generating 16 digit random number in C


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# 1  
generating 16 digit random number in C

Hi,


How can we generate 16 digit random nos in C.
# 2  
Code:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main(void)
{
        int n;
        srand(time(NULL));
        for(n=0; n<4; n++)  printf("%04d", rand()%10000);
}

# 3  
see also lrand48()

But. None of these are going to give you 16 digit random numbers. They are not meant for that. Corona's idea of concatenating output from a PNG is about as good as it gets.
Note that it will cycle (or repeat) 4 times faster because you are using four calls to a PNG to get one number, ie your sequence will start to become non random after a few thousand calls. ie., approximately (RAND_MAX /2) / 4 = ~4096 numbers

If you want something better try the GSL - gnu scientific library

GSL - GNU Scientific Library - GNU Project - Free Software Foundation (FSF)
# 4  
thnks for reply...
But my requirement is like ,we r designing a product that will generate a unique 16 digit no for every call.
# 5  
Does my solution work, or do you have different requirements? Note that my solution will return the same value if called more than once a second!

Last edited by Corona688; 08-14-2009 at 02:44 PM..
# 6  
Another thought - not all unixes have /dev/urandom and /dev/random.
You can read abou them, but they do provide a random bitstream. If your box has them (Soalris & Linux come to mind) read on -

An example of how to play with the bitstream using Corona's idea.:
Code:
#include <stdio.h>

typedef
union
{
	char tmp[sizeof(unsigned long long)];
	unsigned long long myll;
} ll_t;


unsigned long long llrand(void)
{
	FILE *in=fopen("/dev/urandom", "r");
	ll_t ll_u;
	fread(ll_u.tmp, sizeof(ll_u.tmp), 1, in);
	fclose(in);
	return ll_u.myll;
}

int main()
{
	char tmp[64]={0x0};
	char working[64]={0x0};
	int i=0;
		
	for(i=0; i< 20; i++)
	{
		while(strlen(tmp) < 16)
		{
			sprintf(working, "%lu", llrand() );
			strcat(tmp, working);
		}
		tmp[16]=0x0;
		printf("%s\n", tmp);
		*tmp=0x0;
	}
	return 0;
}

This needs work but you get the idea.
# 7  
thanks jim...
its generating 16 digit no, but could please tell me that how the nos can be generated in double format.

i tried using atof() , but did nt work.
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