Set 5

Question 1

What is the advantage of favouring composition over inheritance?

Answer 1

It makes it easier to test each class using unit testing

Question 2

What is the similarity between composition and aggregation?

Answer 2

Both “has a” relationships (ownership)

Question 3

What is the difference between composition and aggregation?

Answer 3

• In composition, if the containing object is destroyed so are the objects it contains
• This is not the case with aggregation

(composition = strong ownership, aggregation = weak ownership)

Question 4

Give five advantages of using subroutines:

Answer 4

• Code re-use
• Allows for modularisation of the program
• It makes it easier to identify bugs
• It makes it easier to test individual tasks NE easier to test
• It makes it easier for other programmers to interpret and understand your code

Question 5

What is an object reference variable?

Answer 5

• A fixed length memory address that points to a memory location (in the heap)
• It is used to refer to an object

Question 6

What is the purpose of a class?

Answer 6

To define the method and property fields that capture the common behaviours and characteristics of objects

Question 7

How is composition represented in UML?

Answer 7

Black diamond

Question 8

How is aggregation represented in UML?

Answer 8

White diamond

Question 9

What is encapsulation?

Answer 9

• The process of combining properties and methods together into an object
• and being able to restrict access to an object’s state / behaviour

Question 10

What is a constructor?

Answer 10

A method that is called when an object is first created

Question 11

What is the purpose of inheritance?

Answer 11

To create a hierarchy of specialisation for classes

Question 12

What is (subtype) polymorphism? What is its purpose?

Answer 12

• When a method in a parent class also exists in a derived class
• The method has the same name and parameter list, but a different implementation
• It allows objects up and down the inheritance hierarchy chain to respond differently to the use of a common interface

Question 13

Give two advantages of “programming to interfaces, not implementation”:

Answer 13

• Allows you to test each part of your program separately
• Allows you to modify your code easily in the future

Question 14

Give two advantages of local variables over global variables:

Answer 14

• You can give local variables the same name in different subroutines because they are only recognised by the subroutine they are declared in
• Local variables are deleted as soon as the subroutine is over and release the memory space which it occupies

Question 15

What is a constant?

Answer 15

A data item that, once declared, retains the same value for the entire duration of the program run

Question 16

Give two advantages of using constants over local variables:

Answer 16

• Makes your code more readable, by clearly indicating that a value should not be changed throughout the program
• Enhances code safety by preventing accidental modifications to values that shouldn’t change

Question 17

What does the access modifier protected mean? Give its UML symbol.

Answer 17

• It can be accessed by any subclasses
• and from within the class itself
• #

Question 18

What does the access modifier public mean? Give its UML symbol.

Answer 18

• It can be accessed by any class
• +

Question 19

What does the access modifier private mean? Give its UML symbol.

Answer 19

• It can only be accessed from within the class
• -

Question 20

What is the purpose of a hierarchy chart?

Answer 20

• To represent the structure of the program
• By showing which subroutine is called from which subroutine

Question 21

Constructing a hierarchy chart aids with which type of abstraction?

Answer 21

Decomposition
(possibly others)

Question 22

What is meant by a structured programming approach?

Answer 22

• When decomposition of a problem occurs by using block structures (subroutines)
• Structured programming makes use of control structures - sequence / selection / iteration

Question 23

What are user-defined data types?

Answer 23

Data types that are derived from existing built-in types in order to create a customised data structure

Question 24

Give three advantages of OOP:

Answer 24

• Classes are highly modular, meaning they are easy to maintain
• Promotes code reuse
• Encapsulation provides a high level of protection for data

Question 25

Give two drawbacks of OOP:

Answer 25

* Can lead to very large and **complex** systems * Objects can take up a relatively large amount of **memory**, which is an issue if there are hardware restrictions

Question 26

What is meant by favouring composition over inheritance?

Answer 26

Using objects composed of other objects rather than a tightly coupled inheritance system as it allows for more flexibility

Question 27

Describe the “encapsulate what varies” design principle

Answer 27

When designing a program in the OOP, any requirements ​which are likely to change in the future ​should be ​encapsulated in a class ​so that any changes can be​ easily made when required​.

Question 28

Name three OOP design principles:

Answer 28

* Encapsulate what varies * Favour composition over inheritance * Program to interfaces, not implementation

Question 29

Steps for binary search on a value **X** (in English/Pseudocode)

Answer 29

1. Look at item in centre of sorted list (or array) *(sort first if needed)* 2. If it isn’t **X**… 2.1. If it is larger than **X**, you can ignore all of the items above 2.2. If it is smaller than **X**, you can ignore all of the items below 3. Check middle of halved list and repeat, until you find **X** or the sublist cannot be halved any more.

Question 30

What is the `while` condition when coding binary search?

Answer 30

`lower<= upper && !found`

Question 31

What does it mean if you reach a leaf node before you find **X** in a binary tree search?

Answer 31

**X** is not in the tree

Question 32

Bubble sort (ascending order) pseudocode on array `A`

Answer 32

`N ← length(A)` `swapped ← True` `while swapped and N > 1:` __`swapped ← False` __`for position = 0, 1, ... , N-2:` ____`if A[position] > A[position+1]:` ______`swap( A[position] , A[position+1])` ______`swapped ← True` __`N ← N-1`

Question 33

What is an abstract data type?

Answer 33

A theoretical description of a way of organising a collection of data, with particular features and access restrictions, that is independent of any particular data structure.

Question 34

What is a data structure?

Answer 34

The concrete realisation of an abstract data type in code.

Question 35

Is a stack FIFO or LIFO?

Answer 35

LIFO (Last in first out)

Question 36

Give the five core operations of a stack:

Answer 36

- **Push** - adding to top of stack - **Pop** - removing from top of stack and returning - **Peek** - returns top item *without* removing it - **IsEmpty** - checking if stack is empty - **IsFull** - checking if stack is full (only relevant when stored in a static structure)

Question 37

Describe the implementation of a stack

Answer 37

- Using an array or list to store the items - Initialise a pointer variable that points to the current top item - The pointer is initialised as -1 - The pointer is incremented if an item is pushed, and vice versa - The pointer will be -1 if stack is empty.

Question 38

What is a recursive subroutine?

Answer 38

One that calls itself (or is defined in terms of itself)

Question 39

Give three advantages of recursion:

Answer 39

1. Can lead to very elegant, short code 2. People think recursively (sometimes) 3. It is sometimes the only way to solve a problem

Question 40

Give two disadvantages of recursion:

Answer 40

1. Can be slower than iterative solutions 2. Can use more memory than iterative solutions

Question 41

What is meant by a ‘base case’?

Answer 41

The **terminating situation** in a recursive procedure that does not use recursion to produce a result

Question 42

What are the steps for linear search on a list?

Answer 42

1. Start at beginning of list 2. Compare each item to one you want until - you find it or - you reach the end of the list *(use an indefinite `while` loop!)*

Question 43

What are the steps for binary search on a value **X** (in English)?

Answer 43

1. Look at item in centre of sorted list (or array) *(sort first if needed)* 2. If it isn’t **X**… 2.1. If it is larger than **X**, you can ignore all of the items above 2.2. If it is smaller than **X**, you can ignore all of the items below 3. Check middle of halved list and repeat, until you find **X** or the sublist cannot be halved any more.

Question 44

What is the `while` condition when coding binary search?

Answer 44

`lower<= upper && !found`