Finding Epsilon

 1/* By David Isaacson, 2003
 2 * Released into the Public Domain
 3 *
 4 * epsilon.c
 5 * The puropse of this program is to find the machine epsilon of this computer, that is,
 6 * the smallest number E such that 1+E != 1 . (using single precision only)
 7 */
 8
 9#include <stdio.h>
10
11int main()
12{
13        float max = 1;		        //This is the lowest value that we know works.     
14					//(By works, I mean that 1+E!=1)
15	float min = 0;			//This is the highest value that we know doesn't work.
16	float test = 0;			//This is our test varible. Duh.
17	
18	do             
19	{
20		test=(max+min)/2;	//Set test to avg of max & min
21		
22		if((1+test)!=1)		//If we're too high, set the max to test and try again
23			max=test;	//(We're trying to center in on the right value)
24		if((1+test)==1)		//If we're too low, set the min to test and try again
25			min=test;
26
27	}while(test!=((max+min)/2));	//If max and min are not changing, they must be so 
28					//close that there is no number in between that the
29					//computer can represent.
30					//So, either test=min= the highest value that
31					//doesn't work, or test=max= the lowest
32					//value that does work.  Either way we're done.
33
34	printf("The epsilon for your machine is %e.\n",max);
35	//max is the lowest value that works. 
36
37	return 0;
38
39}
40
41/* When running this program on my computer (Power Mac G4 533 MHz running Mac OS X 10.2), I
42 * recieved the result:
43 *
44 * The epsilon for your machine is 5.960465e-08.
45 */