Designing Homework 1

• Homework 1 statement.
• This is a little strange because
  • The problem statement is fairly decomposed.
  • There are examples of the decomposition in the previous notes.
• Let's start at the top.
  • My Design

    Ask if they want to encrypt a message
    While the answer is yes
    
         Get the shift amount
         Get the plain text message
    
         Encode the plain text message
         print the crypt text message
    
         Ask if they want to encrypt a message 

  • Other designs are possible.
  • Will this work? Is it correct?
  • Can you implement this?
  • Is this Cohesive?
  • Is this concrete or abstract?
  • Are any of the steps concrete?
  • I believe that every step here is abstract and should be turned into a new sub-procedure and eventually a function.
• One part of functional decomposition is how information is transferred
  • What information needs to be sent from one module/function to another?
  • In general, noting needs to be sent into main from the outside.
  • But main may need to send/receive information from sub functions.
• Consider Ask if they want to encrypt a message.
  • I dislike that name, can we think of something better.
    • How about GetContinue?
  • What does this procedure/function/sub-program do?
  • What Does it need any information from the main routine?
    • What is that information?
  • Should it return any information to the main routine?
    • What is that information?
  • Can you come up with an algorithm for GetContinue?
  • My Algorithm

    GetContinue:
        Input: no input from main
        Output: returns a bool  (yes continue -> true, no don't continue -> false)
    
        While we don't have a response of yes or no
            Print the prompt
            read in the response
            convert the response to lower case
        return the response
           

  • Others are possible.
  • All the questions above
  • Does it work and is it cohesive?
  • Is it concrete or abstract?
  • Can you implement it?
  • In this case, I think I want at least one further function, something to turn a string into lower case.
• I would repeat this process with every function in the main routine.

  
  ToLower:
     Input: a string
     Output: A string where all alphabetic characters are in lower case
  
     for each letter in the string
        convert that letter to lower case.
  
  GetShiftAmount
     Input: no input from main
     Output: returns an integer between 0 and 26 to main
     Assumes no errors on input.
  
     Prompt for number
     Read in number
     Convert the number to the proper range.
     Return the number
  
  Convert a number to the proper range.
     Input: an integer 
     Output: an integer between 0 and 25
  
     While the number is less than 0
         add 26 to the number.
     return the number % 26
  
  GetPlainText
     Input: none
     Output: A string
  
     Prompt the user for the message
     Read in the next line
     Return this line
  
  Encode
     Input: a string, the message
            an int, the shift amount
     Output:
            The encrypted message;
  
     for each letter in the string
         encrypt that letter
     return the encrypted string.
  
  Encrypt Letter
     Input: a character and a shift amount.
     Output: the encrypted character.
  
     If the letter is alphabetic
        Note if the letter is upper case
        Convert the letter to lower case
        Shift the letter by the shift amount.
        if the original was upper case
           convert the shifted letter to upper case
     else
        shifted letter = original letter
     return the shifted letter
  
  Shift a letter
     Input: a lower case character
            a shift amount
     Output: a shifted letter
  
     find the position by subtracting 'a'
     new position = position +   shift amount
     convert the new position to the range 0-25
     return 'a'+new position
  

• This is not the only design possible.
• But it is a working design.
• We will implement this next.