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Computer Science > 4.2 Data structures > Flashcards

4.2 Data structures Flashcards

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1
Q

What is an array?
How are items accessed in an array?

A
  • A collection of items with a fixed size
  • Arrays use index numbers to access individuals elements of the array
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2
Q

What is an abstract data type?

A

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.

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3
Q

What is a data structure?

A

The concrete realisation of an abstract data type in code.

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4
Q

What is a static data structure?

A

A data structure that reserves a fixed amount of memory, specified by the programmer in advance of its use

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5
Q

What is a dynamic data structure?

A

Data structures that have no fixed size. The memory used grows and shrinks as items are added/removed

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6
Q

2 advantages of static data structures over dynamic data structures

A
  • Data is quicker to access directly, with minimal overhead
  • No additional memory is needed to store all of the pointers
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7
Q

2 advantages of dynamic data structures

A
  • There is no wasted memory
  • There is no theoretical limit on the number of items that can be added
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8
Q

Queue: FIFO or LIFO?

A

FIFO (First in first out)

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9
Q

Queue: Four key operations

A
  • Enqueue - adds item to rear of the queue
  • Dequeue - removes item and returns item from the front of the queue
  • IsEmpty - checks if the queue is empty
  • IsFull - checks is the queue is full
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10
Q

Describe briefly the data structures and pointers used by a linear queue

A
  • Linear queues are implemented with arrays (or lists)
  • They use a front and rear pointers to point to:
  • front → the next item to dequeue
  • rear → last item enqueued
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11
Q

Disadvantages of linear queues if you
1. Don’t shuffle down the items
2. If you do

A
  1. There is a limit on the number of items that can be added and removed (maxSize)
  2. Implementing a linear queue where you “shuffle” the items down each time an item is dequeued is very processing intensive
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12
Q

Key difference of circular queue from linear queue?

A
  • Circular queues virtually connect the end to the start of the array
  • This overcomes the problem of reusing spaces in the array
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13
Q

What happens to the rear pointer when enqueuing an item to a circular queue?

A

rear ← (rear + 1) % maxSize

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14
Q

What is a priority queue? How does enqueuing work?

A
  • A queue where each element in the queue has a priority
  • When new elements are enqueued, they are inserted ahead of those of lower priority and behind elements of equal or greater priority
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15
Q

Name an algorithm that can be implemented with a priority queue

A

Dijkstra’s Algorithm

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16
Q

Stack: FIFO or LIFO?

A

LIFO (Last in first out)

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17
Q

Stack: Five core operations

A
  • 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)
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18
Q

Describe implementation of a stack

A
  • 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.
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19
Q

Graph: Static or dynamic?

A

Dynamic, they can grow and shrink in size

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20
Q

What is a graph?

A

Graphs are sets of vertices (nodes) and the edges (arcs) that connect them

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21
Q

What is a graph with a high ratio of edges to vertices called?

A

Dense

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22
Q

What is a graph with a low ratio of edges to vertices called?

A

Sparse

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23
Q

What is a weighted graph?

A

A graph that has weights (number values) on each edge

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24
Q

What is a connected graph?

A

A graph where there is a path between each pair of vertices

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25
Q

Suggest three things that graphs could be used to model

A
  • Social networking: the nodes could be individual people. An edge could represent that two people are acquaintances.
  • Transport networks: the nodes could be towns. An edge could represent that a train line connects two towns.
  • The internet: the nodes could be routers. An edge could represent that two routers are connected.
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26
Q

2 ways to represent graphs

A

Adjacency matrix and adjacency list

27
Q

How do you represent no edge for a weighted graph?

A

“-” or “∞” (can’t use zero)

28
Q

How could an adjacency list be implemented if graph is 1. weighted 2. unweighted?

A
  1. List of dictionaries if weighted
  2. List of lists if unweighted
29
Q

Advantage of adjacency matrix

A

Adding new edges / checking for presence of edges is simple and quick as 2D static arrays can be used

30
Q

2 advantages of adjacency lists

A
  • Adjacency lists are very space efficient, as no memory is needed to store the empty spaces
  • It is easy to add / delete nodes
31
Q

Disadvantage of adjacency list

A

Slow to query (e.g. to check the presence of an edge), as each item in the list must be searched sequentially until the desired edge is found

32
Q

2 disadvantages of adjacency matrices

A
  • If the graph is sparse, then there are lots of empty spaces, which is wasted memory
  • It can be hard to add / delete nodes if a 2D static array is used
33
Q

What is a tree?

