Randomly select index of matrix exactly once - arrays

Question

I have a matrix of 1's (just a 2D vector of ints) and I am trying to randomly select an index so that I can turn that 1 to a 0. My goal is to select every index of the matrix exactly once so that after running a for loop with exactly as many iterations as there are indices, the matrix will be filled with 0s (the 0 doesn't actually matter, the number that replaces the 1 and the 1 itself are arbitrary).
My current approach is very slow. It has been to run a while loop that checks each pass to see if there are still any 1's left. This is obviously incredibly inefficient, but I am not sure how to do this only once for each index and ensure there are no repeats so that I can change to a for loop. Any advice would be very helpful!

Answer 1

Just generate a random sequence of the matrix indices as #Jonny mentioned in the comment. Then iterate through each element of this sequence. The following is a Java implementation I just wrote in case it helps:
import java.util.Random;
public class Test {
public static void randomSelectMatrixIndex(int[][] matrix) {
int rows = matrix.length;
int cols = matrix[0].length;
int[] indices = new int[rows*cols];
System.out.println("Index array before shuffle: ");
for (int i=0; i<indices.length; i++) {
indices[i] = i;
System.out.print(i+" ");
}
System.out.println();
System.out.println();
shuffle(indices);
System.out.println("Index array after shuffle: ");
for (int j=0; j<indices.length; j++) {
System.out.print(indices[j]+" ");
matrix[indices[j]/cols][indices[j]%cols] = 0;
}
System.out.println();
System.out.println();
}
private static void shuffle(int[] indices) {
Random ran = new Random();
for (int i=indices.length; i>0; i--) {
int randomIndex = ran.nextInt(i);
int temp = indices[i-1];
indices[i-1] = indices[randomIndex];
indices[randomIndex] = temp;
}
}
private static void printMatrix(int[][] matrix) {
for (int i = 0; i < matrix.length; i++) {
for (int j = 0; j < matrix[i].length; j++) {
System.out.print(matrix[i][j] + " ");
}
System.out.println();
}
}
public static void main(String[] args) throws Exception {
int[][] matrix = {{1,1,1,1,1}, {1,1,1,1,1}, {1,1,1,1,1}, {1,1,1,1,1}};
System.out.println("Matrix before random select: ");
printMatrix(matrix);
System.out.println();
randomSelectMatrixIndex(matrix);
System.out.println("Matrix after random select: ");
printMatrix(matrix);
}
}

Answer 2

You use this algorithm, which is a bit more efficient, although still not very efficient after a lot of iterations (note that for best result, use a evenly distributed random number generator):
select random index -> check if 1
| | |
| repeat |False |True
|--------------------| |--Change value at index to 0
|
|
the end
I wonder if there is something one could change to make it more efficient...

Answer 3

Simple. Create a vector of pointers using nested loops so that there's a pointer to every single element in the matrix. Randomly select a pointer. Change value to 0. Remove pointer from vector. Repeat until vector is empty. Matrix is now empty.