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Revision (Tom Butler (Defensive design (Why use defensive design …

- - - - We get lexical errors when using the wrong words
      - The instruction you are trying to use is not recognised
    - - Syntax is the computing equivalent of grammar
      - We get syntax errors when using commands incorrecly
    - - You have given the computer commands it understands and used correctly
      - It just does the wrong thing - it does what you have told it to do, but you've told it to do the wrong thing
    - - errors which occur while the program is running
      - These are usually caused by external factors (e.g. Out of memory)
  - - - Length check - Must have a certain number of characters or be above or below some limit
      - Type check - Must be a certain data type such as a character or number
      - Range check - Must fall between certain limits
      - Presence check - must match what is held on file
      - Lookup check - Must confirm to a certain pattern
      - Check digit - Must be exactly the same data as entered previously
  - - - Double entery
        
        Entering data twice and comparing two copies. This effectively doubles the work load, and as most people are paid by the hour, it costs more two
      - Proofreading data
        
        this method involves someone checking the data entered against the original document. This is also time consuming and costly