Random number in C linux

Guptarulz · 10-07-2009, 03:50 PM

Seems a silly question

I want to generate random number between 0-50
any idea how to get this done in C

smeezekitty · 10-07-2009, 04:21 PM

#define MAKE_RANDOM(x) (rand() % (((x)+1)&0xFFFFFFF))
int main(){
srand(time(0) / 5000);
printf("%d", MAKE_RANDOM(50));
}

Sergei Steshenko · 10-07-2009, 05:15 PM

Quote:

Originally Posted by Guptarulz

Seems a silly question

I want to generate random number between 0-50
any idea how to get this done in C

man 3 rand

H_TeXMeX_H · 10-08-2009, 02:54 PM

well, if by random you mean pseudo-random then yes using rand() would be the way to go. But, as there is no truly random number (other than maybe radioactive decay), it depends on how random you want it. I recommend you at least seed rand with say an integer from /dev/random or /dev/urandom.

Here's an example of using rand:
http://www.cplusplus.com/reference/c.../cstdlib/rand/

If you want I can post some code using urandom or random.

lutusp · 10-08-2009, 03:49 PM

Quote:

Originally Posted by smeezekitty

#define MAKE_RANDOM(x) (rand() % (((x)+1)&0xFFFFFFF))
int main(){
srand(time(0) / 5000);
printf("%d", MAKE_RANDOM(50));
}

No, that is not how it's done (I won't catalog all the errors you're making). This is:

Code:

#include <iostream>
#include <cstdlib>

double decentRand(double low = 0, double high = 1)
{
	double x = rand()/((double)RAND_MAX+1);
	return (x * (high-low)) + low;
}

Call it like this:

Code:

double x = decentRand(desired low, desired high);

smeezekitty · 10-08-2009, 04:12 PM

works fine for me.
it has no low range.

lutusp · 10-08-2009, 04:30 PM

Quote:

Originally Posted by smeezekitty

works fine for me.

Yes, on your system, with your word length, and a number of other nonportable assumptions, as well as using the modulo operator to select a subrange, a classic beginner's error.

smeezekitty · 10-08-2009, 04:34 PM

well yours returns a floating point for no reason.
that also means it has to do a (potentially slow)floating point to integer conversion.

smeezekitty · 10-08-2009, 04:35 PM

Code:

long Rand(long low = 0, long high = 1)
{
	long x = rand()/((long)RAND_MAX+1);
	return (x * (high-low)) + low;
}

is better

tuxdev · 10-08-2009, 04:44 PM

No, that's even worse. It'll always return low, except if RAND_MAX+1 overflows.

lutusp · 10-08-2009, 04:45 PM

Quote:

Originally Posted by smeezekitty

Code:

long Rand(long low = 0, long high = 1)
{
	long x = rand()/((long)RAND_MAX+1);
	return (x * (high-low)) + low;
}

is better

That's really brilliant. You do realize, don't you, that the result of dividing a given integer by a larger integer is always zero?

Have you ever considered testing your code before posting it? Here is a test run of your function's output:

Code:

lutusp · 10-08-2009, 04:46 PM

Quote:

Originally Posted by smeezekitty

well yours returns a floating point for no reason.
that also means it has to do a (potentially slow)floating point to integer conversion.

Yes, and most uses for rand() in modern times require a float or a double. There is an equivalent integer version, but it doesn't see much use these days.

smeezekitty · 10-08-2009, 04:49 PM

i have always avoided floating point at all costs (started programming on a 486)

tuxdev · 10-08-2009, 04:59 PM

There *is* a way to get a decent random number out of rand() only using integer ops, but it's painful.

1. Figure out the number of bits B in the next highest power of two of (max - min). If (max -min) happens to already be a power of two, that's nice.
2. Figure out how many meaningful bits rand() can produce. That's relatively easy by counting how many times you have to unsigned right shift to make RAND_MAX zero. Be careful you don't count too many or else entropy goes through the floor.
3. Extract the top B meaningful bits from the rand() call.
4. If that produced a number less than (max - min) you're done. If not, call rand() again until you get a number less than (max - min).

This, of course, assumes that (max - min) < RAND_MAX.

smeezekitty · 10-08-2009, 05:22 PM

Code:

#include <stdlib.h>
#include <time.h>
#include <iostream.h>
long _random(int min, int max){
if(min > max){int j = min; min = max; max = j;}
long r = (long)rand()%(long)((long)max-(long)min);
return min+r;
}
int main(){
srand(time(0) / 2);
cout<<("11 RANDOM NUMBERS FROM 100-150\n");
for(int j = 0;j < 11;j++){
cout<<_random(100, 150) <<'\n';
}
return 0;
}

output:

Code:

11 RANDOM NUMBERS FROM 100-150
102
148
128
107
115
117
131
137
127
120
121