KS3 Computing — Study Pack

Topic: Lists, Arrays & String Handling

Year 7–9 | Programming Fundamentals | UK National Curriculum

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Overview

Most real programs need to store and work with collections of related data, not just a single value. A school register, a set of quiz scores, the letters in a word — all of these are collections. Storing them as separate variables (score1, score2, score3, ...) becomes unmanageable very quickly. Arrays (called lists in Python) solve this by grouping related values under a single name, accessible by their position.

Strings are closely related. A string is actually a sequence of characters — in many ways, it behaves like a list of letters. Python gives programmers a rich set of tools for working with strings: finding their length, extracting portions of them, changing their case, splitting them into words, and much more. These skills are used constantly in real programs: processing user input, building output messages, validating passwords, and working with text data.

This pack covers both lists/arrays and string handling, exploring how to access, modify, and iterate over them — including key operations you are likely to be asked about in assessments.

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Section 1: Arrays and Lists

What is an Array?

An array is an ordered collection of values stored under a single variable name. Each value is called an element, and each element is accessed by its index — its position in the array. Arrays are ideal for storing sets of related values: a list of scores, a set of names, the items in a shopping cart.

In Python, arrays are called lists. They are written using square brackets with elements separated by commas.

scores = [85, 72, 90, 65, 88]
names  = ["Alice", "Bob", "Charlie"]
mixed  = [42, "hello", True]    # Python lists can mix types (arrays often cannot)

Zero-Based Indexing

Array indices in Python (and most programming languages) start at 0, not 1. This is called zero-indexed.

scores = [ 85,  72,  90,  65,  88 ]
index:     0    1    2    3    4

Index	0	1	2	3	4
Value	85	72	90	65	88

print(scores[0])   # 85  (first element)
print(scores[2])   # 90  (third element)
print(scores[4])   # 88  (last element)
print(scores[-1])  # 88  (last element — negative indexing)

Modifying Elements

scores[1] = 99     # Change the second element from 72 to 99
scores[0] = scores[0] + 5   # Increase first element by 5 (85 → 90)

Changing one element does not affect other elements in the list.

Finding the Length

length = len(scores)   # Returns 5 — the number of elements

Common List Operations

Operation	Python code	Effect
Access element	scores[2]	Returns the element at index 2
Modify element	scores[2] = 95	Replaces element at index 2 with 95
Append (add to end)	scores.append(77)	Adds 77 as a new last element
Remove element	scores.remove(65)	Removes the first occurrence of 65
Length	len(scores)	Returns the number of elements
Iterate	for s in scores:	Loops through each element in order

Iterating Over a List

scores ← [85, 72, 90, 65, 88]
FOR i ← 0 TO len(scores) - 1
    OUTPUT scores[i]
ENDFOR

scores = [85, 72, 90, 65, 88]
for score in scores:
    print(score)

Finding the Largest Element

scores ← [85, 72, 90, 65, 88]
largest ← scores[0]
FOR i ← 1 TO len(scores) - 1
    IF scores[i] > largest THEN
        largest ← scores[i]
    ENDIF
ENDFOR
OUTPUT largest

2D Arrays (Awareness Level)

A 2D array is an array of arrays — a grid of values. In Python this is a list of lists.

grid = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]
print(grid[1][2])   # Row 1, Column 2 → 6

2D arrays are used for grids, tables, and game boards (e.g., chess, noughts and crosses).

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Section 2: String Handling

What is a String?

A string is a sequence of characters enclosed in quote marks. Every character in a string has an index, just like a list — starting at 0.

"Computing"
  C  o  m  p  u  t  i  n  g
  0  1  2  3  4  5  6  7  8

String Indexing

word = "Computing"
print(word[0])    # C
print(word[4])    # u
print(word[-1])   # g (last character)

String Length

print(len("Computing"))   # 9
print(len(""))            # 0 (empty string)

String Slicing

Slicing extracts a portion of a string using [start:stop]. The character at start is included; the character at stop is excluded.

word = "Computing"
print(word[0:4])    # "Comp"  (indices 0, 1, 2, 3)
print(word[4:9])    # "uting" (indices 4, 5, 6, 7, 8)
print(word[3:])     # "puting" (from index 3 to end)
print(word[:4])     # "Comp"  (from start to index 3)

String Concatenation

Strings are joined using the + operator.

first = "Hello"
second = "World"
message = first + " " + second    # "Hello World"

Note: you can only concatenate strings with strings. "Score: " + 85 causes a TypeError — you must use str(85) first.

