AQA 7517 · Paper 1 · 4.1.1e

String
Handling

AQA A-Level Computer Science · Section 4.1 Fundamentals of Programming

WHAT YOU'LL LEARN

LEN · SUBSTRING · UPPER/LOWER · CHAR · ASC · Concatenation

AQA SPEC LINK

4.1.1 — Use of string-handling functions in programs

Strings in AQA

What is a String?

A string is an ordered sequence of characters enclosed in double quotes. Strings are indexed from position 0 (zero-indexed) or position 1 — AQA accepts either but be consistent.

AQA INDEXING EXAMPLE

word ← "Python"
— Index 0: 'P', Index 1: 'y', Index 2: 't', Index 3: 'h', Index 4: 'o', Index 5: 'n'

LEN

LEN() — String Length

SYNTAX & EXAMPLES

LEN("Hello") → 5
LEN("") → 0
name ← "Computer Science"
OUTPUT LEN(name) → 16

LEN() counts every character including spaces. Python equivalent: len(string). Useful for validation loops and iterating through characters.

SUBSTRING

SUBSTRING() — Extracting Part of a String

AQA SYNTAX

SUBSTRING(string, start, length)

SUBSTRING("Hello World", 0, 5) → "Hello"
SUBSTRING("Hello World", 6, 5) → "World"

⚠️ AQA NOTE

Parameters are: position (0-indexed), start position, number of characters. Python uses slicing: s[0:5] instead.

UPPER & LOWER

Case Conversion

UPPER()

UPPER("hello") → "HELLO"
UPPER("Python") → "PYTHON"

Python: "hello".upper()

LOWER()

LOWER("HELLO") → "hello"
LOWER("Python") → "python"

Python: "HELLO".lower()

Used for case-insensitive comparisons: IF UPPER(response) = "YES" THEN

CHAR & ASC

Character ↔ ASCII Conversion

ASC() — Char to Code

ASC('A') → 65
ASC('a') → 97
ASC('0') → 48

Python: ord('A')

CHAR() — Code to Char

CHAR(65) → 'A'
CHAR(97) → 'a'
CHAR(48) → '0'

Python: chr(65)

Key values: A=65, Z=90, a=97, z=122, 0=48, 9=57. Used in encryption and character manipulation.

Concatenation

Joining Strings with +

EXAMPLES

first ← "Alice"
last ← "Smith"
full ← first + " " + last → "Alice Smith"
OUTPUT "Score: " + STR(95) → "Score: 95"

⚠️ You can only concatenate string + string. Convert numbers first with STR(). In Python, f-strings (f"Hello {name}") are often cleaner.

String Iteration

Looping Through Characters

COUNT VOWELS EXAMPLE

word ← USERINPUT
count ← 0
FOR i ← 0 TO LEN(word) - 1
  letter ← SUBSTRING(word, i, 1)
  IF letter = "a" OR letter = "e" OR letter = "i" OR letter = "o" OR letter = "u" THEN
    count ← count + 1
  ENDIF
ENDFOR
OUTPUT count

AQA Exam Style

Practice Question

AQA 7517 — Paper 1 Style

What is the output of the following pseudocode?

s ← "AQA Computer Science"
OUTPUT LEN(s)
OUTPUT SUBSTRING(s, 4, 8)
OUTPUT UPPER(SUBSTRING(s, 0, 3))

[3 marks]

1 mark

LEN("AQA Computer Science") → 20

1 mark

SUBSTRING(s, 4, 8) → "Computer"

1 mark

UPPER(SUBSTRING(s, 0, 3)) → "AQA"

Summary

String Functions Reference

Function	Purpose	Python Equivalent
LEN(s)	Length of string	len(s)
SUBSTRING(s,i,n)	Extract n chars from index i	s[i:i+n]
UPPER(s)	Convert to uppercase	s.upper()
LOWER(s)	Convert to lowercase	s.lower()
ASC(c)	Character to ASCII code	ord(c)
CHAR(n)	ASCII code to character	chr(n)

🎉 Lesson complete — move to the quiz!

StringHandling

What is a String?

LEN() — String Length

SUBSTRING() — Extracting Part of a String

Case Conversion

Character ↔ ASCII Conversion

Joining Strings with +

Looping Through Characters

Practice Question

String Functions Reference

String
Handling