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Lesson 2 - Simple 2-Letter Encoding Program

Note: To use the Javascript programs in these lessons, you will need to use a FireFox browser or other non-Internet Explorer browser due to the "feature" in the IE browser disallowing Javascript prompts.

This program does the following:

  1. Prompts you to input a letter. Then outputs the letter (to your screen). This is done for 8 letters.
  2. Assigns a variable to your letter.  For example, the first letter is assigned to the variable letter1a.
  3. Assigns each variable a value of 1 through 26, depending on what letter it equals where A = 1, B = 2, .....Y = 25, Z = 26.  So if letter1a = "C", then letter1a = 3.  If you enter something other than a letter A-through-Z or you enter nothing at all and hit the return, the variable is assigned to the value " * "
  4. Add 2 to each letter variable value using the formulas letter1a = letter1a +2, letter2b = letter1b +2, letter3c = letter3c +2, etc..  So if letter1a=3, 2 is added and letter1a becomes equal to 5. The program "adds" 2 to * by calling it *2
  5. Assigns each new value back to a letter to produce a coded message.  So if letter1a is 5, letter1a is assigned the value becomes "E". Then outputs the letter. This is done for 8 letters. Note that if the letter variable was assigned a value of " *2 " or the new value is something other than a number from 1-to-26, it will output the value " * ".

To see the code used in this program, select View, Page Source in your browser's menu.

Problems With This Encoding Scheme!
What happens when you enter the letter "Y" or "Z"?  Since "Y" corresponds to 25 and "Z" corresponds to 26, when these numeric values are increased by 2, they become 27 and 28 respectively. The program will then output the value "*" for any "Y" or "Z".  For example, the message "ZIG ZAG " encodes as *KI**CI*  as shown below. We have no way of knowing whether the * stands for a space, a "Y", or a "Z".  We need an improved formula! Go to Improved Two Letter Shift to see how this is done.









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