在c中打印二维数组

Question

I am trying to define a two dimensional array by initially defining elements for 5 x 2 matrix and then I am defining again the elements for 6th row. But when I try to print the elements of this matrix I am getting 0 and 5 for the last value. Tried same by defining elements again for 4th or 6th row but then it is working fine.

#include<math.h>
#include<stdio.h>
main()
{
  int arr[ ][2]={{11,12},{21,22},{31,32},{41,42},{51,52}};
  int i, j;
  arr[5][0]=61; arr[5][1]=62;
  for (i=0;i<=5;i++)
  {
    for (j=0;j<=1;j++)
    {
      printf ("%d \n", arr[i][j]);
    }
  }
}

Answer 1

Your initialised array is given exactly enough memory to hold the specified data values. So

int arr[ ][2]={{11,12},{21,22},{31,32},{41,42},{51,52}};

creates the array as int arr[5][2] and then the line

arr[5][0]=61; arr[5][1]=62;

exceeds the array bounds. The maximum index is [4][1] because array indexing is 0 based. If you want to add another element you should specify

int arr[6][2]={{11,12},{21,22},{31,32},{41,42},{51,52}};

and then this line will work.

arr[5][0]=61; arr[5][1]=62;

An alternative would be to use malloc() to allocate memory for the array, and then if you want to add another row you can use realloc() , and this shows how to make a flexible array.

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

#define COLUMNS 2
#define ROWS 5

typedef int rowtype[COLUMNS];

int main() {
    int i, j;
    rowtype *array = malloc(ROWS * sizeof(rowtype));
    if (array == NULL)
        return -1;

    for (j=0; j<ROWS; j++)
        for (i=0; i<COLUMNS; i++)
            array[j][i] = (j+1)*10 + (i+1);

    for (j=0; j<ROWS; j++) {
        for (i=0; i<COLUMNS; i++)
            printf ("%-4d", array[j][i]);
        printf ("\n");
    }

    printf("Add another row\n");
    array = realloc(array, (ROWS+1) * sizeof(rowtype));
    if (array == NULL)
        return -1;
    array[ROWS][0] = 61;
    array[ROWS][1] = 62;

    for (j=0; j<ROWS+1; j++) {
        for (i=0; i<COLUMNS; i++)
            printf ("%-4d", array[j][i]);
        printf ("\n");
    }

    free(array);
    return 0;
}

Program output:

11  12
21  22
31  32
41  42
51  52
Add another row
11  12
21  22
31  32
41  42
51  52
61  62