Part of the "Chess in C" blog post series. The complete code for part 5.

chessInCPart5-complete.c

#include <stdio.h>
#include <stdlib.h>
#define EMPTY 0
#define KING 1
#define QUEEN 2
#define ROOK 3
#define KNIGHT 4
#define BISHOP 5
#define PAWN 6
#define BREADCRUMB 9

#define POTENTIAL_MOVE_KING 10
#define POTENTIAL_MOVE_QUEEN 20
#define POTENTIAL_MOVE_ROOK 30
#define POTENTIAL_MOVE_KNIGHT 40
#define POTENTIAL_MOVE_BISHOP 50
#define POTENTIAL_MOVE_PAWN 60

int main(void) {
    int i = 0;  			/* Used to count rows when printing board */
    int j = 0;				/* Used to count columns when printing board */
	int board[8][8];		/* Represents board */
    int pieceRow = 0;		/* Stores the piece row */
    int pieceColumn = 0;	/* Stores the piece column */
    int pieceType = 0;		/* Stores the piece type - not used yet! */
    int boardValue = 0;		/* Temp value used when drawing the board */
    int badAnswer = 0;		/* Used to control flow when asking questions */
    char input[256];		/* Contains the user input */

    /* Zero out the board */
    for (i=0; i<=7; i++) {
       for (j=0; j<=7; j++) {
          /* Zero out the square */
          board[i][j] = 0;
       }
    }

    /* Zero out the input array */
    /* Any array is full of garbage when initialized */
    for (i=0; i<=255; i++) {
    	input[i] = 0;
    }

    /* Set initial start positions.
     * Keep this here so we can quickly put pieces on the board. */
    /*board[0][0] = ROOK;
    board[0][1] = KNIGHT;
    board[0][2] = BISHOP;
    board[0][3] = QUEEN;
    board[0][4] = KING;
    board[0][5] = BISHOP;
    board[0][6] = KNIGHT;
    board[0][7] = ROOK;
    board[1][0] = PAWN;
    board[1][1] = ROOK;
    board[1][2] = PAWN;
    board[1][3] = PAWN;
    board[1][4] = PAWN;
    board[1][5] = PAWN;
    board[1][6] = ROOK;
    board[1][7] = PAWN;*/

    /* Print and ask for the row number */
    printf("\nRow? (0..7): ");

    /* Get some input */
    if (fgets(input, sizeof(input), stdin)) {

    	/* Make sure it's a number! */
        if (input[0] != '\n' && sscanf(input, "%d", &pieceRow)) {

        	/* Make sure it's between 0 and 7! */
        	if (pieceRow >= 0 && pieceRow <= 7) {

        		/* If you got here, congratulations. It's valid input. */
        		printf("You've entered %i", pieceRow);
        	} else {

        		/* It wasn't between 0 and 7! */
        		printf("ERROR: Not between 0 and 7");
        		badAnswer = 1;
        	}
        } else {

        	/* It wasn't a number! */
        	printf("ERROR: Not a number");
        	badAnswer = 1;
        }
    }

    if (!badAnswer) {
		/* Print and ask for the column number */
		printf("\nColumn? (0..7): ");

		/* Get some input */
		if (fgets(input, sizeof(input), stdin)) {

			/* Make sure it's a number! */
			if (input[0] != '\n' && sscanf(input, "%i", &pieceColumn)) {

				/* Make sure it's between 0 and 7! */
				if (pieceColumn >= 0 && pieceColumn <= 7) {

					/* If you got here, congratulations. It's valid input. */
					printf("You've entered %i", pieceColumn);
				} else {

					/* It wasn't between 0 and 7! */
					printf("ERROR: Not between 0 and 7");
					badAnswer = 1;
				}
			} else {

				/* It wasn't a number! */
				printf("ERROR: Not a number");
				badAnswer = 1;
			}
		}
    }

    /* Calculate a bishop move */
    board[pieceRow][pieceColumn] = BISHOP;

	/* Calculate possible up-left diagonal destinations
	*  This requires the loop to iterate backwards as we are
	*  checking cells BEFORE the bishop */
	for (i=7; i>=0; i--) {
		for (j=7; j>=0; j--){
			/* If 1) the cell to right is inbounds of the array and
			*     2) the cell isn't not on the final row */
			if ((j+1 != 8) && (i != 7)) {
				/* ...AND the above-right cell has a bishop OR a breadcrumb ... */
				if (board[i+1][j+1] == BISHOP || board[i+1][j+1] == POTENTIAL_MOVE_BISHOP) {
					/* ...then leave a breadcrumb (50) */
					board[i][j] = POTENTIAL_MOVE_BISHOP;
				}
			}
		}
	}


    /* Print the board */

    if (!badAnswer) {
		/* New line for the board */
		printf("\n");

		/* For every row */
		for (i=0; i<=7; i++) {
			/* And every column */
			for (j=0; j<=7; j++){
				/* Get the board value */
				boardValue = board[i][j];

				/* And print the contents */
				switch (boardValue) {
				  case EMPTY:                   printf(".");
												break;
				  case KING:                    printf("K");
												break;
				  case QUEEN:                   printf("Q");
												break;
				  case ROOK:                    printf("R");
												break;
				  case KNIGHT:                  printf("N");
												break;
				  case BISHOP:                  printf("B");
												break;
				  case PAWN:                    printf("P");
												break;
				  case POTENTIAL_MOVE_KING:     printf("+");
												break;
				  case POTENTIAL_MOVE_QUEEN:    printf("+");
												break;
				  case POTENTIAL_MOVE_ROOK:     printf("+");
												break;
				  case POTENTIAL_MOVE_KNIGHT:   printf("+");
												break;
				  case POTENTIAL_MOVE_BISHOP:   printf("+");
												break;
				  case POTENTIAL_MOVE_PAWN:     printf("+");
												break;
				  case BREADCRUMB:              printf("+");
												break;
				}
			  }
			  /* At the end of each row, make a new line */
			  printf("\n");
		 }
    } else {
    	printf("\nERROR: Incorrect input. Not drawing board.");
    }

    /* Exit the program */
    return EXIT_SUCCESS;
}