Random numbers in C

Question

for(i = 0; i < n; i++){
        srand(time(NULL));
        printf("%d ", time(NULL));
        for(j = 0; j < (n-1); j++){
            a[i][j] = rand();
        }
    }

I try to generate random numbers, but they are the same... I try srand(i * time(NULL)). No matter.. What should i do?

Array declaration:

int** a;
int i;
printf("Enter array size: ");
scanf("%d", &n);

a = (int**)calloc(n, sizeof(int));
for(i = 0; i < n; i++)
    a[i] = (int*)calloc(n-1, sizeof(int));

Answer 1

srand(time(NULL)); 

for(i = 0; i < n; i++){         
        printf("%d ", time(NULL)); 
        for(j = 0; j < (n-1); j++){ 
            a[i,j] = rand(); 
        } 
    } 

Call srand once outside the loop.

Answer 2

Answer 3

srand(time(NULL));
for(i = 0; i < n; i++){
    for(j = 0; j < (n-1); j++){
        a[i,j] = rand();
    }
}

No matter. The number are the same...

Answer 4

srand is a function that "seeds" the random number generator. In case you don't know, random numbers in computers aren't really random. In effect, the computer just has a list of numbers that seem random in it, and you use srand to tell it where to start in that list, with each call to rand() returning the next item in the list.

The reason you write srand(time(NULL)) is to get the random numbers to start at some point that isn't going to be the same every time you run the program (unless the programs start at the same time).

So what you are doing here is repeatedly telling the program to restart the random number list at the same point (because the time is the same each time you go through the loop). Move the call to srand outside the loop and you will get the correct results.

Answer 5

FAQs 13.15 to 13.20 will be of interest. And I am tempted to create a new tag for such questions.

Answer 6

Don't call srand() every time through the loop - just do it once beforehand.