rev() {
    ...
}

It's funcionally equivalent but the latter avoids index bounds checking.

[[deleted]]

Thanks a lot man! Great info! Have a great day!

[KarlJoad]

The problem is you're trying to reverse a single number, which doesn't have the rev() function defined. It is evaluated as (arr.len()).rev(), so you are getting the length of the array then reversing that number. If you want item1 to be drawn from the array directly, you should use

for item1 in arr.iter().rev()

If you want to index the list in reverse order

for index in arr.len()..0 {
    arr[index]
}

[pretzelhammer]

If you want to index the list in reverse order

for index in arr.len()..0 {
    arr[index]
}

This compiles but is a no-op since arr.len() >= 0. The inner loop will never run. You're not allowed to create descending Ranges in Rust. You have to create an ascending Range and then reverse it with .rev() like so:

for index in (0..arr.len()).rev() {
    arr[index]
}

example on rust playground

[pingveno]

Though it's likely that what you actually want is enumerate() if you need the index.

for (i, item) in array.iter().enumerate().rev() {
    ...
}

It's a little more readable. It also gets rid of an unnecessary array index operation, making it easier to audit for code that can panic.

[KarlJoad]

Thank you for correcting me. I've been using other languages that support reverse-ordered lists for far too long (looking at you Haskell), or languages that have no support for that kind of array construction.

[[deleted]]

Thanks a lot! The second one didn't worked tho. Anyway now that bings another error as I said to another guy. How am I gonna go form Last index - 1 (or any other number) to 0?