A

A connected, undirected graph with no cycles

34
Q

What is a rooted tree? (3*)

A
  • A tree in which one vertex has been designated as the root
  • Rooted trees have parent-child relationships between adjacent nodes
  • The root is the only node with no parent. All other nodes are descendants of the root
35
Q

What is an internal node?

A

A node in a rooted tree which has a parent and at least one child

36
Q

What is a leaf node?

A

A node in a rooted tree that has a parent but is not the parent of other nodes (no child nodes)

37
Q

Suggest a use for rooted trees

A

A game tree, where edges represent moves, and nodes represent all possible positions in the game

38
Q

What is a binary tree?

A

A rooted tree in which each node has at most two child nodes

39
Q

Name a common application of binary trees

A

Binary search trees

40
Q

What is an ordered binary tree? (3*)

A
  • A type of binary tree where the data items are in a particular order
  • Items left of a node have a value less than the node
  • Items right of a node have a value greater than the node
41
Q

What is a hash table?

A

A data structure that creates a mapping between keys and values

42
Q

What is the purpose of a hash table? (2•)

A
  • To provide a way of storing a list of values in which a key is used to locate values in the list without searching through all other values
  • They are used so that records can be retrieved quickly
43
Q

What does a hash function/algorithm do in a hash table?

A

Computes the array index for the value to be stored or retrieved

44
Q

What is hashing?

A

Hashing is the irreversible process of converting data of arbitrary length into data of a fixed length.

45
Q

When does a collision occur (hashing)?

A

When two or more keys hash to the same index

46
Q

When are collisions unavoidable (hashing)?

A

When the number of possible keys is larger than the number of available spaces

47
Q

What is a dictionary? (2•)

A
  • A collection of key-value pairs in which the value is accessed via the associated key
  • There can be no duplicate keys
48
Q

Name an application of dictionaries

A

Dictionary based compression

49
Q

What is the meaning of the symbol ↦ ?

A

Maps to

50
Q

What is important about the components of a vector?

A

That they are all drawn from the same field of values, e.g Reals or Rationals

51
Q

4 ways vectors be represented

A
  • A list of values
  • A one-dimensional array
  • A dictionary (if the vector can be considered to represent a function)
  • Visually (using an arrow)
52
Q

How can a vector be represented in code if the vector is viewed as a function?

A

A dictionary

53
Q

What does vector addition achieve?

A

Translation

54
Q

What does scalar-vector multiplication achieve?

A

Scaling

55
Q

What is another name for dot product?

A

Scalar product

56
Q

What is a convex combination of vectors u and v?

A

αu + βv

such that α , β >= 0
and α + β = 1

57
Q

What is an application of the dot product?

A

Finding the angle between two vectors

58
Q

In a convex combination of u and v, what is the significance of α = β = 0.5?

A

The resultant vector is the midpoint of the position vectors u and v

59
Q

Describe the steps involved in adding a record to a hash table

A
  • Hash algorithm applied to key, which returns the location (often an array index) in table where the record should be stored
  • If location is not empty, a collision has occurred
    – Open addressing is used (the next free location is used, wrapping round at the end of the array)
    – Or rehashing occurs (starting from scratch with a larger array)
    – Or chaining is used (a pointer to a list is stored at each location a collision has occurred)
60
Q

When might it be appropriate to represent a vector using a dictionary?

A

If the vector is used to represent a function

61
Q

Describe the steps involved in rehashing (2•)

A
  • A larger hash table is created
  • Each value in the existing table is inserted into the new table in a position determined by a new hashing algorithm
62
Q

Name three methods of collision handling (for a hash table)

A
  • Rehashing
  • Chaining
  • Open addressing
63
Q

How are collisions resolved with chaining?

A

A pointer to a list is stored at each location that points to a list of items that have collided at that location

64
Q

How are collisions resolved with open addressing?

A

The item is stored in the next available location in the hash table (wrapping around at the end)

Computer Science (11 decks)

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