C：将数组（指针）传递给函数 [英] C: passing an array (pointer) to a function

问题描述

我有一个函数，我想要一个函数来接收2D数组（H），读取指定的列（col）并将其传递给另一个数组（b）。
如果我希望打印1，则下面的代码似乎不正确。
非常感谢任何指导。

I have a function I would like to have a function to receive a 2D array (H), to read a specified column (col) and to pass it to another array (b). It appears that the code below is not OK has I was expecting to get a 1 printed. Any guidance is very much appreciated.

#define nline 5
#define ncol 4

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

void count0(int **M,int col, int *a);

void main(){

    int i;     
    int **H;

    H=(int**)malloc(nline*sizeof(int*));
    for(i=0;i<nline;i++){
        H[i]=(int*)malloc(ncol*sizeof(int));
    }

    H[0][0]=8;
    H[0][1]=5;
    H[0][2]=6;
    H[0][3]=0;
    H[1][0]=7;
    H[1][1]=5;
    H[1][2]=4;
    H[1][3]=0;
    H[2][0]=5;
    H[2][1]=1;
    H[2][2]=1;
    H[2][3]=7;
    H[3][0]=0;
    H[3][1]=0;
    H[3][2]=0;
    H[3][3]=2;
    H[4][0]=1;
    H[4][1]=0;
    H[4][2]=1;
    H[4][3]=4;

    int *b;

    int col=1;

    count0(H,col,&b); 

    printf("num 0=%d\n",b[2]); getchar();

}

/////////////////////////////////////////////////////////////////////////////////

void count0(int **M,int col, int *a){

    int i;

    a=(int*)malloc(1*sizeof(int));

    for(i=0;i<nline;i++){
        a[i]=M[i][col];
        a=realloc(a,(i+2)*sizeof(int));
    }
}

推荐答案

As您已经知道count0函数中矩阵中的行数= nline 。因此，您只需要一次在count函数中为数组 a 分配所有内存，而无需调用realloc（）函数。

As you already knows the number of rows = nline in your matrix in count0 function. So you should simply allocate all memory for array a in count function one time, you don't need to re-call realloc() function.

void count0(int **M, int col, int** a){
   (*a) = malloc(nline * sizeof(int));
   for(i = 0; i < nline; i++){ 
      (*a)[i] = M[i][col];
   }
}

注意：的优先级[ ] 比 * 高，因此您需要（）大约 * a

Note: precedence of [] is higher then * so you need () around *a

只需将此函数调用为： count0（H，col，& b）;

just call this function as: count0(H, col, &b);

和 free（b）; 在printf语句之后的main（）中显式释放内存。

and free(b); in main() after printf statement to deallocate memory explicitly.

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