Question about `position_{max,min,…}`: should we add `position_first_max` and `position_last_min`?

[dmyTRUEk]

In the docs for position_max it is stated:

If several elements are equally maximum, the position of the last of them is returned.

I encountered a problem where I would need something like position_first_max, which returns index/position of the first max value.

And same goes for position_min. Stated:

If several elements are equally minimum, the position of the first of them is returned.

So my question is: should we add position_first_max and position_last_min, which correspondingly return index/position of first max value and index/position of last min value?

If yes I can try making pull request, though I'm not very experienced with Rust.

Usage example:

//  index  0  1  2  3  4  5  6  7  8
let arr = [3, 0, 4, 9, 5, 9, 6, 0, 7];

assert_eq!(Some(5), arr.iter().position_max());
assert_eq!(Some(3), arr.iter().position_first_max());

assert_eq!(Some(1), arr.iter().position_min() );
assert_eq!(Some(7), arr.iter().position_last_min() );

[ronnodas]

Alternatively, you can use std::cmp::Reverse: eg arr.iter().map(Reverse).position_min() gives you position_first_max().

[scottmcm]

Or you can make them not equal any more, if you find that more readable, like arr.iter().zip((0..).map(cmp::Reverse)).position_max() for position_first_max.

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Maybe start with a simple PR to add examples like these to the docs? ("If you instead want the first item equal to the max, then ...") That's helpful without needing to make decisions about APIs.

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Musing: since maintaining this would be nearly free, it makes me ponder what it would be like if position_min returned MinMaxResult<usize>. Then you'd get

• NoElements if the iterator is empty
• OneElement(i) if there's a distinct maximum at i
• MinMax(i, j) if the first one is at i and the last one is at j (with potentially others between them)

Dunno that it'd be worth the breaking change, though.

[ronnodas]

There's also room for {min,max}_position_set analogous to {min,max}_set.

[phimuemue]

Musing: since maintaining this would be nearly free, it makes me ponder what it would be like if position_min returned MinMaxResult<usize>. Then you'd get

* `NoElements` if the iterator is empty

* `OneElement(i)` if there's a distinct maximum at `i`

* `MinMax(i, j)` if the first one is at `i` and the last one is at `j` (with potentially others between them)

Dunno that it'd be worth the breaking change, though.

Nice idea. However, in my experience in (conservatively) 70% of the cases I do not care which max/min/etc. I get (or the result is unique anyway). Thus, I'd prefer this:

• By default, iter.max() just returns any maximum (maybe debug_asserts the result is unique to make the user aware of non-uniqueness).
• For special behavior users can supply "tie-breakers": iter.max<GetFirst>(), iter.max<GetFirstAndLast>() or iter.max<GetVec<_>>().

These tie-breakers could be reused for other functions (min, position_min, theoretically find, ...) and would alleviate the need for things like max_set et al.

afaik Rust currently does not allow my proposed API, but maybe there's a close enough alternative.