声明和初始化动态分配的二维数组的最佳方法是什么？

Question

I tried in this way, but I don't know if it's the best way to do this:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define ROW 5
#define COL 5

int *matrix;

int main(int argc, const char * argv[])
{
    matrix = malloc(ROW * COL * sizeof(int));
    memset(matrix, 0, ROW * COL * sizeof(int));

    return 0;
}

Answer 1

(I'm ignoring the word "best" in your question title)

1. Try to avoid global variables . If you need matrix in main() , then declare it in main() and pass it to other functions as necessary.

2. As @Vlad's answer mentions - if you specifically want to set your array to zero, you can combine your malloc() and memset() calls into a single calloc() ("clear-allocate" if you will) call.

3. If you know the matrix dimensions in advance, you might just allocate the matrix on the stack, eg:

    int main(int argc, const char * argv[]) { int matrix[ROW * COL] = { 0 }; // etc. etc. }

   (which should set the whole array to zero as well.)

   if, on the other hand, the matrix is quite large, then even for dimensions known apriori it makes more sense to allocate dynamically, because stack space is limited.

4. It is not a good idea to allocate every column or every row separately with malloc() . Even if you wanted your matrix to be used in the form matrix[i][j] , you would still use a single allocation - but allocate space for the actual data as well as a sequence of pointers to column beginnings.

Further reading:

• How do I correctly set up, access, and free a multidimensional array in C?
• comp.lang.c FAQ 6.16: How can I dynamically allocate a multidimensional array?

Answer 2

It is better to deal with a matrix when it is represented as a two-dimensional array.

Instead of calling malloc and then memset manually you can substitute them for a call of calloc .

So you can write in main (there is no need to declare the pointer in the file scope)

int ( *matrix )[COL] = calloc( 1, sizeof( int[ROW][COL] ) );

Using such an array makes code more clear. For example to output the matrix you can write

for ( size_t i = 0; i < ROW; i++ )
{
    for ( size_t j = 0; j < COL; j++ )
    {
        printf( "%d ", matrix[i][j] );
    }
    putchar( '\n' );
}

To conclude about difficulties of using a one-dimensional array imagine how will look a code of multiplication of two matrices.