A First Example

• Some questions
  • How do you know when to write a function?
  • What should that function do?
  • What do you pass to the function?
  • What should the function return?
• These are all good questions.
  • The short answer is whatever you need.
  • You write a function when you need one
    • To simplify the code
    • To make it easier to read
    • To make it so you can reuse the code (function) later
    • To make code easier to maintain/modify
  • You make a function do whatever you need it to do!
  • You pass the function whatever data it needs to accomplished the assigned function.
  • You return whatever you need to get back from the function.
• There are deeper theories, but for now
  • Let's try to learn by an example.
• An example problem
  • You are to write program to balance your bank account. To do this you need to read data from a file. This file contains
    • Your opening balance on the first line.
    • Every other line contains
      • A transaction number
      • The date in the form mm/dd/yy
      • A transaction type (D for deposit, W for withdraw)
      • An amount in the form $ddd.cc
        • dd may be between 0 and 1,000,000
        • cc is two decimal places.
  • An example file
  • Process this file and compute the new balance.
• Let's build an algorithm
  • Write a top level function with very little code.
    • We would like all functions (including the main) to be cohesive (see page 166)
      • All code is at the same level (abstract or concrete)
      • The code accomplishes a single purpose
    • For now, tend to build too many functions not too few.
  • Answer

    Open the file
    Get starting balance
    Get transaction amount
    While valid transaction
        process transaction
        Get transaction amount
    Print final balance
        

• After we build an algorithm for main, build the other algorithms
  • Open the file
    • Answer

      do 
          Ask the user for the file name
          attempt to open the file
      until successful at opening a file
      return the open file handle
                    

  • Get Starting Balance
    • Answer

      Read the $ as a character
      Read the number as a float
      Return the number
                    

  • Get a transaction amount
    • Answer

      Read the transaction number
      Read the date as a string
      Read A Number