CODEWITHSUNDEEP
crandom
xacobe97
xacobe97

Reputation: 51

Create random numbers

this is my first question here, i'm making a little game, and for assigning the roles, i need random numbers. I have this code, but is possible that the numbers don't be repeated? Thanks.

void giveroles() {
    srand(time(NULL));
    int r = rand() % i + 1; 
    switch ( r ) { /* ... */ }
}

Upvotes: 1

Views: 152

Answers (3)

JasonD
JasonD

Reputation: 16582

If you want to randomly assign a small set of numbers, rather than generate them at random, create a list of the numbers you want, and then randomise the order of the list (iterate over the list, randomly swapping entries).

For example:

int cards[52];
for(int i = 0 ; i < 52 ; i++)
{
    cards[i] = i;
}
for(int i = 0 ; i < 1000 ; i++)
{
    int r1 = rand()%52;
    int r2 = rand()%52;
    int t = cards[r1];
    cards[r1] = cards[r2];
    cards[r2] = t;
}
for(int i = 0 ; i < 52 ; i++)
{
    printf("%d\n", cards[i]);
}

For completeness, it has been pointed out that shuffling in this fashion is biased. Here's a variation which should be unbiased:

cards[0] = 0;
for(int i = 1 ; i < 52 ; i++)
{
    int r = rand() % (i+1);
    cards[i] = cards[r];
    cards[r] = i;
}

(it should be further noted that taking the module of rand() is also likely to be biased, as the range of rand() will not be an even multiple)

Upvotes: 4

Mats Petersson
Mats Petersson

Reputation: 129314

If you don't want to repeat random numbers, the solution is to keep track of which random numbers you have previously used, and if you get a "hit" in your list of existing numbers, try again.

Also, using srand(time(NULL)); should only be done once. I suspect this is more the question you were actually asking.

Upvotes: 3

Paul R
Paul R

Reputation: 212929

Take out the line srand(time(NULL));. You should in general only ever do this once in your program, e.g. at the start of main(), in order to randomize the seed used by subsequent calls to rand().

Upvotes: 3

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