D12 Answer

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

#define NUMCOLS 7
#define NUMROWS 4

/* Puzzle D12 -- Initialize a 2D array so that its diagonal is 0,
                 elements above the diagonal are 1,
                 and elements below the diagonal are -1. */

int initDiagonal( int x[][NUMCOLS], int nrows )
{
  int r,  c;

  /* Initialize the Diagonal */
  for ( r=0; r<NUMCOLS && r<nrows; r++ )
    x[r][r] = 0;

  /* Initialze elements above the Diagonal */
  for ( r=0; r<nrows; r++ )
    for ( c=r+1; c<NUMCOLS; c++ )
      x[r][c] = 1;

  /* Initialze elements below the Diagonal */
  for ( r=0; r<nrows; r++ )
    for ( c=0; c<r; c++ )
      x[r][c] = -1;
}

void print2DArray ( int x[][NUMCOLS], int nrows )
{
  int r, c;  /* row and column indexes for the array */

  /* Print elements in row major order */
  for ( r=0; r<nrows; r++ )
  {
    for ( c=0; c<NUMCOLS; c++ )
      printf("%3d ", x[r][c]  );
    printf("\n");
  }
}

int main(int argc, char *argv[])
{
  int x [NUMROWS][NUMCOLS];

  initDiagonal( x, NUMROWS );
  printf("\n\nInitialized:\n");
  print2DArray( x, NUMROWS  );

  system("PAUSE");
  return 0;
}

Comments:

An element x[r][c] is on the diagonal if r==c.

An element x[r][c] is above the diagonal if r<c.

An element x[r][c] is below the diagonal if r>c.

Another way to write the function is:

int initDiagonal2( int x[][NUMCOLS], int nrows )
{
  int r,  c;

  for ( r=0; r<nrows; r++ )
    for ( c=0; c<NUMCOLS; c++ )
      if      ( r==c ) x[r][c] =  0;
      else if ( r<c  ) x[r][c] =  1;
      else             x[r][c] = -1;
}