#include<stdio.h>
#include<math.h>
#include<stdlib.h>
#define MAX 1000
int n,N,i, val;

float a,b,x[MAX], y[MAX],sx,sy,sx2,sxy,siga,sigb,sig,sig2, den;
/* sx=sum_0^N x[n]
   sy=sum_0^N y[n]
   sxy=sum_0^N x[n]*y[n]
   sx2=sum_0^N x[n]*y[n] */
   
FILE *fptr;  /* the data file */
main(int argc, char *argv[]){

    if(argc < 2 || argc > 2){
        printf("lg datafile \n");
        exit(0);
    }

    if( (fptr=fopen(argv[1],"r")) == NULL){
        printf("There is no such file in your current directory\n");
        exit(0);
    }
    sx=0.0;
    sy=0.0;
    sx2=0.0;
    sxy=0.0;
    sig2=0.0;
    do{
        fscanf(fptr,"%f\t%f\n",&x[n],&y[n]);
        sx=sx+x[n];
        sy=sy+y[n];
	 sx2=sx2+x[n]*x[n];
        sxy=sxy+x[n]*y[n];
        n=n+1;}  while(!feof(fptr));
    N=n;  
    printf("%d\n",N);
    den=((float)N*sx2-sx*sx);
    a=(float)N*sxy-sx*sy;
    a=a/den;
    b=sx*sxy-sx2*sy;
    b=b/den;
/* get sig*sig */
for(i=0;i<N;i++){
sig2=sig2+(y[i]-(a*x[i]+b))*(y[i]-(a*x[i]+b));}
sig=sqrt(sig2/(float)(N-1));

    siga=sig*sqrt((float)N/((float)N*sx2-sx*sx));
    sigb=sig*sqrt(sx2/((float)N*sx2-sx*sx));
printf("The best fit slope is a= %f, intercept is b=%f\n", a,b);
printf("With sig_a= %f, sig_b=%f\n", siga,sigb);


}