Areas of Circles and Triangles

Write a program that calculates areas of circles
and triangles, where the use chooses which kind of
object he wants to caclulate areas for.

The area of a circle is Pi r^2, where r is the
radius and Pi is approximately 3.14159265358979324.
The area of a triangle is 1/2 b h, where b is the
length of the base, and h is the height.
#include "si204.h"

int main() {
  // Read type of object: circle or triangle
  cstring s;
  fputs("Do you have a circle or a triangle? ", stdout);
  readstring(s, stdin);

  // declare this variable outside the blocks, so that it
  // can be used afterwards
  double area;

  // remember, strcmp returns 0 if the strings are equal
  if (strcmp(s, "circle") == 0) {
    // Compute area of circle

    // These variables are only needed inside the block!
    double Pi = 3.14159265358979324;
    double radius;

    fputs("Enter the radius of your circle: ", stdout);
    radius = readnum(stdin);

    area = Pi * radius * radius;
  else {
    // Compute area of triangle

    // These variables are only needed inside this block!
    double base;
    double height;

    fputs("Enter base length: ", stdout);
    base = readnum(stdin);

    fputs("Enter height: ", stdout);
    height = readnum(stdin);

    area = 0.5 * base * height;

  // print out the computed area
  fputs("Area equals ", stdout);
  writenum(area, stdout);
  fputs("\n", stdout);

  return 0;