#define an input number in C - c

Question

I need to get the user to input a number into a program and then need to be able to use that number in many other parts functions in the program. Any way to do that?
Here is the code:
#include <stdio.h>
#include <math.h>
#define W 8.
#define H 4.
double ellipse(double);
typedef double (*DfD) (double);
double simpsons_int (DfD, double, double, int);
int main()
{
double len, w, h, volume;
printf("Please enter a length, width and height (in meters) of the an elliptical storage tank \n");
scanf("%lf %lf %lf", &len, &w, &h);
double a = h/2.*-1., r;
for (double depth=10; depth<=400; depth=depth+10)
{
r=a+(depth/100);
volume = len*simpsons_int(ellipse, a, r, 10000);
printf("depth is %.1f, volume is %f\n", depth, volume);
}
}
double ellipse(double y)
{
double x;
double A=W/2.;
double B=H/2.;
x=2*sqrt((1-(y*y)/(B*B))*(A*A));
return x;
}
double simpsons_int(DfD f, double y0, double y1, int n)
{
double y, sum, dy = (y1 - y0)/n;
sum = f(y1) + f(y0);
for(y = y0; y <= y1-dy; y += dy)
sum += 2.0 * f(y+dy) + 4.0 * f(y + dy/2);
return sum * dy / 6.0;
}
but I need H and W to be number that are input by the user not 8 and 4.

Answer 1

You can either pass it as argument of the function, or declare it as global variable. I'd rather use the first, depending on the application.
1) passing as parameter. Your function should be:
double ellipse(double y, double W, double H )
{
double x;
double A=W/2.;
double B=H/2.;
x=2*sqrt((1-(y*y)/(B*B))*(A*A));
return x;
}
And then you declare and scanf W and H within main()
2) Just declare W and H before main();
double W,H;
int main()
{
double len, w, h, volume;
printf("Please enter a length, width and height (in meters) of the an elliptical storage tank \n");
scanf("%lf %lf %lf", &len, &w, &h);
scanf("%lf %lf",&W,&H);
double a = h/2.*-1., r;
for (double depth=10; depth<=400; depth=depth+10)
{
r=a+(depth/100);
volume = len*simpsons_int(ellipse, a, r, 10000);
printf("depth is %.1f, volume is %f\n", depth, volume);
}
}

Answer 2

Preprocessor directives like #define must be known at compile-time. Think of them as constants: you can set them, but as soon as you run the program they're set in stone.
You should be using your variables to do this; you could possibly define w and h to be global variables, but better practice would be to pass them in as parameters to the ellipse function.