Proper use of rand()? - c++

Question

So the idea with this is that it takes in the 9x9 array that's in main, and swaps around rows. It can only swap rows 1-3 with 1-3, 4-6 with 4-6 and 7-9 with 7-9. For some reason every once in awhile it will swap one from 4-6 with one from 7-9, and also sometimes it will give me absolute garbage for one of the rows 7-9. I've spent the better part of 2 hours trying to figure out the proper way to use rand() in this context and I am sure I am not doing it correctly. Any ideas?
#include <iostream>
#include <time.h>
#include <stdlib.h>
using namespace std;
void printSudoku(int square[9][9]) // Prints out the 9x9 array
{
for(int i = 0; i < 9; i++)
{
for(int j = 0; j < 9; j++)
{
cout << square[i][j];
}
cout << endl;
}
}
void swapRows(int square[9][9]) // Randomly generates numbers, within bounds, and swaps those rows with each other
{
int temp[1][9];
srand(time(NULL));
int n = (rand() % 2) + 0;
int m = (rand() % 2) + 0;
for(int i = 0; i < 9; i++)
{
temp[0][i] = square[n][i];
}
for(int j = 0; j < 9; j++)
{
square[n][j] = square[m][j];
}
for(int k = 0; k < 9; k++)
{
square[m][k] = temp[0][k];
}
int a = (rand() % 5) + 3;
int b = (rand() % 5) + 3;
for(int i = 0; i < 9; i++)
{
temp[0][i] = square[a][i];
}
for(int j = 0; j < 9; j++)
{
square[a][j] = square[b][j];
}
for(int k = 0; k < 9; k++)
{
square[b][k] = temp[0][k];
}
int c = (rand() % 8) + 6;
int d = (rand() % 8) + 6;
for(int i = 0; i < 9; i++)
{
temp[0][i] = square[c][i];
}
for(int j = 0; j < 9; j++)
{
square[c][j] = square[d][j];
}
for(int k = 0; k < 9; k++)
{
square[d][k] = temp[0][k];
}
}
int main() {
int square[9][9] = {1,2,3,4,5,6,7,8,9,
4,5,6,7,8,9,1,2,3,
7,8,9,1,2,3,4,5,6,
2,3,4,5,6,7,8,9,1,
5,6,7,8,9,1,2,3,4,
8,9,1,2,3,4,5,6,7,
3,4,5,6,7,8,9,1,2,
6,7,8,9,1,2,3,4,5,
9,1,2,3,4,5,6,7,8,};
printSudoku(square);
swapRows(square);
cout << endl;
printSudoku(square);
return 0;
}

Answer 1

When you call rand(), it gives you a number between 0 and RAND_MAX. When you use something like rand() % 8, it gives you a random number between 0 and 7. This is more range than you want the random numbers to span. You only want the random numbers to be from 0 through 2 (0, 1, or 2), then add the offset for the first row.
For example, (rand() % 8) + 6 gives you a random number from 0+6=6 through 7+6=13. Instead, use (rand() % 3) + 6 to give you 0+6=6 through 2+6=8.