Puzzle 2D15

Rotate the elements of a 2D array by N positions Down

[M-15] Write a function that moves every element of an array down by N positions in raster order.

Allow N to be in the range 0 <= N <= NUMROWS*NUMCOLS. If N is out of range, do nothing to the array and return 0. Otherwise, rotate the array and return 1.

Here is an example:

Original:
  0   1   2   3   4   5   6   7   8   9
 10  11  12  13  14  15  16  17  18  19
 20  21  22  23  24  25  26  27  28  29
 30  31  32  33  34  35  36  37  38  39
 40  41  42  43  44  45  46  47  48  49

Rotated by 13:
 13  14  15  16  17  18  19  20  21  22
 23  24  25  26  27  28  29  30  31  32
 33  34  35  36  37  38  39  40  41  42
 43  44  45  46  47  48  49   0   1   2
  3   4   5   6   7   8   9  10  11  12

An easy way to do this is to call the previous function N times. But for this puzzle, perform the rotation by moving each element only once. Think of the array as a 1D sequence of ints. Use a local variable inside the function to hold the first N elements, shift the remaining elements, then copy the first N back to the array. (Some compilers will not allow this.)

Here is a testing framework:

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

int rotate ( int nrows, int ncols, int x[nrows][ncols], int N )
{
  int temp[N];  /* buffer for the first N elements in row-major order */
  int j ;
  . . . .
}
 
void fill2DArray ( int nrows, int ncols, int x[nrows][ncols] )
{
  int r, c, val = 0;
  for ( r=0; r<nrows; r++ )
  {
    for ( c=0; c<ncols; c++ )
       x[r][c] = val++ ;
  }
}

void print2DArray ( int nrows, int ncols, int x[nrows][ncols] )
{
  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<ncols; c++ )
      printf("%3d ", x[r][c]  );
    printf("\n");
  }
}
 
int tester( int nrows, int ncols, int rot ) 
{
  int x[nrows][ncols];
  
  fill2DArray( nrows, ncols, x );
  printf("\n\nOriginal:\n");
  print2DArray( nrows, ncols, x );
  
  rotate( nrows, ncols, x, rot);
  printf("\n\nRotated:\n");
  print2DArray( nrows, ncols, x );
  return 0;
}

int main(int argc, char *argv[])
{
  int nrows, ncols, rot;
  
  printf("Number of rows: ");
  scanf("%d", &nrows);
  printf("Number of cols: ");
  scanf("%d", &ncols);
  printf("Rotation: ");
  scanf("%d", &rot);

  tester( nrows, ncols, rot );  
}