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Subject: Re: [std-proposals] Finding the index of an element (std::find_first_index)
From: Arthur O'Dwyer (arthur.j.odwyer_at_[hidden])
Date: 2020-10-01 11:25:14

On Thu, Oct 1, 2020 at 11:43 AM Kosher Yosher via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> Hi,
> Quick thoughts on adding a new function to <algorithm> which returns the
> index of an element, matched by a predicate, in a container? The goal is to
> stop writing FindX scanning functions and make it as concise as possible to
> do this common operation.

I think from the classic STL's point of view that'd be simply

    auto it = std::find_if(v.begin(), v.end(), pred);
    if (it != v.end()) {
        auto index = std::distance(v.begin(), it);
        // do something with index

The thing is, in terms of the classic STL, you never actually want to know
indices because they're simply not as useful as iterators. Anything you
might want to do with "that element" requires an iterator. Anything you
might want to do "for each element up to that one" requires an iterator.
All you can do with an integer index is use it to increment an existing
iterator, which might be slow if the iterator type isn't random-access.

However, the STL does provide std::count and std::count_if, which do
something similar to what you want and return integers. So I guess maybe
the classic-STL name for what you want is `count_until`. The problem is
that it's not clear what value `count_until` could return to indicate
failure. The obvious answer is "length of the entire range."

    auto trim(string_view sv) {
        auto leading_spaces = nonstd::count_until(sv.begin(), sv.end(),
        auto trailing_spaces = nonstd::count_until(sv.rbegin(), sv.rend(),
        return sv;

Compare to one way you might write that today (if I haven't made an
off-by-one error):

    auto trim(string_view sv) {
        auto first_nonspace = std::find_if(sv.begin(), sv.end(),
        sv.remove_prefix(first_nonspace - sv.begin());
        auto last_nonspace = std::find_if(sv.rbegin(), sv.rend(),
        sv.remove_suffix(sv.end() - last_nonspace);
        return sv;

template<class InputIterator, class UnaryPredicate>
> constexpr std::size_t find_first_index(InputIterator first, InputIterator last, UnaryPredicate pred)
I think you should use std::iter_difference_t
<https://en.cppreference.com/w/cpp/iterator/iter_t><InputIterator> instead
of std::size_t.
OTOH, it's a little weird to talk about the difference_type of an input
OTOOH, it seems that all standard input iterators have `ptrdiff_t` as their
difference_type, so that's good.

my $.02,

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