Takes 2 different random number in C language

Question

Possible Duplicate:
calling rand() returning non-random results

In my workshop I need to takes 2 different random numbers but I get 2 same random number.

#include<stdio.h>
#include<stdlib.h>
#include<time.h>
int random1_6(){
 int k;
 srand(time(0));
 k=((rand()%6)+1);
 return k;
}
int main(void){
 int a,b;
    a=random1_6();
    printf("%d.\n",a);
    b=random1_6();
    printf("%d.\n",b);
return 0;
}

How to get 2 different random number?

ckruse · Accepted Answer · 2012-07-08T14:33:16.360

5

A non-cryptographic random number generator (RNG) is not truely random but it generates random-like numbers based on a seed.

What you do is initializing the RNG with the same seed two times, so you get the same results. Seed the RNG just once, e.g. at program start, and you will get random-like different results.

Edit: a code like follows should work:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int random1_6(){
  return ((rand() % 6) + 1);
}

int main(void){
  int a,b;
  srand(time(NULL));

  a = random1_6();
  printf("%d.\n",a);

  b=random1_6();
  printf("%d.\n",b);

  return 0;
}

edited Jul 08 '12 at 14:33

answered Jul 08 '12 at 14:18

ckruse

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+1 for explaining what is going on. – ArjunShankar Jul 08 '12 at 14:19
Can you edit my code to get diiferent result? – Conan9x1111 Jul 08 '12 at 14:28
@Conan9x1111 edited answer and added example – ckruse Jul 08 '12 at 14:33
@Conan9x1111 It may be, since a value range from 1 to 6 is not very big. When I execute the code above I get sometimes two times the same value and some times different values. That's how random works. – ckruse Jul 08 '12 at 14:39
Thank you! Your code is usefull. – Conan9x1111 Jul 08 '12 at 14:39
If we need sum of two different random number is 11, it takes 218504 times call 2 random numbers. Can you have effective way? – Conan9x1111 Jul 08 '12 at 14:51
Generate a random number and subtract it from 11 to get your second number? – tangrs Jul 08 '12 at 15:02

score 3 · Answer 2 · answered Jul 08 '12 at 14:18

3

Don't do srand(time(0)); on every call. Only call it once.

answered Jul 08 '12 at 14:18

Luchian Grigore

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score 1 · Answer 3 · answered Jul 08 '12 at 14:18

You must initialize the random number generator seed with srand() only once : upon each call you are re-initializing the RNG with the same seed since is it most likely that the two subsequent calls to time(0) will return the same timestamp (seconds-level precision), hence rand() will return the same number twice.

If you call srand() only once at the beginning of your program (in the main() entry-point), then every call to rand() will return a different number.

score 0 · Answer 4 · answered Jul 08 '12 at 14:20

You always initialize the random number generator with the same seed, so you'll get the same random sequence, it is pseudo random anyway. Typically you will only want to call srand once in the beginning to initialize the generator.

Also, you only have 6 different possible outcomes, so it is perfectly legitimate to get same number twice, there is 1/6 chance for that.

Takes 2 different random number in C language

4 Answers4