1. Coding Concepts (Data Structures)
Topic 2: Programming
Coding Concepts (Data Structures)

2. Programming: Data Structures
Whether we’re creating simple or complex programs, we always make use of data structures
They are different ways of storing pieces of data
The simplest data structure we’ve used (often) is the variable
It is a data structure that stores a single value
However, there are more
Programming: Data Structures

3. Records/Dictionaries
Data Structures
There are three complex data structures that we need to examine for this course
We have used arrays (both explicitly and implicitly) already
From creating arrays of numbers
To using string values (which are arrays of characters)
The other two need to be looked at in quite a bit of detail
As they are very useful
1D Arrays
2D Arrays
Records/Dictionaries
Programming: Data Structures

4. Programming: Data Structures
1D Arrays
Arrays are data structures that can store multiple values
They’re like cells in a table, or spaces in a car park
We have to say, beforehand, how many spaces we want
We can then put whatever we’d like in those spaces
Each space has its own ID, called an index
It’s how we get to, meaning access, those spaces
At any point, we can
Read a space (and use its value any way we want)
Write to a space (saving a value at that position)
Programming: Data Structures

5. Programming: Data Structures
1D Arrays
The D stands for Dimension
Like ‘3D’ stands for ‘Three Dimensions’
Whenever we create an array, we can actually specify how many dimensions we want
1D arrays create a ‘line’
2D arrays create a ‘rectangle’
3D arrays create a ‘cuboid’
We’d use different dimensions for different uses
A series of random numbers works best in a 1D array
A table of information works best in a 2D array
Positions of objects in space works best in a 3D array
Programming: Data Structures

6. Programming: Data Structures
1D Arrays
Creating an array is as simple as using a pair of square brackets []
Most programming languages recognise this as ‘array’ syntax
For Python, we assign these square brackets to a ‘variable’
For C# and Java, we need to create a ‘array’ variable and either
Give it an array with the values already in it
Create a new array with a specific number of spaces
Programming: Data Structures

7. Programming: Data Structures
1D Arrays
Here are examples of creating arrays in the different programming languages
Note that in C# and Java, we need to state the data-type of the contents of the array as well
Python C#
Java
Programming: Data Structures

8. Programming: Data Structures
1D Arrays
In the previous coding examples, we saw two ways of creating an array
By specifying the actual values to put in
By stating the number of spaces that we want to use
The latter option works best when we don’t know what we’re putting in the array
Like random values, or user-input values
C# and Java have this support built in (we simply put the number inside the square brackets)
Python doesn’t support this natively
Instead we make a ‘default’ value and multiply it by the size we want
Programming: Data Structures

9. 1D Arrays One really important thing to remember about arrays
When we’ve made an array
By giving it values or spaces
The array will only have that many spaces from then on
This is known as a static data-structure
We CANNOT add or remove spaces after an array has been made!
Programming: Data Structures

10. Programming: Data Structures
1D Arrays
We can re-create an array if we wanted to
That means keeping the array’s name
But, change the number of spaces
Or give specific values
Note that this is just like creating the array from scratch
Any values will be lost when doing this
Programming: Data Structures

11. Create the following arrays in a program An array of 3 country names
An array of 3 integers (from 1 to 10) for the ‘cute factor’ for the following animals
An array of 6 random integers (from 0 to 99)
Aardvaark Axolotl Fennec Fox
Programming: Data Structures

12. Programming: Data Structures
2D Arrays
The next dimension up is the 2D array
Rather than having a single line of values, we now have a rectangle
The best way of imagining 2D arrays is
the first dimension goes down to a specific row
the second dimension goes across to a specific column
Columns
[0]
[1]
[2] 10 7 5 4 8 12 13 54 40
[0]
Rows
[1]
[2]
e.g. [0][2] = 5
Programming: Data Structures

13. Programming: Data Structures
2D Arrays
We could use 2D arrays if we’re trying to
Represent anything in a table format
Save pixel data for bitmaps
Store positions in a grid-based 2D game
Adding this second dimension to arrays is rather simple
Using the second dimension, however, takes a bit more work
Programming: Data Structures

14. Programming: Data Structures
2D Arrays
Here are some examples C#
Python
Java
Programming: Data Structures

15. Programming: Data Structures
2D Arrays
In Python and Java, we can think of 2D arrays as arrays of arrays
Thus the use of two pairs of square brackets
In fact, we can add another pair of square brackets for more dimensions
C#, however, thinks of it as a table
It only has one pair of square brackets
But use a comma to separate each dimension
When we’re specifying the values that go into a 2D array, we add nested brackets for each row
Programming: Data Structures

16. Programming: Data Structures
Create the following 2D arrays in a program
A String array with 2 rows and 4 columns (as a ‘nickname’ generator)
The first row: names (e.g. “Max” or “Aisha”)
The second row: adjectives (e.g. “Happy” or “Thoughtful”)
An integer array to represent the 10 times- table
Programming: Data Structures

17. Programming: Data Structures
Records
The final data structure is the record
Rather than storing values in indexed boxes, we store values by keys
This allows us to create key-value pairs
We give a value a key, and store it in the record
We can then get the same value using the same key
These are also known as dictionaries and associative arrays
Programming: Data Structures

18. Programming: Data Structures
Records
For example, we could store lots of different pieces of data on a single thing
We could create a record for a student
We can store their name (using the key “name”)
We can store their age (using the key “age”)
In Python, we can use different data-types for different values
In C# and Java, we’re restricted to using a single data-type
We also use different structures for making these
In C#, we use the Dictionary class
In Java, we use the HashMap class
Programming: Data Structures

19. Programming: Data Structures
Records
Here’s how we use records in the different programming languages
Pay close attention to the {} in Python, [] in C#, and the function calls Java C#
Python
Java
Programming: Data Structures

20. Programming: Data Structures
Create a record for a pet (real or made-up)
Make the record itself
Give it keys and values for the following
The pet’s species
The pet’s name
The pet’s age
Output this information to the console (in a nicely formatted manner)
Programming: Data Structures

21. Programming: Data Structures
Is it possible to make a 1D array of records?
See if you can work out how to create this yourself!
Try an example like an array of pets
Each index points to a single pet
Each pet as the keys and values from the previous exercise
For Python, we simply add another pair of square brackets (like for 2D arrays) for access, and multiple sets of curly-braces for the values:
pets = [{“species”: “cat”, “name”: “Toby”, “age”: 5, {“species”: “dog”, “name”: “Sir Woofles”, “age”: 8}}]
For C#, we add a pair of square brackets after the Dictionary definition (and then access like a 2D array) and have to initialise each element:
Dictionary<string, string>[] pets = new Dictionary<string, string>[2];
pets[0] = new Dictionary<string, string>();
pets[0][“species”] = “cat”; …
For Java, we add a pair of square brackets after the HashMap definition (and then access like a 2D array) and have to initialise each element:
HashMap<String, String>[] pets = new HashMap[2];
pets[0] = new HashMap<>();
pets[0].put(“species”, “cat”);
Programming: Data Structures