Eight queens in C++

Question

The idea behind this problem is to represent a configuration of 8 queens with a one-dimensional array. So a[0] represents the first row, a[1] the second row etc. and a[row]=column position, where column position represents the "row position" of the queen.

Is there anything I can improve?

#include <iostream>
#include <math.h>
using namespace std;

int main() {

    int a[7];
    int m, b, i=0, x, y, k=0, c,u;

    for(u=0; u<8; u++){
        cin>>a[u];
    }

    for(m=0; m<7; m++){

        for(b=m+1; b<=7; b++){

            if(a[m]==a[b]) i++;
        }

    }   
    for(x=0; x<7; x++){
        c=0;
        for(y=x+1; y<=7; y++){
            c++;
            if(abs(a[x]-a[y])==c) k++;

        }
    }

    if(i==0 && k==0) cout<<"valid";
    else cout<<"invalid";
}

Answer 1

• using namespace std is always wrong. In this case it is unjustified as well.

• int a[7] only has room for 7 elements. You put 8 there. This is plain UB.

• Avoid magic numbers. Define a const int board_size = 8. board_size and board_size - 1 are understood much better than a random mix of 7 and 8.

• An idiomatic counting for loop terminating condition is expressed with <, not <=:

      for (....; b < board_size; ....)
  

• Avoid single letter variables. Variable name usually conveys some meaning. In this case, board coordinates are traditionally called rank and file.

• Declare variables as close the use as possible (e.g. for (int x = 0; ...)).

• Terminate early. Instead of i++ you may immediately say std::cout << "invalid"; return 0; (same with k), thus eliminating unneeded variables.