Thinking ahead
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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 3 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 e.g. name, author • Borrow details e.g. name, library card number Outputs: • Expected waiting time • Confirmation
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 2 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 not 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 delay
What are the disadvantages 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
What are some advantages of caching?
• Allows for faster use in the future than fetching if from secondary storage or online usage • Speeds up loading and minimises system resources needed to load a page
Give a limitation of caching
• Accuracy of the algorithms used • Effectiveness of algorithm in managing the cache • Size of the cache
Give 3 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 2 examples of reusable program components
• Abstract data structures e.g. queues and stacks • Classes • Subroutines • Use of external libraries