How to get the index of an item in std::array without using a loop?

Question

How can I get the index an item in a std::array without running any loops?

#include <iostream>
#include <array>

std::array<int, 10> some_array = { 89, 56, 78, 96, 4, 34, 77, 2, 48, 3};

unsigned int GetIndexOfValue(unsigned int some_value) {
    // How get the index of some_value here without running a loop?
}

int main() {
    unsigned int some_value = 34;
    std::cout << "The index of value passed is " << GetIndexOfValue(some_value) << std::endl;
}

Is it possible to do it using std::find?

cigien · Accepted Answer · 2020-08-02 17:21:35Z

4

You can use the functionality from the <algorithm> header, so you can avoid writing a raw loop, like this:

unsigned int GetIndexOfValue(unsigned int some_value) {
    return std::distance(std::begin(some_array),
             std::find(std::begin(some_array), std::end(some_array), some_value));
}

Here's a demo.

answered Aug 2, 2020 at 17:21

cigien

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Comments

Igor Tandetnik · Accepted Answer · 2020-08-02 17:16:32Z

1

std::find(some_array.begin(), some_array.end(), some_value) - some_array.begin()

should do the trick (not tested).

answered Aug 2, 2020 at 17:16

Igor Tandetnik

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1 Comment

Igor Tandetnik Over a year ago

Yes, std::find returns an iterator - that's why I subtract some_array.begin() from it, to arrive at an index. You could use std::distance instead if you are so inclined.

max66 · Accepted Answer · 2020-08-02 17:47:38Z

1

Is it possible to do it using std::find?

Yes: with a std::array it's possible.

std::array<int, 10> some_array = { 89, 56, 78, 96, 4, 34, 77, 2, 48, 3};

auto idx =   std::find(some_array.cbegin(), some_array.cend(), 34)
           - some_array.cbegin();

std::cout << "The index of value passed is " << idx << std::endl;

With std::find() you get an iterator that, in the case of std::array (and std::vector) is a random access iterator and support the difference; so you can subtract the cbegin() iterator getting the index

But this doesn't mean that you can avoid a loop: the loop is inside std::find().

See also the answer from cigien that works also with containers not supporting random access iterators: you can use std::distance().

But, in case of a container supporting a non-random access iterator, I suppose that std::distance() can introduce a second loop. So, in this case, I suppose is better if you directly write a single loop.

edited Aug 2, 2020 at 17:47

answered Aug 2, 2020 at 17:20

max66

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4 Comments

max66 Over a year ago

@vsoftco - in case of a random access iterator (std::array and std::vector, at least) is guarantee, as far I know.

vsoftco Over a year ago

Not 100% sure. It's certainly true that the Distance type is ptrdiff_t, en.cppreference.com/w/cpp/iterator/iterator, but cannot find anywhere an operator- for random access iterators.

max66 Over a year ago

@vsoftco - well... I see that now is named "LegacyRandomAccessIterator". I suppose it's to avoid confusion with the new (C++20) std::random_access_iterator concept... C++ become more complicated with every passing version.

max66 Over a year ago

@vsoftco - anyway, from this page you can see that an std::array iterator has to be a "LegacyRandomAccessIterator" (before C++20) and from this page you can see that must support the difference

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