编程问题，精确度！！这点应如何精确啊

#include "stdio.h"
#include "math.h"
void main()
{float i,pi=0,j=1;
for(i=1;fabs(pi)<(1e-4);i++)
{pi=pi+pow(-1,i+1)*1/j;
j+=2;}
pi*=4;
printf("%f\n",pi);

}

#include "stdio.h"
#include "math.h"
void main()
{float i=1,pi=0,j=1;

float tpi;

do
{

tpi=pi;

pi=pi+pow(-1,i+1)*1/j;

j+=2;}while(fabs(pi-tpi)>(1e-4))
pi*=4;
printf("%f\n",pi);

}