Method of referencing each row of 2d array with for loop in C++

Question

I was practicing array problems and I stuck by this one:

Given a declaration of 2D array:

int a[][2] = { {2,2}, {3,3}, {4,4} };

write a nested for loop to print all the values of a.

First, since 2D array is an array of rows (means each element of this array is a row vector),

I tried a for loop like this:

for (int& x[]: a)
    for (int y: x)
        cout << y << " ";

The outer for-loop means I want to reference each row of a, give it a name "x"; the inner for-loop means I want to reference each element of x, give it a name "y".

I thought the declaration in the outer for-loop is valid as I specified x as array in integer type, but error showed up while compiling. I checked out the solution and it indicated that x has to be declared as auto type, which means I should write the outer loop as " for(auto& x: a) ". The solution also indicated that this is the only way, but I was not sure whether it is true or not.

Hence, I want to figure out couple things:

1. Why it was not working when I wrote a line like " for (int& x[]: a) " ?
2. What is the data type of x in the line " for (auto& x : a) " ? What did auto detected?
3. Is using auto really the only way in this situation?

Thank you!

Answer 1

1. In for (int& x[] : a) x is an array of references. Arrays of references are not legal C++.

2. The type is int[2].

3. You can avoid auto by writing for (int (&x)[2] : a). The extra parentheses around &x are crucial, without the parens you have an array of references (not legal), with the parens you have a reference to an array.

Answer 2

As john noted:

#include <iostream>
int a[][2] = { {2,2}, {3,3}, {4,4} };

int main()
{
    for (int(&x)[2] : a) // one row at a time
    {
        for (int x2 : x) // print each col in row
        {
            std::cout << x2 << " ";
        }
        std::cout << '\n';
    }
}

or, just use auto

#include <iostream>
int a[][2] = { {2,2}, {3,3}, {4,4} };

int main()
{
    for (auto& x: a) // one row at a time
    {
        for (auto x2 : x) // print each col in row
        {
            std::cout << x2 << " ";
        }
        std::cout << '\n';
    }
}