Procedure Oriented Programming: Example

//cshapes.h

#define T_CIRCLE 1
#define T_RECTANGLE 2
#define T_TRIANGLE 3

typedef struct CIRCLE_SHAPE
{
   short type;
   double x,y;
   double radius;

} CIRCLE_SHAPE;

typedef struct RECTANGLE_SHAPE
{
   short type;
   double x1,y1;
   double x2,y2;

} RECTANGLE_SHAPE;

typedef struct TRIANGLE_SHAPE
{
   short type;
   double x1,y1;
   double x2,y2;
   double x3,y3;

} TRIANGLE_SHAPE;

typedef union SHAPE
{
  short type;
  CIRCLE_SHAPE circle;
  RECTANGLE_SHAPE rectangle;
  TRIANGLE_SHAPE triangle;
} SHAPE;

double compute_area(SHAPE *pShape);
void draw_shape(SHAPE *pShape);


//cshapes.c
#include<stdio.h>
#include"cshapes.h" 
#include<math.h>


double compute_area(SHAPE *pShape)
{
   double area;

   switch(pShape->type)
   {
      case T_CIRCLE:
      {
         area = M_PI *pShape->circle.radius * pShape->circle.radius;
         break;
      }

      case T_RECTANGLE:
      {
         area = fabs(
    (pShape->rectangle.x2 - pShape->rectangle.x1) *
    (pShape->rectangle.y2 - pShape->rectangle.y1) 
      );
  break;
      }

      case T_TRIANGLE:
      {
        //double d1,d2,d3;
 // Hero's formula
 //d1 = (pShape->triangle.x1)
         area = 0.5; //TODO:
         break;
      }

      default: printf("Unknown shape\n");
   }

   return area;
}

void draw_shape(SHAPE *pShape)
{
   printf("Draw: ");
   switch(pShape->type)
   {
      case T_CIRCLE:
      {
         printf("Circle of radius %f at (%f,%f)\n", pShape->circle.radius, pShape->circle.x,pShape->circle.y);
         break;
      }

      case T_RECTANGLE:
      {
         printf("Rectangle with corners:  (%f, %f)and (%f,%f)\n", pShape->rectangle.x1, pShape->rectangle.y1,pShape->rectangle.x2, pShape->rectangle.y2);
  break;
      }

      case T_TRIANGLE:
      {
 printf("Triangle with coordinate: (%f,%f)", pShape->triangle.x1, pShape->triangle.y1);
      break;
      }

      default:
      printf("Unknown shape in draw!\n");
   }
}


//main.c
#include<stdio.h>
#include "cshapes.h"


int main()
{
  int i;
  SHAPE s[3];

  s[0].type = T_RECTANGLE;
  s[0].rectangle.x1 = 80.0;
  s[0].rectangle.y1 = 30.0;
  s[0].rectangle.x2 = 120.0;
  s[0].rectangle.y2 = 50.0;

  s[1].type = T_CIRCLE;
  s[1].circle.x = 200.0;
  s[1].circle.y = 100.0;
  s[1].circle.radius = 50.0;

  s[2].type = T_TRIANGLE;
  s[2].triangle.x1 = 4.0;
  s[2].triangle.y1 = 4.0;
  s[2].triangle.x2 = 4.0;
  s[2].triangle.y2 = 4.0;
  s[2].triangle.x3 = 4.0;
  s[2].triangle.y3 = 4.0;


  for (i = 0; i < 3; ++i)
 printf("Areas of shape[%d] = %f\n", i, compute_area(&s[i]));

  for (i = 0; i < 3; ++i)
    draw_shape(&s[i]);

  return 0;
}

How to run the program?

$gcc -lm -c cshapes.c -o cshapes.o
$gcc cshapes.o main.c -o shapes
$./shapes
Foot Note:
-lm links math library.
-c compile only option to generate object file.