代码似乎正确，但是程序在打印数组的第一行后挂起

Question

I am trying to implement matrix using pointer. The code seems correct but the program hangs after printing the first row of the array.

int main ()
    {
        int **a;
        int i,j;
        int count = 1;

        for (i = 0; i < 4; i++)
        {
            for(j = 0; j < 4; j++)
            {
                *(*(a + i) + j) = count;
                count += 1;
                printf("%d\t", *(*(a + i) + j));
            }
        }
    }

Answer 1

This will work!

#include<stdio.h>
#include<stdlib.h>
int main ()
{
    int **a;
    int i,j;
    int count = 1;
    // Allocate memory and populate 
    a = malloc(4*sizeof(int*));
    for (i = 0; i < 4; i++)
    {
        a[i] = malloc(4*sizeof(int));
        for(j = 0; j < 4; j++)
        { 
            *(*(a + i) + j) = count;
            count += 1;
            printf("%d\t", *(*(a + i) + j));
        }
        printf("\n");
    }
    //Now free the allocated memory
    for ( i = 0; i < 4; i++)
    {
        free(a[i]);
    }
    free(a);
    return 0;
}

**a is just a pointer to pointer, however in your code you have not specified where this pointer should point to.

First you need to allocate memory before using it and then perform the desired operation (populate, read, write etc) and once you no longer require it, free the allocated memory.

Answer 2

For the code to stand a chance of working, you must allocate space for the array, and space for the pointers. For example:

#include <stdio.h>

int main(void)
{
    int   m[4][4];
    int  *p[4] = { m[0], m[1], m[2], m[3] };
    int **a = p;
    int count = 1;

    for (int i = 0; i < 4; i++)
    {
        for (int j = 0; j < 4; j++)
        {
            *(*(a + i) + j) = count++;
            printf("%d\t", *(*(a + i) + j));
        }
        putchar('\n');
    }
    return 0;
}

There are other ways of specifying m and p . For example:

int m[16];
int *p[4] = { &m[0], &m[4], &m[8], &m[12] };

Answer 3

What is missing from your code is the allocation of memory. When you say int** a; you only get the double pointer variable, no space for the pointer that it should point to, and no space for the integer that the intermediate pointer should point to. Dereferencing such an uninitialized pointer is undefined behavior.

Now, the easiest way to allocate a matrix in C, is to allocate an array of arrays like this:

int main() {
    int n = 4, count = 1;
    int (*matrix)[n] = malloc(n*sizeof(*matrix));
    for(int i = 0; i < n; i++) {
        for(int j = 0; j < n; j++) {
            matrix[i][j] = count++;
        }
    }
    free(matrix);
}

malloc() allocates the memory, free() gives it back so that it can be reused. matrix is a pointer to a line array of the matrix, and the malloc() call allocates enough memory for n elements of whatever matrix points to (that is n line arrays of n elements in this case). You can't do the allocation more concise.