Software Development Method

1. Problem Specification
Get a statement of the problem
Understand what is being asked for.

Example: Write a program that reads in the miles of a 
trip and displays the equivalent mileage in kilometers. 
Modify this program to read the mileage for your car 
and the cost of a gallon of gasoline and display the 
total cost of the trip.

Questions:

1. Do you still want to display the distance in 
kilometers?
2. What exactly is "the mileage for a car"?
3. Are any of these items "constants" or should they 
all be read as data?

Analysis
List the problem inputs
List the problem outputs
List other variables you may need to keep track of.
List any formulas that may be relevant.

Problem Inputs
	kilometers	distance in kilometers
	mileage	miles per gallon for your car
	galcost		cost of a gallon

Problem Outputs
	miles	distance in miles
	tripcost		cost of trip
	

Additional Variables
	gallons	number of gallons used

Formulas
	1 mile = 1.609 kilometers
	gallons = miles / mileage
	tripcost = gallons x galcost

3.	Design
Write an algorithm (list of steps) to solve the problem.
Don't attempt to solve every detail at once. First, 
provide an overview similar to an outline. Add 
detailed refinements for each step that needs 
clarification separately. ("Divide and conquer")

Algorithm
1. Read the miles, mileage, and cost of gasoline
2. Calculate the distance in kms.
3. Calculate the cost of the trip
4. Display the distance in kms and the cost of the 
trip.

Refine step 3:
3.1 Calculate the number of gallons needed.
3.2 Calculate the cost of the trip.


Implementation
	Write (or code) each step in C.

Testing
	Compile and debug the program. Validate the program 
output for several representative test cases that you 
calculate by hand.


Implementation (partial)

/* Figure 1.9  Miles-to-Kilometers  
 * Conversion Program
 * Converts distance in miles to kms
 */
#include <stdio.h>   
/* printf, scanf definitions */
#define KMS_PER_MILE 1.609 

int
main(void)
{
      double miles;
          /* input - dist. in miles. */
      double kms;    
           /* output - dist. in kms  */

      /* List other variables here */
      ...

      /* Read miles, mileage, and 
         cost of a gallon. */
      printf("Enter dist in miles> ");
      scanf("%lf", &miles); 

      ...

      /* Calculate dist. in kms. */
      kms = KMS_PER_MILE * miles;

      /* Calculate cost of trip */

      ...

      /* Display dist in kilometers. */
      printf("That equals %f kms.\n", 
     kms);

      ...

      return (0);
}


Different kinds of statements:

Compiler directives

#include <stdio.h>

Include the source code for library file stdio.h. Enables C 
compiler to recognize printf and scanf from this library.

#define KMS_PER_MILE 1.609

Substitute 1.609 for the name KMS_PER_MILE wherever 
it appears.

Comments - statements that clarify the program - ignored 
by compiler but "read" by humans.

/* Calculate cost of trip */


Declaration of identifiers or variables in the program. 
These are names of memory cells that will store 
information.

double miles;

Declares the variable miles that can store a real number 
(with a fractional part).

Other declarations:

int kids, courses;

Declares the variables kids and courses that can store 
integer values.

char initial;

Declares the variable initial that can store a single 
character.


Calls to library functions

printf("Enter dist in miles> ");
scanf("%lf", &miles); 
printf("That equals %f kms.\n", kms);

Each call to printf and scanf begins with a format string in 
double quotes. 

printf(format string);
printf(format string, output list);
scanf(format string, input list);

After the format string (following comma) comes an input 
list (for scanf) or an output list (for printf). The variables 
named in the input list are preceded by an & which means 
address of (e.g., &miles).

%lf and %f are placeholders - They indicate the data type 
and position of a value in a format string. Use:

%lf in scanf for type double value
%f in printf for type double value
%d in printf and scanf for type int value
%c in printf and scanf for type char value

The two-character sequence \n represents the "newline" 
character - carriage return, line feed.


Assignment statements

Assign a value to a variable. The value to be assigned is 
written on the right hand of the assignment operator =. The 
variable getting the value is on the left hand side.

sum = x + y;

sum gets the value of x + y. Variables x and y must be 
"defined" beforehand - they must have data stored in them.

kids = kids + 1;

kids gets the value of the "current value of kids" + 1. If 
kids was 5, its new value will be 6 (strange, but true!)

hypotenuse = sqrt(side1 * side1 +
                  side2 * side2);

Assignment statements can be quite complex!

Notice that 

x + y = sum;

is invalid. Why?