How does rand() and srand() together?

**bos1234** · 08-28-2013

Can someone verify if my understanding is correct?

I have read some document that says:
rand() function computes a sequence of pseudo-random integers in the range 0 to RAND_MAX. Returns a pseudo-random integer.

srand(seed) : uses the argument for a new sequence of pseudo-random numbers to be returned by subsequent calls to rand.

Questions:
1. How many numbers are generated when srand is seeded. For example, in the following code, it seems as if srand() has generated a huge list of numbers and rand() returns a number based on some formula. Is this right?

Code:

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

int main()
{
	srand(1);

	printf("%d\n", rand());
	printf("%d\n", rand());
	printf("%d\n", rand());
	printf("%d\n", rand());
	printf("%d\n", rand());
	printf("%d\n", rand());

	getchar();getchar();
	return 0;


}

**oogabooga** · 08-28-2013

All that srand() does is initialize a static parameter to the rand() function called. It doesn't generate a bunch of numbers. Here's an example implementation (from the C standard).

Code:

static unsigned long int next = 1;

int rand(void) // RAND_MAX assumed to be 32767
{
    next = next * 1103515245 + 12345;
    return (unsigned int)(next >> 16) & 0x7fff;
}

void srand(unsigned int seed)
{
    next = seed;
}

**rcgldr** · 08-28-2013

microsoft version:

Code:

int rand(void){
    return(( (seed = seed * 214013L + 2531011L) >> 16) & 0x7fff );
}

wiki article with a list of examples:

https://en.wikipedia.org/wiki/Linear...tial_generator

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