Chapter 3

Using existing information in programs

1. Using code from previous programs
2. Using functions from libraries (sqrt, cos, log)
3. Building your own functions for reuse


Problem:

Write a program to calculate the area and 
circumference of a circle given its radius.

Analysis

Problem Inputs

radius


Problem Outputs

area

circumference


Formulas

area = PI * r * r
circumference = 2 * PI * r


Design

Algorithm

1. Read the radius
2. Calculate the area
3. Calculate the circumference
4. Display area and circumference


Implementation


/* 
 * Calculates area & circumf of circle 
 */

#include <stdio.h>
#define PI 3.14159

int main()
{
    double radius; /* input - radius of 
                            a circle */
    double area;   /* output - area of 
                            a circle */
      double circumf; /* output - 
                       circumference */

      /* Get the circle radius */
      printf("Enter radius> ");
      scanf("%lf", &radius);

      /* Calculate the area */
      area = PI * radius * radius;

      /* Calculate the circumference */
      circumf = 2 * PI * radius;

      /* Display area & circumf */
      printf("The area is %.4f\n", 
              area);
      printf("The circumf is %.4f\n", 
              circumf);

      return (0);
}

Enter radius> 5.0
The area is 78.5397
The circumf is 31.4159



Problem

Calculate the price per square inch of a circular pizza 
given its size (diameter) and its cost.

Analysis

Problem Inputs

diameter
cost

Problem outputs

price_psqi

Other variables

radius
area

Formulas

radius = diameter / 2
area = PI * r * r
price per square inch = price / area


Design

Algorithm
1. Read pizza size and cost
2. Calculate price per square inch
3. Display price per square inch

Refine step 2
2.1 Calculate radius
2.2 Calculate area
2.3 Calculate price per sq. inch

Implementation


/*
 * Calculates pizza unit price */
 */

#include <stdio.h>
#define PI 3.14159

int main()
{
    double diameter; /* input - size */
    double cost; /* input - cost */
    double area;   /* output - area of 
                      a circle */
     double circum; /* output - 
                    circumference */
     double radius; /* radius of a 
                    circle */

      /* Get pizza size and cost */
      printf("Enter diameter > ");
      scanf("%lf", &diameter);
      printf("Enter cost $ ");
      scanf("%lf", &cost);


      /* Calculate price per sq in */
      radius = diameter / 2;
      area = PI * radius * radius;
      price_psi = cost / area;

    /* Display the cost per sq. inch */
      printf("The unit cost is %.2f\n", 
             price_psqi);

      return (0);
}

Enter diameter > 12.0
Enter cost $ 20.0
The unit cost is $0.18