C random number generator generate sometimes the same numbers

Question

i have strange behavior in which i try to generate random numbers and they sometimes totally the same , or there is problem if they created on the same time? using VC 2012

for (int i = array_size; i < (array_size); i++)
{

        srand((unsigned int)time(NULL));
        int rw = rand() % 960 + (20); //returns a pseudo-random integer between x resolution size 0 -> width
        int rh = rand() % 640 + (20);    //returns a pseudo-random integer between x resolution size 0 -> height
        lwsl_notice("c:%d width %d height %d\n",c, rw, rh);
        c++;
}

The output is :

[2016/06/30 09:39:09:7274] NOTICE: c:0 width 606 height 567
[2016/06/30 09:39:09:7274] NOTICE: c:1 width 606 height 567
[2016/06/30 09:39:09:7274] NOTICE: c:2 width 606 height 567
[2016/06/30 09:39:09:7274] NOTICE: c:3 width 606 height 567

oops the more correct dup is [Same random numbers every loop iteration](http://stackoverflow.com/q/9251117/995714), [rand() is consistent across multiple function calls](http://stackoverflow.com/q/29478555/995714), [srand(time(NULL)) doesn't change seed value quick enough](http://stackoverflow.com/q/5574914/995714), [srand() — why call it only once?](http://stackoverflow.com/q/7343833/995714) — phuclv, Jul 30 '16 at 06:54

score 5 · Answer 1 · answered Jul 30 '16 at 06:45

5

Move srand((unsigned int)time(NULL)); out of the for loop.

answered Jul 30 '16 at 06:45

artm

17,291
6
38
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7

This is the correct answer but you should explain why that's the case. – Michael Jul 30 '16 at 06:48
It would be useful to tell explain where exactly can srand be called and what are the pros and cons of possible solutions. – gdziadkiewicz Jul 30 '16 at 06:58

score 3 · Accepted Answer · answered Jul 30 '16 at 06:50

Your loop is so fast that time(NULL) always have the same value. This means that your generator always starts from the same seed, so it will always generate the exact same values.

srand should be called once and only once in your program (outside the for loop). Commonly, people call it once from themain function.

score 1 · Answer 3 · answered Jul 30 '16 at 06:48

1

Inside for loop you initialized i as array_size and you are looping it till i<array_size with incrementing i. Whats that? Please post some more code.

answered Jul 30 '16 at 06:48

Saurabh Badola

148
7

That's not an answer – jpo38 Jul 30 '16 at 06:54
it's **without** incrementing `i` and the loop won't execute a single time. Anyway that's not the main problem here because even if the OP fixes the loop it won't work anyway – phuclv Jul 30 '16 at 06:59

C random number generator generate sometimes the same numbers

3 Answers3