-1 (inverse)

brazilnut · 10-17-2008, 10:17 AM

Hi, i'm having a play with random number generators and was looking at this.

My question is, how do you do 'inverse' in C?
I've found some functions for 'inverse modulus', but i don't think thats right.

Heres a template i'm playing with:

Code:

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

int prng(int x, int a, int c, int m)
{
	/*
		Based upon 'Inversive Congruential Generator'
		x - last / start number (seed)
		a - increment
		c - multiplier
		m - modulus
	*/
	return (int)((1/(a*x))+c)%m;
}

int main(int argc, char *argv[])
{
	int i;
	int tot = 20;
	int x = 3;
	int a = 5;
	int c = 3;
	int m = 255;
	
	printf("nums: ");
	for(i=0;i<tot;i++)
	{
		x = prng(x, a, c, m);	//	x, a, c, m
		printf("%d ", x);
	}
	printf("\n");
	
	return(0);
}

brazilnut · 10-17-2008, 10:22 AM

oops, this seems to be a solution (well the results match my calculator):

Code:

#include <math.h>
	printf("1: %.8f\n", pow(1,-1));
	printf("1: %.8f\n", pow(2,-1));
	printf("1: %.8f\n", pow(3,-1));
	printf("1: %.8f\n", pow(4,-1));

IsharaComix · 10-17-2008, 10:23 AM

x^(-1) is equal to 1/x

Just put whatever you want to inverse under a one. That should work if C does math like normal people...

brazilnut · 10-17-2008, 10:40 AM

yes thats what i was trying first 1/x, i think my problem is lying in the choice of numbers, because all my results currently are repetitive, so i'm looking at this and attempting to figure out a way to generate from a single seed...

graemef · 10-17-2008, 06:30 PM

You will need to use float numbers at some point because an inverse of an int will always result in zero. (1/3 = 0 remainder 1)