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)

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() % rows;
            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);
}