Dumb 8 Queens problem in C++ using goto and backtrack

Question

I have been working on multiple 8 queens problems in my C++ class. I got the bazic 8 queens problem. Now I am having an issue printing out the solutions using the goto and backtrack statements.

// Queens2.cpp.cpp : Defines the entry point for the console application.
// 8 Queens problem using goto and backtrack.

#include "stdafx.h"
#include <iostream>
using namespace std;


int main()
{
    int b[8][8] = {0};
    int r, c, i;
    b[0][0] = 1;
    c =0;

NC: c++;
    if (c == 8) goto print;

    r = -1;
NR: r++;
    if (r == 8) goto backtrack;

    // Row Test
    for(i = 0; i < c; i++)
        if(b[r][i] == 1) goto NR;
    // Up Diagonal Test
    for(i = 1; (r-i) >= 0 && (c-i) >= 0; i++)
        if(b[r-i][c-i] == 1) goto NR;
    // Down Diagonal Test
    for(i = 1; (r+i) <= 7 && (c-i) >= 0; i++)
        if(b[r+i][c-i] == 1) goto NR;

    b[r][c] = 1;
    goto NC;

backtrack:
    c--;
    if(c == -1) return 0;

    r = 0;
    while(b[r][c] != 1){
        r++;
    }

    b[r][c] = 0;
    goto NR;

print:
    for(i = 0; i <= 7; i++){
        for(int j = 0; j <= 7; j++){
            cout << b[i][j];
        }
        cout << endl;
    }

    return 0;
}

Answer 1

The goto statement performs a one-way jump to (it "goes to") another line of code; here, the destination is specified by a label. The labels in the code you posted are groups of letters followed by a colon (:), so in this particular case, the labels are:

• NC
• NR
• backtrack
• print

Note that backtrack isn't a statement or a keyword, it's just a label.

Under normal circumstances, code will execute line-by-line, starting from the top of the program, moving downward. However, when a goto statement is executed, the next line executed is not the line below the goto statement, but the line below the label that the goto specifies.

For example, the code

NC: c++;
    if (c == 8) goto print;

    r = -1;

does the following when c is not equal to 8:

1. Increment c
2. Set r equal to -1

However, when c is equal to 8, this happens:

1. Increment c
2. Jump to the print label, causing these lines (immediately below the label) to be executed:

for(i = 0; i <= 7; i++){
    for(int j = 0; j <= 7; j++){
        cout << b[i][j];
    }
    cout << endl;
}

return 0;

Hopefully, that clarifies the goto statement for you.

Answer 2

Your code as given correctly finds one solution and prints it out. It sounds like your assignment is to make the code print all 92 solutions.

When a solution is found and printed (at the print label), what happens next in the given code? Answer: The return at the end of main() is hit, and the whole program exits.

I was able to modify the given code to print all 92 solutions by adding just one line of new code. Hint: Think about what you might want to do next after printing one solution.