2.1.2: thinking ahead

• What is the purpose of thinking ahead?

  To make programs easy and intuitive for users to use.
• Define inputs.


  Any data that is required to solve a problem, usually entered into the system by the user.
• Define outputs.

  The results that are passed back once the inputs have been processed and the problem solved.
• What three considerations do programmers need to make about inputs and outputs when thinking ahead?

  • Method of input/output (device used)
  • Data structures used
  • Data types used
  • Order of data
• Identify the inputs and outputs that are likely to be used in a book reservation system i.e. at a library.

  inputs:

  • book details: name, author
  • ISBN
  • borrower details, name, library card number
  • collection point

  outputs:

  • expected waited time
  • confirmation
  • availability at other libraries
• What are preconditions?


  Requirements which must be met before a program can be executed.
• Where can preconditions be defined?

  Within the code or within documentation
• Give an example where preconditions are required.

  Stack functions:

  • Check that a stack is not empty when popping an element from a stack
  • Check that a stack is not full when pushing an element onto a stack

  factorial function

  • The number passed to the function cannot be negative
• State two advantages of including preconditions within the documentation accompanying a subroutine.

  • Reduces the length of the program
  • Reduces the complexity of the program
  • Saves time needed to debug and maintain a longer program
  • Makes subroutine more reusable
• Define caching


  The process of storing instructions or values in cache memory after they have been used, as they may be used again.
• How is caching used in storing web pages?

  Web pages that a user frequently accesses are cached, so the next time one of these pages is accessed, content can be loaded without any delay.
• What are the advantages of caching web pages?

  • Content can be loaded without delay
  • Images and text do not have to be downloaded again multiple times
  • Frees bandwidth for other tasks on a network
  • Less time is spent waiting
• What is the name given to the technique in which algorithms are used to predict which instructions are likely to soon be used?

  Prefetching
• Give a limitation of caching

  • Accuracy of the algorithms used
  • Effectiveness of algorithm in managing the cache
  • Size of the cache
• Give three advantages of using reusable program components.

  • More reliable than new components, as they have already been tested.
  • As developing from scratch is not required, this saves time, money and resources.
• Give two examples of reusable program components.

  • Abstract data structures eg. queues and stacks
  • Classes
  • Subroutines