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

c random srand

173,281

Solution 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;

Solution 2

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

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

Solution 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.

173,281

Author by

Admin

Updated on July 05, 2022

Comments

Admin almost 2 years
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.
Admin almost 11 years

Please write me the code, it would be really helpful if you put it
Paul R almost 11 years

randomize is not a standard function - perhaps you mean srand ?
Ze.. almost 11 years

Paul R, yes, i mean srand() from the "stdlib.h". Thank you for adding comment. My mistake.
Ze.. almost 11 years

cplusplus.com/reference/cstdlib/srand has an example where srand is called more than once.
Daniel Fischer almost 11 years

That example is to illustrate that not calling srand() has the same effect as calling srand(1). It should be made clearer, though.
Ze.. almost 11 years

@Ðeepak, may this information will help you: stackoverflow.com/questions/7343833/srand-why-call-only-once
Ze.. almost 11 years

@Daniel Fischer, thank you for the hint.
R.M.VIVEK Arni over 9 years

this method encryption and decryption progamming used subpart
Jiminion about 7 years

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