2.1.2 Thinking Ahead

Question 1

What is the purpose of thinking ahead?

Answer 1

To make programs easy and intuitive for users to use.

Question 2

Define inputs.

Answer 2

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

Question 3

Define outputs.

Answer 3

The results that are passed back once the inputs have been processed and the problem solved.

Question 4

What are some considerations that programmers need to make regarding inputs and outputs when thinking ahead?

Answer 4

• Method of input/output (device used),
• Data structures used,
• Data types used,
• Order of data.

Question 5

Identify the inputs and outputs that are likely to be used in a book reservation system i.e. at a library.

Answer 5

Inputs

• Book details: name, author,
• ISBN,
• Borrower details: name, library card number,
• Collection point.

Outputs

• Expected waiting time,
• Confirmation,
• Availability at other libraries.

Question 6

What are preconditions?

Answer 6

Requirements which must be met before a program can be executed.

Question 7

Where can preconditions be defined?

Answer 7

Within the code or within documentation.

Question 8

Give an example of where preconditions are required.

Answer 8

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.

Question 9

State some advantages of including preconditions within the documentation accompanying a subroutine.

Answer 9

• 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.

Question 10

Define caching.

Answer 10

The process of storing instructions or values in cache memory after they have been used, as they may be used again.

Question 11

How is caching used in storing web pages?

Answer 11

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.

Question 12

What are the advantages of caching web pages?

Answer 12

• 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.

Question 13

What is the name given to the technique in which instructions are fetched based on algorithms which predict which instructions are likely to soon be used?

Answer 13

Prefetching.

Question 14

Give a limitation of caching.

Answer 14

• Accuracy of the algorithms used,
• Effectiveness of algorithm in managing the cache,
• Size of the cache.

Question 15

Give some advantages of using reusable program components.

Answer 15

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

Question 16

Give some examples of reusable program components.

Answer 16

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