unit 8 - robust programs

• Anticipating Misuse


  Planning ahead to take steps against potential misuse (e.g Twitter preventing the same tweet sent twice in a row)
• Input Sanitisation

  Cleaning up data that has been input, (e.g. removing special characters to prevent a SQL injection)
• Validation

  Checking whether input data follows criteria and should be accepted (e.g. length check on a password)
• Verification

  Checking whether data has been entered correctly (e.g. double entry)
• Authentication

  Ensuring only authorised users can gain access to a system (e.g. passwords)
• Maintainable code

  Allowing other programmers to understand the code (e.g. comments, indentation, meaningful variable names)
• Range Check

  Checks data is within a certain range
• Type Check

  Checks data is a certain data type
• Format Check


  Checks data is entered in a certain way
• Presence check


  Checks that data has actually been entered and not left blank
• Lookup table


  A table of acceptable entries, also known as a list
• Length check


  Checks the length of the input is within a certain amount
• Modularisation


  Using subprograms to reuse code and make them easier to test
• Appropriate variable names


  Using a naming convention to ensure the purpose of a variable is immediately understood
• Indentation


  Improves readability and clearly shows each 'block' of code
• Comments


  Enable a programmer to understand the purpose of each line of code, crucial when working in a team
• Robust programs can handle unexpected inputs and data without crashing and without being tricked into malicious action.
• Reliable programs will work as intended under many different conditions or situations.
• Contingency planning


  It is impossible to predict all possible cases of misuse, but there are ways to help reduce errors. For example:

  • Providing helpful prompt messages.
  • Providing error-recovery routes (e.g. an undo feature).
  • Preventing access from certain parts of the system.
  • Using exception handling.
• An exception is a description of the error that arose.
• A comment is a line of code that is completely ignored by the computer.
• Purpose of testing


  • Ensure a program works correctly no matter what input has been entered by the user
  • Ensure user requirements have been met
  • Ensure errors have been removed
  • Ensure the program doesn't crash while running
• Iterative Testing


  1. Programmer develops a module

  2. Tests it

  3. Repeats until module works as expected
• Final Testing

  1. Takes place after development and before program is released to end user

  2. Ensures whole program works as originally expected
• Syntax Error


  • Mistake in the grammar or spelling of the program
  • Prevents the program from being compiled
  • translators needs the code to be syntactically correct
• example of syntax errors

  • Incorrect Spelling: pront ("hello")
  • Incorrect punctuation: print ("hello"(
• Logical Error


  • Mistake made by the programmer
  • Program still works but displays the wrong output
• Logical Errors

  
  

  -Incorrect calculation:

  total = num1 - num2,

  print (total)

  
  

  -Incorrect variable printed:

  age = 16

  name = "Steve"

  print ("Nice to meet you" , age)
• Test Data


  • Used to test whether a program is functioning correctly
  • Covers a range of possible and incorrect inputs, each designed to prove a program works or to highlight any flaws
• Types of Test Data

  • Normal data - Sensible data that the program should accept and be able to process
  • Boundary data - Data at the extreme boundary of any data ranges
  • Invalid (AKA Erroneous) data - Data that the program cannot process and should not accept
• Debugging is the process of finding and correcting existing and potential errors (also called bugs) in software code. Bugs can cause software code to behave unexpectedly or crash.
• Common causes of logic errors

  • Not correctly understanding what the program needed to do
  • Using the incorrect logical operator in a selection statement
  • Missing or incorrect positioning of brackets in mathematical calculations, which means that the incorrect result is returned
  • Loops that execute more or fewer times than intended
• Common causes of syntax errors

  • Spelling or typing errors
  • Missing parentheses, ( ) or { }
  • Missing colons (:), or semicolons, (;), in statements in which they are required by the language
  • Missing or unexpected indentation in Python
  • Printing a value without declaring it
• Common causes of runtime errors

  • Trying to access a list item that does not exist
  • A mathematical calculation that results in the program trying to divide by 0
  • Trying to perform numerical operations on strings
  • Linking to a file or resource that has been moved or no longer exists
• Logical operators


  Symbols used to represent circuits of transistors within a computer
• Common logical operators

  • NOT
  • AND
  • OR
• Truth table


  A visual way of displaying all possible outcomes of a logical operator
• Truth table


  • The input and output values must be a Boolean value - usually 0 or 1 but occasionally True or False
• Order of precedence

  • NOT
  • AND
  • OR
• AND gate


  • An AND logical operator will output 1 only if both inputs are also 1
  • AND is also known as Conjunction.
  • ​The symbol for AND is ∧