Add an row and a column around an multilinear array-CodePudding

I want to add a row and a column of 1 around an multidimensional array in C.

For the input 1,2,3,4,5,6,7,8,9 - this is how I want it to be added. Is there in C a function like append in python, or how this can be done?

The code I already have for asking the user for elements and printing:

#include <stdio.h>
int main()
{
    int n,m;
    printf("Lines: ");
    scanf("%d" , &n);
    printf("Columns: ");
    scanf("%d", &m);
    int arr[n][m];

//    Input Elements
    printf("Input array elements: \n");
    for (int i=0 ; i<n ; i  )
    {
        for(int j=0 ; j<m ; j  )
        {
            printf("arr[%d][%d]=",i,j);
            scanf("%d" , &arr[i][j]);
        }
    }
//    Show Elements
    for (int i=0 ; i<n 1 ; i   ) {
        for(int j=0; j<m 1; j   ) {
            printf("%d\t", arr[i][j]);
        }
        printf ("\n");
    }

    return 0;
}

Thanks

CodePudding user response：

The size of array does not change during its lifetime. Even though you use Variable-Length Array, word "variable" mean that the size is kept in a variable, not that the size itself is variable.

Therefore you must create a new padded array and fill it:

// add 2 to each dimension due to 1-element-wide strips on each side
int padded[n   2][m   2];

// copy data from the old array
for (int i = 0; i < n;   i)
for (int j = 0; j < m;   j)
    padded[i   1][j   1] = arr[i][j];

// fill padded area
for (int j = 0; j < m   2;   j)
  padded[j][0] = padded[j][m   1] = 1;
for (int i = 0; i < n   2;   i)
  padded[0][i] = padded[n   1][i] = 1;

This code can be easily moved to a bit more generic helper function that uses VLA parameters:

void pad_matrix(int n, int m,
                int pad_size, int pad_val,
                int src[n][m],
                int dst[n   2 * pad_size][m   2 * pad_size]) {

  for (int i = 0; i < n   2 * pad_size;   i) {
  for (int j = 0; j < m   2 * pad_size;   j) {
    // position in src
    int ii = i - pad_size;
    int jj = j - pad_size;
    if (0 <= ii && ii < n && 0 <= jj && jj < m)
      dst[i][j] = src[ii][jj];
    else
      dst[i][j] = pad_val;
  }}
}

Usage could be:

int padded[n   2][m   2];
pad_matrix(n, m, 1, 1, arr, padded);