range functions should iterate backwards as well

[brson]

Right now we have a bunch of range functions in mod int, etc that all iterate from low to high. It would be convenient if they would detect that the arguments were in high to low order and provide a reverse range. If not, then we need rrange functions.

[andreisavu]

Is this what you are thinking about?

#[doc = "Iterate over the range [`begin`..`end`)"]
fn range(begin: i16, end: i16, it: fn(i16)) {
    let i = begin;
    if begin < end {
        while i < end { it(i); i += 1i16; }
    } else {
        while i > end { it(i); i -= 1i16; }
    }
}

Is there a test suite for libcore?

[brson]

Yes, that's what I was thinking. The core tests are located in the core crate, near the code they test (marked with the #[test] attribute).

[brson]

Although maybe it makes sense for the reverse range to cover the same numbers, so it would still include the low value, but not the high value. Not sure. I wonder if there's a precedent in another language.

[andreisavu]

Would it be a better idea to have an implementation like in Python?

range([start,] stop[, step]) -> list of integers

Return a list containing an arithmetic progression of integers.
range(i, j) returns [i, i+1, i+2, ..., j-1]; start (!) defaults to 0.
When step is given, it specifies the increment (or decrement).
For example, range(4) returns [0, 1, 2, 3].  The end point is omitted!
These are exactly the valid indices for a list of 4 elements.

[brson]

Python's range function does it the way you wrote if you give it a -1 step value, so I guess that makes sense. If you provide a low value that is greater than the high value to python's range it returns an empty list. Maybe there's a good reason for that and we should have a range_step function or somesuch that requires you to be explicit. I'd be interested in hearing other opinions.

[brson]

I would be fine with copying python.

[andreisavu]

Ok. I will attach a pull request later today.

[jruderman]

I think a separate rrange function would be safer. One programmer’s reverse range is another’s base case empty range.

[catamorphism]

@andreisavu -- are you still working on this?

[andreisavu]

No, I'm busy with other things.
On Apr 13, 2012 2:30 AM, "Tim Chevalier" <
reply@reply.github.com>
wrote:

@andreisavu -- are you still working on this?

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#1817 (comment)

[catamorphism]

Ok, then this is up for grabs.