Generate two different random numbers in C

Question

I am trying to generate two different random numbers in C within a given range.The range is

0 to nk-1

But my following code sometimes works and sometimes fails.What is the reason behind this?

n1=rand()%nk;
n2=rand()%nk;
while(n1==n2)
{
 srand(time(0));
 n2=rand()%nk;
}

`srand()` should be called only once to seed the random generator. Do not call it inside the while loop ! — francis, Sep 20 '15 at 17:55
The canonical duplicate is: [`srand()`: why call it only once?](http://stackoverflow.com/questions/7343833/srand-why-call-it-only-once/). — Jonathan Leffler, Sep 20 '15 at 17:57
If you like to generate two different random numbers in that range you should use n1 = rand()%nk; n2 = rand()%(nk-1); if(n1 == n2) n2 += 1; You should avoid the while loop. — Christian Fries, Sep 20 '15 at 18:04
I don't see that this is a duplicate (given that he states in a comment, that he get's a segmentation fault). He should clarify what "fails" means. — Christian Fries, Sep 20 '15 at 18:43

score 3 · Answer 1 · answered Sep 20 '15 at 17:55

3

You're supposed to seed the RNG once,

srand(time(0));
n1=rand()%nk;
n2=rand()%nk;
while(n1==n2)
{
 n2=rand()%nk;
}

answered Sep 20 '15 at 17:55

Aldehir

Cool,now its working.Can you please throw some light on the reason.Explanation would be great. – Sumeet Sep 20 '15 at 17:58
Take a look at the answer for [this question](http://stackoverflow.com/questions/5574914/srandtimenull-doesnt-change-seed-value-quick-enough). – Aldehir Sep 20 '15 at 18:00
I am sorry ,I only ran it 4 to 5 times. But when I ran it for 12 th time still segmentation fault. I don't think it correct. – Sumeet Sep 20 '15 at 18:04
@Dante. The segmentation fault is occurring somewhere else. I suggest you run it through a debugger to determine the cause. – Aldehir Sep 20 '15 at 18:20

ameyCU · Accepted Answer · 2015-09-20T18:05:08.320

1

srand(time(0));

This has to be outside loop.

In loop you reseed the RNG , whereas you should be doing it once.

edited Sep 20 '15 at 18:05

answered Sep 20 '15 at 17:56

ameyCU

I moved it outside the loop,but this only increased the probability of successful running.Meaning that program is still seg faulting but less as compared to my code. – Sumeet Sep 20 '15 at 18:05
@Dante. Is this your excat code. – ameyCU Sep 20 '15 at 18:06
Yes it is my exact code. – Sumeet Sep 20 '15 at 18:12
@Dante.: The segfault is somewhere else in the code you haven't shown. – Blastfurnace Sep 20 '15 at 18:16

2 Answers2