Answer 2D9

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

/* Puzzle D09 -- add two arrays */
#define COLS 6
#define ROWS 6

/* Generate a random integer  min < = r < = max */ 
int randInt( int min, int max )
{
  return rand()%(max-min+1) + min ;
}

void print2DArray ( int nrows, int ncols, int x[nrows][ncols] )
{
  int r, c;
  for ( r=0; r<nrows; r++ )
  {
    for ( c=0; c<ncols; c++ )
      printf("%3d,", x[r][c] );
    printf("\n");
  }
}

void fill2Drandom( int nrows, int ncols, int x[nrows][ncols], int low, int high )
{
  int r, c;
  for ( r=0; r<nrows; r++ )
  {
    for ( c=0; c<ncols; c++ )
       x[r][c] = randInt( low, high );
  }
}

void  add2DArrays( int rows, int cols, int x[rows][cols] , 
                   int y[rows][cols],  int sum[rows][cols]   )
{
  int r, c;
  for ( r=0; r<rows; r++ )
    for ( c=0; c<cols; c++ )
       sum[r][c] = x[r][c]+y[r][c];
}

int main()
{
  int x[ROWS][COLS] ;
  int y[ROWS][COLS] ;
  int sum[ROWS][COLS] ; 
  
  /* Fill the arrays with random integers */
  fill2Drandom( ROWS, COLS, x, -10, +10 );
  fill2Drandom( ROWS, COLS, y, -10, +10 );
  
  /* Add the two arrays */
  add2DArrays(  ROWS, COLS, x, y, sum  );
      
  /* Print the arrays */
  printf("Array x:\n");
  print2DArray( ROWS, COLS, x  );
  printf("\n\nArray y:\n");
  print2DArray( ROWS, COLS, y  );
  printf("\n\nSum:\n");
  print2DArray( ROWS, COLS, sum );

  /* Print the arrays, but with curious parameters */
  printf("Array x:\n");
  print2DArray( ROWS/2, COLS*2, x  );
  printf("\n\nArray y:\n");
  print2DArray( ROWS/2, COLS*2, y  );
  printf("\n\nSum:\n");
  print2DArray( ROWS/3, COLS*3, sum );
  
  printf("\n");
  return 0;
}

It would be greatly informative to make some deliberate mistakes in the above program:

1. Call the function add2DArrays with confused parameters.
2. Change the dimensions of the sum array so it is different from the others.
3. Reverse the dimensions of one of the arrays.
4. Give the three arrays incompatible dimensions.

Often, mistakes such as reversing the dimensions don't have an effect on the program. In figuring out why, recall that a 2D array is stored as a linear sequence of ints in main memory. A 4x7 array takes up 28 ints in memory. The same 28 ints could be regarded as a 7x4 array or a 14x2 array or a 2x14 matrix. The parameters supplied to (say) print2DArray could be any of the above.

Often programs deal with square arrays: arrays with the same number of rows and columns. You might test your program and find that it works perfectly with your sample arrays (all of which are square), only to discover when you release your code that it fails with non-square arrays.