Random characters from array without repetition

Question

I try to create a program in C that selects random characters from the array and stores them in the second array but without repetition.

Code:

int main() {
    srand(time(NULL));
    char characters[] = { 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I' };
    char array[20];
    int size = strlen(array);

    int random, random_character;

    for (int i = 0; i < 4; i++) {
        random_character = rand() % 4;
        random = characters[random_character];
        array[i] = random;
        for (int j = 0; j < 4; j++) {
            if (array[i] == array[j]) {
                array[i] = random;
            }
        }
    }

    for (int i = 0; i < 4; i++) {
        printf("%c ", array[i]);
    }
}

My output still has at least two equal characters.

1. Copy the array as-is to your target that will hold the random sequence. 2. rando-shuffle the target using any proper shuffle algorithm you want (Fisher-Yates for example). 3. Done; the target will now contain the sequence without duplicates (assuming there were none in the source array to begin with) in random order. — WhozCraig, Nov 12 '21 at 20:53
Select 4 random numbers as in https://stackoverflow.com/questions/69888283/random-4-digit-number-with-non-repeating-digits-in-c Then use them as indices to pick the characters. — Eugene Sh., Nov 12 '21 at 21:01
You're calling `strlen()` on an uninitialized char array, which is bad news and the dreaded undefined behavior. At least you're not then trying to use that `size` variable. — Shawn, Nov 12 '21 at 21:27

0___________ · Accepted Answer · 2021-11-12T22:22:27.493

int size = strlen(array); undefined behaviour as array is not initialized. Additionally wrong type.
Your random index is wrong as you probably want to select from all characters in the characters array, not only 4 first.
Your check for a duplicate is wrong.
Many small issues like wrong types.

int main(void)
{
    srand ( time(NULL) );
    char characters[] = {'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I'};
    char array[20];

    int random, random_character;
    size_t j;

    for(size_t i = 0; i < 4; )
    {
        random_character= rand() % sizeof(characters);
        random = characters[random_character];
        array[i] = random;
        for(j = 0; j < i; j++)
        {
            if(array[j] == random) break;
        }
        if(j == i) 
        {
            i++;
        }
    }

    for(size_t i = 0; i < 4; i++){
        printf("%c ", array[i]);
    }
}

https://godbolt.org/z/M9b73KK4r

Random characters from array without repetition

1 Answers1