//to estimate regression equation form sampled data and evaluate values of standard deviation regression coffecient for atleast two independent variables
#include<stdio.h>
#include<string.h>
#include<math.h>
float mean(float *a,int n);
void deviation(float  *a,float mean,int n,float *d,float *s);
void main()
{
float a[20],b[20],dx[20],dy[20];
float sy=0,sx=0,mean_x=0,mean_y=0,sum_xy=0;
float corr_coff=0,regcoff_xy=0,regcoff_yx=0;
char type_coff[7];
int n=0,i=0;
printf("enter the value of n \n");
scanf("%d",&n);
printf("enter the value of x and y \n");
for(i=0;i<n;i++)
scanf("%f %f",&a[i],&b[i]);
mean_x=mean(a,n);
mean_y=mean(b,n);
deviation(a,mean_x,n,dx,&sx);
deviation(b,mean_y,n,dy,&sy);
for(i=0;i<n;i++)
sum_xy=sum_xy+dx[i]*dy[i];
corr_coff=sum_xy/(n*sx*sy);
printf("enter the type of regression coffecient as 'x on y'or 'y on x' \n");
fflush(stdin);
gets(type_coff);
if(strcmp(type_coff,"x on y")==0)
{
regcoff_xy=corr_coff*(sx/sy);
printf("\n the value of linear regression coff is %f",regcoff_xy);
}
else if(strcmp(type_coff,"y on x")==0)
{
 regcoff_yx=corr_coff*(sy/sx);
printf("\n the value of linear regression coff is %f",regcoff_yx);
}
else
printf("enter the correct type of regression coffecient \n");
getch();
}
float mean(float *a,int n)
{
float i=0,sum=0;
for(i=0;i<n;i++)
sum=sum+a[i];
sum=sum/n;
return(sum);
}
void deviation(float  *a,float mean,int n,float *d,float *s)
{
float sum=0,t=0;
int i=0;
for(i=0;i<n;i++)
{
d[i]=a[i]-mean;
t=d[i]*d[i];
sum=sum+t;
}
sum=sum/n;
*s=sqrt(sum);
}