Is there something wrong with this shuffling algorithm?

Question

I do a little fun computing. My mini-project was a simulation of the Italian game "tomboli". A key building block was the simulation of the following process:

The game is controlled by a person with a bag of 90 marbles with numbers from 1 to 90. He draws balls one after another randomly from the bag, each time calling the marble number to the players.

After a little thought, I wrote the following code for this building block:

// NBR marbles, numbered 1...NBR are in a bag. Simulate randomly
//  pulling them from the bag, one by one, until the bag is empty
void bag( int random_sequence[NBR] )
{
    int i;

    // Store each marble as it is pulled out
    int *store = random_sequence;

    // Array of marbles still in the bag
    int not_yet_pulled[NBR];
    for( i=0; i<NBR; i++ )
        not_yet_pulled[i] = i+1;    // eg NBR=90; 1,2,3 ... 90

    // Loop pulling marbles from the bag, one each time through
    for( i=NBR; i>=1; i-- )
    {
        int x = rand();
        int idx = x%i;  // eg i=90 idx is random in range 0..89
                        // eg i=89 idx is random in range 0..88
                        //            ...
                        // eg i=1  idx is random in range 0..0
                        //    (so we could optimize when i=1 but not worth the bother)
        *store++  = not_yet_pulled[idx];

        // Replace the marble just drawn (so it cannot be pulled again)
        //     with the last marble in the bag. So;
        //     1) there is now one less marble in the bag
        //     2) only marbles not yet pulled are still in the bag
        // If we happened to pull the last marble in the *current subarray*, this is
        //    not required but does no harm.
        not_yet_pulled[idx] = not_yet_pulled[i-1];
    }
}

I know that while modeling games with random numbers there are subtleties and traps, therefore, although I am very pleased with my code, my confidence is slightly less than 100%. So my questions are:

1) Is there something wrong with my code?

2) [if the answer is 1) no] Am I unconsciously using the standard shuffling algorithm?

3) [ 2) ] ?

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Answer 1

Fisher-Yates.

Answer 2

Fisher-Yates-Knuth shuffle:

public static void shuffle(int[] array) 
{
    Random rng = new Random();       // java.util.Random.
    // n is the number of items left to shuffle
    for (int n = array.length; n > 1; n--) 
    {
        // Pick a random element to move to the end
        int k = rng.nextInt(n);  // 0 <= k <= n - 1.
        // Simple swap of variables
        int tmp = array[k];
        array[k] = array[n - 1];
        array[n - 1] = tmp;
    }
}

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Answer 3

int idx = x%i;  // eg i=90 idx is random in range 0..89

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Answer 4

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std:: random_shuffle()?

 void bag( int random_sequence[NBR] )
 {
     for(int i=0; i<NBR; ++i) 
     {    random_sequence[i] = i+1;
     }
     std::random_shuffle(random_sequence,random_sequence + NBR);
 }

std:: random_shuffle():

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Answer 5

rand() % i, ( ), (int) ((rand() / (double) (RAND_MAX+1)) * i).

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Answer 6

:

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Answer 7

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Answer 8

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Earlier versions of Netscape encryption were compromised because hackers were able to predict the "random" number used, since the pseudo-random number generator was populated by the current time. See the Wikipedia entry .