分配2d阵列

Question

My aim is to allocate a 2d array with only using 1 line for efficiency. Since my prof is expecting it to be efficient. the code gives me an error saying that it can't convert from void* to int.

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

#define NUMOFCOL 4

int **addtwoarr(int (*A)[NUMOFCOL], int (*B)[NUMOFCOL]);

int main(void){
    int firstarr[4][4]={{1,1,1,1},
                        {1,1,1,1},
                        {1,1,1,1},
                        {1,1,1,1}},
        secondarr[4][4]={{1,1,1,1},
                        {1,1,1,1},
                        {1,1,1,1},
                        {1,1,1,1}}, **receiver;
    receiver = addtwoarr(firstarr, secondarr);
    printf("%d", receiver[3][3]);
}

int **addtwoarr(int (*A)[NUMOFCOL], int (*B)[NUMOFCOL]){
    int col, row, **arr;
    (*arr)[NUMOFCOL] = malloc(NUMOFCOL * sizeof(*arr)); /*this line in particular gives the error */
    for(row=0; row<NUMOFCOL; row++){
        for(col=0;col<NUMOFCOL; arr[row][col]=A[row][col]+B[row][col], col++){}
    }
    return arr;
}

The allocation happens in the addtwoarr function which is where the error occurs.

Answer 1

I seriously don't recommend this, as there are a ton of assumptions in your code about top-end sizing. But if you really want to do it, the rather cryptic syntax for returning pointers to fixed length arrays in C looks something like this:

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

#define NUMOFCOL 4

int (*addtwoarr(int A[][NUMOFCOL], int B[][NUMOFCOL]))[NUMOFCOL];

int main(void)
{
    int firstarr[][NUMOFCOL] = {
        {1,1,1,1},
        {1,1,1,1},
        {1,1,1,1},
        {1,1,1,1}},

    secondarr[][NUMOFCOL] = {
        {1,1,1,1},
        {1,1,1,1},
        {1,1,1,1},
        {1,1,1,1}};

    int (*receiver)[NUMOFCOL] =  addtwoarr(firstarr, secondarr);
    printf("%d\n", receiver[3][3]);
    free(receiver);
}

int (*addtwoarr(int A[][NUMOFCOL], int B[][NUMOFCOL]))[NUMOFCOL]
{
    int col, row;
    int (*arr)[NUMOFCOL] = malloc(NUMOFCOL * sizeof(*arr));
    for(row=0; row<NUMOFCOL; row++){
        for(col=0;col<NUMOFCOL; arr[row][col]=A[row][col]+B[row][col], col++);
    }
    return arr;
}

Output

2

Best of luck.