How to use function srand() with time.h? [duplicate]

Question

My program contains code that should generate a random positive integer number every time I execute it. It generates random numbers but only once. After that, when I execute same code, it gives me same values, and it is making my code useless.

I started with the rand function, and then I used the srand() function with the time.h header file, but still it is not working properly.

#define size 10
for(i=0;i<size;i++)
    Arr[i] = rand()%size;

First call (random):

6 0 2 0 6 7 5 5 8 6

Second call (random but same as previous):

6 0 2 0 6 7 5 5 8 6

---

Later I visited Stack Overflow questions and I read about the srand() function, and I used it as:

#include<time.h>
for(i=0;i<size;i++)
    Arr[i] = srand(time(NULL));

First call:

-10327 -10327 -10327 -10327 -10327 -10327 -10327 -10327 -10327 -10327

Second call:

-10326 -10326 -10326 -10326 -10326 -10326 -10326 -10326 -10326 -10326

It is giving me different (but not random values). I've defined Arr[i] as unsigned int, and still I am getting negative values.

Centurial

did you set your seed? Set your seed with the time function. – Semite

You only need to call srand once to seed the rand generator. Don't call srand each time to get your numbers – Spragens

You probably ought to read the manpage, which explains how these functions work, and provides a working example. – Quietude

@Magn3s1um I don't know what you are talking about – Centurial

You set your seed with the current time: srand(time(null)). Then you call rand. That way you get a unique seed every single time. The seed changes the random algorithm. Do you understand now? – Semite

is there any specific reason for using srand(), if not may be you can follow [How to use /dev/random or urandom in C? stackoverflow ](#2572866) – Welladvised

@Magn3s1um: Clear and concise explanation :) – Demonolater

Answer 1

You need to call srand() once, to randomize the seed, and then call rand() in your loop:

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

#define size 10

srand(time(NULL)); // randomize seed

for(i=0;i<size;i++)
    Arr[i] = rand()%size;

Chape

srand() takes an unsigned int as an input, so time(NULL) should be recast. – Kort

Answer 2

Try to call randomize() before rand() to initialize random generator.

(look at: srand() — why call it only once?)

Belgium

Please write me the code, it would be really helpful if you put it – Centurial

randomize is not a standard function - perhaps you mean srand ? – Chape

Paul R, yes, i mean srand() from the "stdlib.h". Thank you for adding comment. My mistake. – Belgium

cplusplus.com/reference/cstdlib/srand has an example where srand is called more than once. – Belgium

That example is to illustrate that not calling srand() has the same effect as calling srand(1). It should be made clearer, though. – Fraley

@Ðeepak, may this information will help you: https://mcmap.net/q/64620/-srand-why-call-it-only-once – Belgium

@Daniel Fischer, thank you for the hint. – Belgium

Answer 3

If you chose to srand, it is a good idea to then call rand() at least once before you use it, because it is a kind of horrible primitive psuedo-random generator. See Stack Overflow question Why does rand() % 7 always return 0?.

srand(time(NULL));
rand();
//Now use rand()

If available, either random or arc4rand would be better.

Fabron

Answer 4

#include"stdio.h"
#include"conio.h"
#include"time.h"

void main()
{
  time_t t;
  int i;
  srand(time(&t));

  for(i=1;i<=10;i++)
    printf("%c\t",rand()%10);
  getch();
}

Father

Answer 5

#include"stdio.h"//rmv coding for randam number access

#include"conio.h"

#include"time.h"

void main()
{
    time_t t;
    int rmvivek;

    srand(time(&t));
    rmvivek=1;

    while(rmvivek<=5)
    {
        printf("%c\t",rand()%10);
        rmvivek++;
    }
    getch();
}

Father

this method encryption and decryption progamming used subpart – Father