Iterating over an Array using iterators

Question

I want to iterate over a simple double array using iterators. I wonder how I can do that actually.

What I would like to have is something like this:

void foo(double* array, size_t size)
{
    std::vector<double> real(size);
    std::vector<double> imag(size);
    auto arrBegin = std::begin(array); // this of course doesn't work
    auto arrEnd = std::end(array); // this of course doesn't work

    bool toggle = false;

    std::partition_copy(arrBegin, arrEnd, std::begin(real), std::begin(imag),
                [&toggle] (int) {return toggle != toggle});

    //Do other stuff
}

As some of you might guess, the array is an interleaved complex number, that I would like to split up into arrays of real and imaginary part.

Is there a way to do that using an array that I receive as a pointer if I know the size of the array, or do I need to use a separate class like std::vector for the array in order to work with iterators?

I would appreciate to use std::partition_copy(), but I am open to other ideas.

Note: I cannot use std::array because the array size is not known during compile time. std::vector would be possible, but I actually want to see whether it's possible to do it without std::vector as I receive the array from an API and want to use it as it is.

Note that your vectors aren't big enough to hold the output ranges. — Kerrek SB
– Kerrek SB, Commented Apr 12, 2016 at 8:57
On an unrelated note, if you have a size use size_t instead. On a 64-bit system it's most likely an alias of an unsigned 64-bit integer type just like uint64_t, but it conveys more semantical information to the reader of the code. — Some programmer dude
– Some programmer dude, Commented Apr 12, 2016 at 9:00

Kerrek SB · Accepted Answer · 2016-04-12 09:00:01Z

2

Use array and array + size as your iterators.

You probably also want to use inserter-iterators for the vectors.

#include <algorithm>
#include <iterator>

std::partition_copy(array, array + size,
                    std::back_inserter(real),
                    std::back_inserter(imag),
                    [&toggle](int t){ return t != toggle; });

edited Apr 12, 2016 at 9:00

answered Apr 12, 2016 at 8:56

Kerrek SB

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8 Comments

Chiel Over a year ago

This answer requires some explanations. What are you doing, this is pretty advanced.

Kerrek SB Over a year ago

@Chiel: Which part of this is unclear and not covered by the respective documentations?

Chiel Over a year ago

Why the back inserters? Why the modification to the lambda function? Your solution looks elegant if only I understood it.

Kerrek SB Over a year ago

@Chiel: Inserters because you want to insert (check the documentation of partition_copy. The OP's lambda code was ill-formed. I'm assuming that the OP knows how the algorithms work; the question gave me no reason to believe otherwise.

FreddyKay Over a year ago

Thanks man. Works like a charm. Btw: are the back_inserters in any way preferable to std::begin / std::end if I preallocate the vectors? Oh and my lambda function actually works, as I have shown it above, What did I do wrong?

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