Sorting complex numbers by their argument

Question

I have a vector of complex numbers and I need to sort them by their argument. Sadly, the numbers have type complex<int>, so function arg(c) returns an integer in range [-3,3] instead of a float and the numbers can't be sorted properly.

I've tried also

typedef complex<int> ci;
typedef complex<double> cd;

vector<ci> a;
sort(a.begin(), a.end(), [](ci v, ci u) { return arg(cd(v)) < arg(cd(u)); });

but it does not work either (compilation error: no matching function for call to ‘std::complex<double>::complex(ci&)). Can I sort these numbers without changing their type?

and the error is ? please post a [mcve] and the compiler error message — 463035818_is_not_an_ai, Mar 25 '22 at 22:22
@463035818_is_not_a_number I've edited the question. and yes, I mean by the angle — Michał Dobranowski, Mar 25 '22 at 22:25
btw type aliases are typically used to make the code more readable. `ci` and `cd` are rather cryptic — 463035818_is_not_an_ai, Mar 25 '22 at 22:25
@ThomasWeller then we can say that they are equal to each other — Michał Dobranowski, Mar 25 '22 at 22:27
i find it odd that `arg( complex)` returns `T`. Though why don't you use `complex` ? — 463035818_is_not_an_ai, Mar 25 '22 at 22:30
I'd rather not use `complex` because later on I need to sum up a lot of those numbers and I'm a little worried about floating point errors. Probably I even need `complex`, but it does not change anything in my problem with sorting. — Michał Dobranowski, Mar 25 '22 at 22:38
`std::complex` does not even seem to be intended for use with anything other than floating point types. The standard says that instantiation with any other types has unspecified effects. You should reconsider whether to use `std::complex` at all. — user17732522, Mar 25 '22 at 22:45
See also [Why does C++ mandate that complex only be instantiated for float, double, or long double?](https://stackoverflow.com/questions/11108743/why-does-c-mandate-that-complex-only-be-instantiated-for-float-double-or-lon.) — user17732522, Mar 25 '22 at 22:47

463035818_is_not_an_ai · Accepted Answer · 2022-03-25T22:52:04.400

3

You get the error because there is no converting constructor from std::complex<int> to std::complex<double> you have to construct the std::complex<double> by passing real and imaginary parts to the constructor:

#include <vector>
#include <complex>
#include <algorithm>

int main() {

    std::vector<std::complex<int>> a;
    std::sort(a.begin(), a.end(), [](const auto& v,const auto& u) {
        return std::arg(std::complex<double>(v.real(),v.imag())) < std::arg(std::complex<double>(u.real(),u.imag())); 
    });
}

Note that you can also use atan2 directly without constructing the std::complex<double> as mentioned by user17732522.

Last but not least, reconsider if you really need int with std::complex. From cppreference:

T - the type of the real and imaginary components. The behavior is unspecified (and may fail to compile) if T is not float, double, or long double and undefined if T is not NumericType.

Basically this means that you need to check the implementation you are using whether it supports std::complex<int> at all.

edited Mar 25 '22 at 22:52

answered Mar 25 '22 at 22:34

463035818_is_not_an_ai

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You can also use `std::atan2` and skip the construction of `std::complex`. (Just be careful, the arguments are the other way around.) – user17732522 Mar 25 '22 at 22:37
That is funny, default implementation of template has such conversion constructor, but specialization for floating points do not have it (conversion is limited only between floating point types). So conversion from `std::complex` to `std::complex` works. IMO should work other way around. – Marek R Mar 25 '22 at 22:50
@MarekR As the answer now mentions, instantiating `std::complex` at all already has unspecified behavior. I am not sure why the definition of the primary template definition with e.g. this converting constructor was left in the standard. Maybe this was just meant to allow implementations room to define the behavior of `std::complex` and similar in whatever way they see fit? – user17732522 Mar 25 '22 at 23:12

Sorting complex numbers by their argument

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