String Methods

Python provides many built-in methods for working with strings:

Method	What it does	Example	Result
.upper()	Converts to uppercase	"hello".upper()	"HELLO"
.lower()	Converts to lowercase	"HELLO".lower()	"hello"
.strip()	Removes leading/trailing whitespace	" hi ".strip()	"hi"
.replace(old, new)	Replaces all occurrences of old with new	"cat".replace("c","b")	"bat"
.split(delimiter)	Splits string into a list at each delimiter	"a,b,c".split(",")	["a","b","c"]
.find(sub)	Returns index of first occurrence of sub	"hello".find("l")	2

Type Conversion Between Strings and Numbers

num_string = "42"
num_int    = int(num_string)      # "42" → 42
num_float  = float("3.14")       # "3.14" → 3.14
back_to_str = str(99)             # 99 → "99"

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Key Vocabulary

Term	Definition
Array	An ordered collection of values stored under one name, accessed by index
List	Python's implementation of an array; can hold multiple values of any type
Element	An individual value stored in an array/list
Index	The numerical position of an element in an array/list; starts at 0
Zero-indexed	Indexing that begins at 0; the first element is at index 0, not index 1
len()	Python function that returns the number of elements in a list, or characters in a string
.append()	Python list method that adds an element to the end of the list
2D array	An array of arrays; used to represent a grid or table
String	A sequence of characters enclosed in quotes; treated as a data type for text
Concatenation	Joining two or more strings together using the + operator
Slicing	Extracting a portion of a string or list using [start:stop] notation
String method	A built-in function associated with string objects (e.g., .upper(), .split())
Type conversion	Converting a value from one data type to another (e.g., int("5"), str(99))

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Common Misconceptions — Corrected

Misconception	Correction
The first element of an array is at index 1	Arrays are zero-indexed: the first element is at index 0, the second at index 1, and so on.
Strings cannot be indexed like lists	Strings support all the same indexing and slicing operations as lists. "Computing"[0] gives "C".
Changing one element of a list changes the whole list	Each element is independent. scores[2] = 99 only changes the element at index 2.
You can concatenate a string and an integer with +	"Score: " + 85 causes a TypeError. You must convert the integer first: "Score: " + str(85).
.split() splits a string into individual characters	.split() splits at whitespace (spaces) by default, or at a specified delimiter. To split into characters, use list("hello").
len("Computing") returns 8	"Computing" has 9 characters (C-o-m-p-u-t-i-n-g), so len("Computing") returns 9.
scores[-1] is an error	Negative indexing is valid in Python. -1 refers to the last element, -2 to the second-last, etc.

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Diagrams / Code Examples

List Index Diagram

 List:    scores = [ 85,  72,  90,  65,  88 ]

 Index:              0    1    2    3    4
 Neg index:         -5   -4   -3   -2   -1

 scores[0]  →  85
 scores[2]  →  90
 scores[-1] →  88

String Index Diagram

 String:  word = "Computing"

 Index:           0  1  2  3  4  5  6  7  8
 Character:       C  o  m  p  u  t  i  n  g

 word[0]      →  'C'
 word[0:4]    →  'Comp'
 word[4:]     →  'uting'
 len(word)    →  9

Trace Table: List Manipulation

Program:

marks ← [60, 75, 80]
marks[0] ← marks[0] + 10
marks[2] ← marks[1] - 5
OUTPUT marks[0]
OUTPUT marks[1]
OUTPUT marks[2]

Line	Action	marks[0]	marks[1]	marks[2]	Output
Init	marks ← [60, 75, 80]	60	75	80
2	marks[0] ← 60 + 10	70	75	80
3	marks[2] ← 75 - 5	70	75	70
4	OUTPUT marks[0]	70	75	70	70
5	OUTPUT marks[1]	70	75	70	75
6	OUTPUT marks[2]	70	75	70	70

Iterating a List with FOR

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print("I like", fruit)

Output:

I like apple
I like banana
I like cherry

String Operations Example

sentence = "  Hello, World!  "

print(sentence.strip())           # "Hello, World!"
print(sentence.strip().lower())   # "hello, world!"
print(sentence.strip().upper())   # "HELLO, WORLD!"
print(sentence.strip().replace("World", "Python"))  # "Hello, Python!"

words = "red,green,blue".split(",")
print(words)    # ["red", "green", "blue"]
print(words[1]) # "green"

Finding Largest in List (Pseudocode)

numbers ← [34, 12, 78, 56, 23]
largest ← numbers[0]
FOR i ← 1 TO len(numbers) - 1
    IF numbers[i] > largest THEN
        largest ← numbers[i]
    ENDIF
ENDFOR
OUTPUT "Largest: " + str(largest)

Trace:

i	numbers[i]	largest
start	—	34
1	12	34 (12 not > 34)
2	78	78 (78 > 34)
3	56	78 (56 not > 78)
4	23	78 (23 not > 78)

Output: Largest: 78

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Exam-Style Questions

Q1 [1 mark] A list is defined as: myList = [10, 20, 30, 40, 50] What is the index of the first element?

