Classnotes

eXercise #20

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Some Simple Visual Projects
featuring iteration

index: for..to..do | repeat..until | while..do

1. Cryptography is the art of substituting encoded symbols for common characters. This project uses a simple displacement cypher to encode a string.
   
   Imagine a section of the ASCII table, together with the same section of the ASCII table displaced by a set number of characters:
   

   a

   b

   c

   d

   e

   f

   g

   h

   i

   j

   k

   l

   m

   n

   o

   d

   e

   f

   g

   h

   i

   j

   k

   l

   m

   n

   o

   p

   q

   r

   Because the letters are arranged ordinally in the table, we can 'advance' them by a fixed amount and therefore seemingly scramble them as shown below:
   
   In this project, you will construct a form as follows:
   
   run | cheat
   
   The initial text of each of the edit boxes is largely irrelevant as eventually the cleartext box will generate the crypttext when the encrypt button is pressed.
   
   In this cypher - each character has been advanced 1 in the ASCII table: t becomes u, a becomes b and so on.
   
   The project comprises:
   
   • 1 spin edit box named displacement
   • 2 edit boxes called cleartext and crypttext
   • an encrypt button and a done button
   • some labels for eye-candy value only
   
   Each letter in the cleartext.text can be accessed individually (as it is a string). The fifth letter, for example is referred to as cleartext.text[5].
   
   You will write the event handler behind the [encrypt] button so that firstly it correctly converts the first letter (cleartext.text[1]) to it's encrypted form Use ord to turn it into ASCII and char to convert it back to a char)
   
   The process you use will be generalised and a loop added to step from the first letter to the last letter, encrypting as you go. We need a loop starting at 1 and ending at length(cleartext.text) similar to:
   

   for counter := 1 to length(cleartext.text) do ...

   each character of the cleartext can then be accessed by something similar to: cleartext.text[counter]
   
   The loop should add each encrypted character onto the crypttext (use the string catenator '+' to do this).
   
   If your program is working correctly, a message encrypted using a displacement of 3, say, can be decrypted (if you paste the gobbldegook into the cleartext) using a displacement of -3
   
   
2. Bang the Weasel.
   
   Construct a form as follows:
   
   run | cheat
   
   The project (initially) comprises:
   
   • 1 TImage containing star.wmf
   • a timer

   
   Write in the ONTIMER a process that randomly relocates the star and randomly determines if it is visible or not.
   
   To randomly make an object vanish, you need to set the .visible property to FALSE. In this program, you could achieve this using something like:
   
   

   star.visible := random(100)<50;

   To randomly relocate an object, you need to change it's .left and .top property. In this program, you could change the .top using something similar to:
   

   star.top := random(clientheight)+1;

   The onclick process for the star TImage should be something simple like close.
   
   *** The POINT of this exercise ***
   When you have your SINGLE star working correctly, copy and paste lots of stars onto the form. Delphi actually keeps track of all the things on a form using the COMPONENTS array. It is indexed 0..n-1, where n= the number of components on the form. You can use a loop to cycle through the things on the form, asking if it is an image, if it is you then relocate it and randomly make it invisible. A FOR..TO..DO loop makes most sense to do this.
   
   You can modify your single star process to be smart enough to work for as many stars as you like by simply including a loop similar to the following:
   
   

   for counter := 0 to componentcount do
   if (components[counter] is TImage)
   then (components[counter] as TImage).top := .....
   

   Experts:
   
   • You should ensure the stars are only randomly located on visible parts of the form
   • Bias the visibility and change the timer speed so it is actually playable
   • You should add some element of game play - the handler that registers a hit should increment some counter and the game should terminate only after you kill x stars
   • You could add an additional timer that limits the time of the game - making it necessary to click-frenzy to win before time runs out.
     
     
3. CASE STUDY: Hi-Lo Guessing Game
   
   
   run | cheat
   
   Your Mission, Jim, should you choose to accept it....
   Write a program that invents a random number between 1 and 100, then repeatedly asks the user for a guess reporting either 'Too High' or 'Too Low' until the user gets it right
   
   Guidelines:
   
   • The number should be stored as a BYTE variable
   • The prompt for a new guess should be an inputbox (not an edit box)
   • a REPEAT...UNTIL loop should be used here as we are not easily able to predict how many guesses are necessary, but at least one is required.
   • The progress of the guessing should be displayed somehow as captions on the form.
   
   Extra for Experts:
   
   • Some track should be kept of the number of goes taken - terminate the game if more than 10 guesses are taken, say.
   • The boundaries could be displayed somehow. If the initial guess is 50, say, then the program could respond "too high, the mystery number is between 1 and 50"

   
   

4. Rework Question 3 using a WHILE..DO instead of the REPEAT..UNTIL