c++ dynamic array initialization with declaration

Question

I have function like this:

void findScarf1(bool ** matrix, int m, int n, int radius, int connectivity);

and in main function I create 2d dynamic array to pass in this function

    bool matrix[6][7] = {
    {0, 0, 1, 1, 1, 0, 0},
    {0, 0, 1, 1, 1, 0, 0},
    {0, 0, 1, 1, 1, 0, 0},
    {0, 0, 1, 1, 1, 0, 0},
    {0, 0, 1, 1, 1, 0, 0},
    {0, 0, 1, 1, 1, 0, 0}
};

The problem is:

findScarf1(matrix, 6, 7, 3, 4);

causes error C2664: 'findScarf1' : cannot convert parameter 1 from 'bool [6][7]' to 'bool **'

How to initialize array compactly(simultaneously with declaration)?

p.s. sorry if it's duplicate question but I've spent 1.5 hours figuring it out

Arrays are not pointers. You're passing a pointer (after decay) to an array where a pointer to a pointer is expected. — chris, Jul 10 '13 at 09:12
try: `bool matrix[6][7]`, and `bool matrix[][7]`, and `bool (*matrix)[7]` — Grijesh Chauhan, Jul 10 '13 at 09:14
possible duplicate of [passing 2D array to function](http://stackoverflow.com/questions/8767166/passing-2d-array-to-function) — Daniel Daranas, Jul 10 '13 at 09:15
Or just use `std::array`: `std::array, 7>`. Also, this is not a dynamic array. — chris, Jul 10 '13 at 09:15

score 3 · Answer 1 · answered Jul 10 '13 at 09:16

If you look at how your array is laid out in memory, and compare it how a pointer-to-pointer "matrix" is laid out, you will understand why you can't pass the matrix as a pointer to pointer.

You matrix is like this:

[ matrix[0][0] | matrix[0][1] | ... | matrix[0][6] | matrix[1][0] | matrix[1][1] | ... ]

A matrix in pointer-to-pointer is like this:

[ matrix[0] | matrix[1] | ... ]
  |           |
  |           v
  |           [ matrix[1][0] | matrix[1][1] | ... ]
  v
  [ matrix[0][0] | matrix[0][1] | ... ]

You can solve this by changing the function argument:

bool (*matrix)[7]

That makes the argument matrix a pointer to an array, which will work.

And by the way, the matrix variable you have is not dynamic, it's fully declared and initialized by the compiler, there's nothing dynamic about it.

score 2 · Accepted Answer · answered Jul 10 '13 at 09:15

2

Technically, a 2D array is an array of 1D arrays. So it cannot convert into pointer to pointer. It can convert into pointer to array, though.

So this should work:

void findScarf1(bool (*matrix)[7], int m, int n, int radius, int connectivity);

Here bool (*matrix)[7] declares a pointer to array of 7 bool.

Hope that helps.

answered Jul 10 '13 at 09:15

Nawaz

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You have to add that in function `findScaf1()` he can access elements `(*matrix)[i][j]` and `matrix[i][j]` will be wrong, I am sure otherwise OP make mistake. – Grijesh Chauhan Jul 10 '13 at 09:17
1

@GrijeshChauhan, How would it be wrong? That's what it decays to when indexing it normally. – chris Jul 10 '13 at 09:20
2

@GrijeshChauhan: `matrix[i][j]` is absolutely correct syntax. In fact, `(*matrix)[i][j]` is WRONG!!! – Nawaz Jul 10 '13 at 09:22
@chris , Nawaz I was wrong, you both are correct :), but let me keep this comment undeleted may be helpsome one – Grijesh Chauhan Jul 10 '13 at 09:28
1

@GrijeshChauhan: Yes, don't delete it. It is important for programmers to learn what *not to do*. :-) – Nawaz Jul 10 '13 at 09:31

score 1 · Answer 3 · answered Jul 10 '13 at 09:12

You may define the function as:

void findScarf1(bool * matrix, int m, int n, int radius, int connectivity);

Or

void findScarf1(bool matrix[][7], int m, int n, int radius, int connectivity);

No matter how many dimensions an array has, it's just a block of linear memory.

When you use the first manner, you may need to do a cast when call this function:

findScarf1((bool *)marix, 6, 7, 3, 4);

score 1 · Answer 4 · answered Jul 10 '13 at 09:57

My Following example may be helpful for you:

#include<stdio.h>
void f(int (*m)[7]){  // As Nawaz answred
 printf("%d\n", m[3][3]);

}
void _f(int m[6][7]){ // As I commented to your question
 printf("%d\n", m[3][3]);

}
void _f_(int m[][7]){// Second form of Nawaz's answe 
 printf("%d\n", m[3][3]);

}
void f_(int (*m)[6][7]){// Pointer of 2D array
 printf("%d\n", (*m)[3][3]);
}

int main(){

    int matrix[6][7] = {
    {0, 0, 1, 1, 1, 0, 0},
    {0, 0, 1, 3, 1, 0, 0},
    {0, 0, 1, 4, 1, 0, 0},
    {0, 0, 1, 5, 1, 0, 0},
    {0, 0, 1, 6, 1, 0, 0},
    {0, 0, 1, 7, 1, 0, 0}
    };
    f(matrix);
    _f(matrix);
    _f_(matrix);    
    f_(&matrix);
    return 1;
}

question not tanged to c, but I compiled with gcc (I have not installed g++).

~$ gcc  -Wall -pedantic 2d.c
~$ ./a.out 
5
5
5
5

_{I was not intended to post an answer, but because I commented wrong to Nawaz answer so during an experiment I written this code.}

Here one can find it working at codepacde

c++ dynamic array initialization with declaration

4 Answers4

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