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Q2 [1 mark] Using the same list as Q1, what is the output of print(myList[2])?

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Q3 [2 marks] The following list stores student scores: scores = [45, 78, 62, 91, 55]

a) Write one line of Python code that adds the value 88 to the end of the list. [1 mark] b) After running your code, what is len(scores)? [1 mark]

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Q4 [3 marks] Explain what each of the following string operations does, and state the result when applied to text = "Computing":

a) text[0] b) text[0:4] c) len(text)

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Q5 [3 marks] Write a pseudocode program that:

• stores the numbers [3, 7, 1, 9, 4] in an array called values
• uses a FOR loop to add up all the numbers
• outputs the total

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Q6 [4 marks] Trace through the following program, completing the trace table.

data ← [5, 10, 15]
total ← 0
FOR i ← 0 TO 2
    total ← total + data[i]
ENDFOR
OUTPUT total

i	data[i]	total before addition	total after addition
0		0
1
2
Output

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Q7 [2 marks] Describe three string methods available in Python. For each, state what it does and give an example.

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MCQ [1 mark] What is the output of the following code?

word = "algorithm"
print(word[3:6])

A) "algo" B) "ori" C) "gor" D) "rith"

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Fill in the blank [1 mark] The Python method that splits a string into a list of substrings is ________.

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Model Answers

Q1: Index 0 [1]

Q2: 30 [1] (index 2 is the third element: 10 at 0, 20 at 1, 30 at 2)

Q3: a) scores.append(88) [1] b) 6 [1] (was 5 elements, now 6)

Q4: a) text[0] — accesses the character at index 0 — result: "C" [1] b) text[0:4] — slices from index 0 up to (not including) index 4 — result: "Comp" [1] c) len(text) — returns the number of characters in the string — result: 9 [1]

Q5:

values ← [3, 7, 1, 9, 4]
total ← 0
FOR i ← 0 TO 4
    total ← total + values[i]
ENDFOR
OUTPUT total

[1 for correct initialisation of total; 1 for correct loop structure; 1 for correct output — total should be 24]

Q6:

i	data[i]	total before	total after
0	5	0	5
1	10	5	15
2	15	15	30
Output			30

[1 per correct row + output]

Q7: Any three from:

• .upper() — converts all characters to uppercase. Example: "hello".upper() → "HELLO" [1]
• .lower() — converts all characters to lowercase. Example: "HELLO".lower() → "hello" [1]
• .strip() — removes leading and trailing whitespace. Example: " hi ".strip() → "hi" [1]
• .split(",") — splits string into a list at each comma (or specified delimiter). Example: "a,b,c".split(",") → ["a","b","c"] [1]
• .replace("old","new") — replaces all occurrences. Example: "cat".replace("c","b") → "bat" [1] (Max 2 marks for descriptions, 1 for examples — total 2 marks for 3 correct methods with examples)

MCQ: C — "gor" [1] "algorithm": a(0) l(1) g(2) o(3) r(4) i(5) t(6) h(7) m(8). [3:6] → indices 3,4,5 → "ori". Wait — index 3='o', 4='r', 5='i'. So "ori". Answer: B — "ori". (Correction: the answer is B. The string "algorithm" at indices 3,4,5 gives 'o','r','i' = "ori".)

Fill in the blank: .split() [1]

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Revision Checklist

• I can explain what an array (list) is and why it is useful compared to separate variables.
• I understand zero-based indexing and can identify the correct index for any element.
• I can access, read, and modify individual elements of a list using their index.
• I can use len() to find the number of elements in a list.
• I can use .append() to add an element to the end of a list.
• I can iterate over a list using a FOR loop, both with an index and using for item in list.
• I can write pseudocode/Python to find the largest (or smallest) value in a list.
• I understand what a 2D array is and can access elements using two indices.
• I understand that a string is a sequence of characters and can be indexed like a list.
• I can use string indexing (word[0]) and slicing (word[0:4]) correctly.
• I can use len() on a string to find its length.
• I can concatenate strings using + and explain why "text" + number causes a TypeError.
• I can describe and use at least five string methods: .upper(), .lower(), .strip(), .replace(), .split().
• I can convert between strings and numbers using int(), float(), and str().
• I can trace a program that uses lists through a trace table.