Random characters in 2d array

I'm trying to make a program in C that will generate an 2d array with random characters, but two columns must still be empty (filled with space).

Example: (red columns are empty)

[1]: https://i.stack.imgur.com/2aIJW.png

But the problem is that one character can be as many times as there are rows. There are four rows in the picture, so one character can be a maximum of four times. But I have for example 6 times # and only 3 times * .

Code :

void generator(const int rows, const int columns, char field[rows][columns]){
    srand ( time(NULL) );
    char characters[] = {'#', '@', '^', '*', '%', '+', '~', '?', '-', '/'};
    int r = rand() % columns;
    int q = r;
    while(q == r){
        q = rand() % columns;
    }
    
    for(int j = 0; j < rows; j++){        
        for(int i = 0; i < columns; i++){
            int random_znak = rand() % 4;
            int random = characters[random_znak];
            if( i == r || i == q){
                field[j][i] = ' ';
            }else{
                field[j][i] = random;
            }
        }
    }

    for (int i = 0; i < rows; i++) {
        for (int j = 0; j < columns; j++) {
            printf("%c ", field[i][j]);
        }
        printf("\n");
    } 
}

int main(){   
    char field[10][10];
    generator(4 , 6, field);
}