Nested for loop in a two-dimension array

Question

I have learned how to print out each element in a two-dimensional array

int arr[3][3] = {....};
for ( auto &row : arr){
     for ( auto col : row)
        cout<<col<<endl;
}

I understand that the &row in the outer for loop has to be a reference. Otherwise, row will become a pointer pointing to array arr's first element which is an array of 3 ints.

Based on this, I thought the following code could work but it didn't

for( auto row : arr ){
    for ( auto col:*row)
         cout<<col<<endl;
}

It gives me the error about the inner for loop

no callable 'begin' function found for type 'int'

Did I miss something here?

row is an array of 3 ints, which can be iterated. *row is an int, you can't loop through an integer. — tnt
– tnt, Commented Mar 9, 2019 at 14:04
@tntxtnt row is a pointer, not an array (in the code that gives the error). — interjay
– interjay, Commented Mar 9, 2019 at 14:11
Ah I didn't see row is not a reference in the second example. — tnt
– tnt, Commented Mar 9, 2019 at 14:29

interjay · Accepted Answer · 2019-03-09 14:09:44Z

2

Each element of arr has type int[3].

When row is a reference, it gets type int (&) [3], which can be iterated over. But when it isn't a reference, the int[3] array decays to a pointer to its first element, so row has type int*, which can't be used in a range-for loop.

Your code is attempting to iterate over *row, which has type int, leading to the error.

answered Mar 9, 2019 at 14:09

interjay

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