C26 Answer

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

/* Puzzle C26 -- look thru an array for a
|   sequence of x immediately followed by y
|
|  Return the index of the first element,
|  or -1 if the sequence is not found.
*/
int findSequence( int arr[], int size, int x, int y )
{
  int j;
  
  for ( j=1; j < size; j++ )
  {
    if ( arr[j-1] == x && arr[j] == y ) return j-1;
  }

  return -1;
}

void printArray( int arr[], int size )
{
  const int N = 10;
  int j;
  
  for ( j=0; j < size; j++ )
  {
    if ( j%N == N-1 )
      printf("%4d\n", arr[j] );
    else
      printf("%4d ", arr[j] );    
  }
}

/* Get user input, x and y. Return 0 if
|  the user wants to quit, otherwise return 1
*/
int userInput( int *x, int *y)
{
  char inputX[32], inputY[32];

  printf("x y: ");
  scanf("%s", inputX );
  
  if ( !isdigit((int)inputX[0]) && inputX[0] != '-' )
    return 0;
  else
  {
    scanf("%s", inputY );
    *x = atoi( inputX );
    *y = atoi( inputY );
    return 1;
  }
}

int main(int argc, char *argv[])
{
  const int SIZE = 10;
  int arr[] = { -5, -3, 0, 6, 4, 16, -3, 0, 7, 9 };
  int loc, x, y;
   
  printArray( arr, SIZE );
  
  while ( userInput( &x, &y) )
  {
    loc = findSequence( arr, SIZE, x, y );
    
    if ( loc != -1 )
      printf("%d, %d found at index %d\n",
      x, y, loc );
    else
      printf("  sequence not found\n");
  }
  
  system("PAUSE");	
  return 0;
}

Comments: The user interaction in the testing program is tricky enough to move into a separate function. The function assumes that if the first group of characters does not start with a digit or a minus sign, then the user wishes to quit. However, the function is easily confused by input that starts with a digit, but then contains no-digit characters.