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 three considerations do 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

Requirement 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 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
• Factorial function
• The number passed to the function cannot be negative

Question 9

State two 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 three 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 two examples of reusable program
components.

Answer 16

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