Foreach loop with multidimensional arrays in c++

Question

I am getting an error while compiling the following cpp code:

int x[][2]{{1, 2}, {3, 4}};

for (int e[2] : x) {
    std::cout << e[0] << ' ' << e[1] << '\n';
}

This gives the following error:

error: array must be initialized with a brace-enclosed initializer

I did replaced int e[2] with auto e and that worked but I want to work with the actual type.

Is there any workaround?

This would be easier with std::array

catnip
– catnip

2022-12-16 16:45:49 +00:00
Commented Dec 16, 2022 at 16:45 — catnip
– catnip, Commented Dec 16, 2022 at 16:45

apple apple · Accepted Answer · 2022-12-16 16:33:19Z

5

the correct fixed-size declaration is

for (int(&e)[2] : x) {}

or you can use auto& to deduce it

for (auto& e : x) {} // same as above

note: auto doesn't deduce the same type

for (auto e : x) {} // e is int*

edited Dec 16, 2022 at 16:33

answered Dec 16, 2022 at 16:31

apple apple

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6 Comments

463035818_is_not_an_ai Over a year ago

fwiw godbolt.org/z/4shcEvvaa

AK-CHP Over a year ago

It works but it seems like we are using a reference type like a reference(int(&e)[2]) or pointer (int *). Though I know that C-style arrays are actually pointers internally. Can't we just use an actual array type like we do in a one dimensional array? Thank you.

apple apple Over a year ago

@AK-CHP then you'd copy the content, which probably is not what you want. (and yes, you cannot copy raw array type)

AK-CHP Over a year ago

I am just hacking. Surely I won't do it in an actual program. I just want to explore whether it can be done :).

apple apple Over a year ago

@AK-CHP no, you cannot copy raw array that way, in range-based-for or not.

|

Ted Lyngmo · Accepted Answer · 2022-12-16 16:35:38Z

2

interpret the inner array as a pointer:

#include <iostream>
   
int main() {
    int x[][2]{{1, 2}, {3, 4}};

    for (int* e : x) {
        std::cout << e[0] << ' ' << e[1] << '\n';
    }
}

edited Dec 16, 2022 at 16:35

Ted Lyngmo

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answered Dec 16, 2022 at 16:26

Frobeniusnorm

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2 Comments

Ted Lyngmo Over a year ago

Surely working, but there's no need to decay the array into a pointer here. auto& would be the better option. +1 nevertheless since it's actually a working solution.

463035818_is_not_an_ai Over a year ago

you loose the information of the size of the array. With the pointer you cannot eg use range based loop on the inner array like this godbolt.org/z/e4vE4vcdo

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