2-Dimensional Arrays

Question

I've looked at other posts regarding to this problem but can't really seem to apply it to my situation particularly this post: Passing a 2D array to a C++ function

int compute(int size, int graph[][size], int u, int v)

and get the following error:

candidate function not viable: no known conversion from
  'int [size][size]' to 'int (*)[size]' for 2nd argument
int compute(int size, int graph[][size], int u, int v)

When I call this function in my main method I do it the following way:

compute(size,matrix,0,size-1)

Any suggestions as what my problem is?

try defining `int compute(int size, int graph[][], int u, int v)` — jsantander, Apr 01 '14 at 12:53
`matrix` has to be declared as `int *matrix[size]` and not as `int matrix[size][size]` or you have to change the definition of your function as jsantander said. — user2076694, Apr 01 '14 at 12:56
even if my array is 2D, i should declare it as *matrix[size] ?user3018144?? — user3485137, Apr 01 '14 at 13:02
i defined it as int matrix[size][size]. I just set it to all 0's. — user3485137, Apr 01 '14 at 13:02
Compiler cannot make function that takes two-dimentional variable-length array (VLA - which is non-standard to ISO C++, by the way). If you really want to use VLA, make it one-dimentional and pass size separately - VLA won't be passed to function without decaying to pointer. — keltar, Apr 01 '14 at 13:37

score 1 · Answer 1 · edited Apr 01 '14 at 13:35

1

template<size_t sizex, size_t sizey>
int compute(int (&graph)[sizex][sizey], int u, int v);

This behaves the way people expect, for example sizeof returns the correct size without needing to worry about tricky pointer conversions. It will also only let you pass 2d-Arrays, not pointers.

edited Apr 01 '14 at 13:35

keltar

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answered Apr 01 '14 at 13:10

nwp

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@Bathsheba In a way, yes. It is to date not possible in C++ to have variable length arrays so it is not possible to pass arrays that do not have fixed compile time sizes to this function. If VLAs were supported this may break. As of C++14 this will work with any 2D-Array. – nwp Apr 01 '14 at 13:23
So, consider yourself having a forward contract on an upvote for C++14 expiry, settled immediately. – Bathsheba Apr 01 '14 at 13:24
@Bathsheba I did not really understand your comment. Are you saying I should not use this code because it may break in a future version of C++? – nwp Apr 02 '14 at 08:59

score 1 · Answer 2 · answered Apr 01 '14 at 13:17

1

The following worked for me on g++

const int SIZE=3;
int compute(int s, int graph[][SIZE], int u, int v) {
        return 0;
}


int main(int argc,char **argv) {
    int matrix[SIZE][SIZE];
    compute(SIZE,matrix,0,SIZE-1);
}

Note that SIZE is an integer constant, and is not to be confused with the first parameter of compute.

answered Apr 01 '14 at 13:17

jsantander

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can you change the value of size based up on user input? – user3485137 Apr 01 '14 at 13:24

2-Dimensional Arrays

2 Answers2