1. Arrays & Loops

2. Learning Objectives Understand the difference between arrays and lists
Understand and be able to use an index for a one-dimensional array or list
Be able to use a loop structure to iterate through an array or list

3. Arrays or Lists
An array is a data structure that allows you to hold many items of data, all of the same datatype with one name.
In python Arrays are called Lists, where each element can be a different datatype
An list of 10 student names could be stored in a list:
In most programming languages the “index” starts at 0:
userName[0], userName[1], …. userName[9]
The elements of the array are typically referred to as
userName[1], userName[2], …. userName[10]

4. Advantages of using Lists
Imagine that you want to input, sort and print 100 user names. It would be very inconvenient to use 100 different variable names
With a list you can have as many data items as you need stored using one variable name
Non-sequential nature – a program can jump between non- neighbouring elements instantly without having to browse through the whole array

5. Advantages of using Lists
Ease of iterating through an array – which allow programmers to manipulate multiple items of data with very few lines of code, replacing many separate variables.
FOR number = 1 TO 100
userName[number] = Input(“ Enter a user name”)
NEXT number
username = sort(userName)
Print (userName[number])
Next number

6. Recognising Lists or Arrays
Not all languages use square brackets as Python does for indices
Some use round brackets which might cause confusion because functions and procedures also have round brackets next to their identifiers.
For example in Visual Basic getPupilData(4) is a function call and pupils(4) is the 5th element (assuming zero-based array) of an array named pupils.
In Python you can identify arrays by the index that appears next to array identifiers. E.g. pupils[2]

7. Activity1 : spot an array
Counter = 3
Counter = getData(5)
OUTPUT Widgets[4]
Names[2] = findMax(2)
OUTPUT Sentence.SPLIT(“ “)
FOR Counter = 1 TO LEN(Lines)-1

8. Activity1 : spot an array answer
Counter = 3 Just a variable
Counter = getData(5) A var  a function
OUTPUT Widgets[4] Widgets is Array!
Names[2] = findMax(2) Names is Array!
OUTPUT Sentence.SPLIT(“ “) Array is output
FOR Counter = 1 TO LEN(Lines)-1 Lines is an array

9. Activity 2 The user inputs 5 numbers
The program outputs the first and the last number.
Things to consider:
Initialising the list
Using prompts and input
Datatype of stored data
Here is an example of what the program should look like it when it runs:
ARRAY Numbers[5]
FOR count = 0 TO 4
OUTPUT ”Data Item” + count
INPUT Numbers[count]
NEXT count
OUTPUT Numbers[0]
OUTPUT Numbers[4]
save as For Loop Data Fixed Size

10. Activity 3 Design and code solutions for the following tasks:
Task A. Modify the code in Activity1 to allow the user to enter the number of data items before the loop starts.
Task B. Modify the code to allow the user continue entering data items until a break code such as -1 is entered
Task C. Modify the code from Task B so the the user enters data in the same manner with the added feature that allows the list to be searched for an item of data. Your code will ask the user to enter a number to be searched for. If it is found a message displaying the index position will be displayed or if it doesn’t exist in the list display an appropriate message
Task D. Modify the code in Task B to use an alternative type of loop. Eg if you have used a For loop, change it to a While or vice versa
Note: for additional Python resources using loops and lists checkout this link:
save as For Loop Variable Size
save as List and Loop break
save as List and Loop Find

11. Common Errors
Since traditional arrays have fixed sizes, it is quite common to lose track of the length of the array and attempt to assign a value to a non-existent index:
ARRAY Numbers[4]
Numbers[5]:=9  will cause a crash with a message “Index not found”
Off By One (OBO) errors
Forgetting to take account of the fact that array indices start at 0 and for example using the len() function without subtracting 1
Only Python supports negative indices !

12. Validating for array size
Best practice before writing to or reading from an array is to find out its length:
Wrong:
ARRAY Numbers[4]
FOR count = 0 TO 5
Numbers[count] = RAND (1,100)
NEXT count
Right:
ARRAY Numbers[4]
FOR count = 0 TO LEN(Numbers)-1
Numbers[count] = RAND (1,100)
NEXT count
In Python:

13. Useful Info Programmers count from ZERO!
The first item in a list is therefore at position ‘0’
Setting up an empty list:
Adding a variable or literal to a list:
To count the number of items in a list use the len() function e.g.
Lists always use square brackets
Setting up a list of strings when the list is initialised