kahantest.c

//総和の実験
//0.1を5億回足す
#include<stdio.h>

#define N 500000000

int main(int argc,char **argv){
	int i;
	double s1=0.0;
	double s2=0.0;
	double c;
	double t,y;

	//単純ループ
	for(i=0;i<N;i++){
		s1+=0.1;
	}

	puts("simple finished");

	//カハンの加算アルゴリズム
	c=0.0;
	for(i=0;i<N;i++){
		y=0.1-c;
		t=s2+y;
		c=(t-s2)-y;
		s2=t;
	}

	puts("kahan finished");

	printf("Simple = %23.15lf\n", s1);
	printf("Kahan  = %23.15lf\n", s2);

	return 0;
}