OCR J277 · Component 2 · Topic 2.2.1e

Programming Techniques
String Manipulation

Length, indexing, substring, case conversion, ASCII — in OCR ERL and Python

CSZone OCR GCSE Computer Science J277

Learning Objectives

By the end of this video you will be able to...

Use concatenation to join strings together and explain how strings are stored as a sequence of characters

Find the length of a string using length / len(), and access individual characters using 0-based indexing

Extract part of a string using substring() in OCR ERL and slicing in Python

Convert string case using upper() and lower(), and convert between strings and numbers using str(), int(), float()

Use ASC() and CHR() in OCR ERL (and ord() / chr() in Python) to work with ASCII character codes

⚡ String manipulation lets programs process text — usernames, passwords, search terms, output formatting — all rely on these operations.

Strings

What is a string — and concatenation

WHAT IS A STRING?

A string is a sequence of characters enclosed in speech marks. Characters include letters, digits, spaces, and symbols. Each character occupies one position in the sequence, numbered from index 0.

EXAMPLE — "Hello"

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5 characters — indices 0 to 4

OCR ERL — STRING DECLARATION

greeting ← "Hello" name ← "Alice" digit ← "5" ← this is a string, not an int!

CONCATENATION

Concatenation joins two or more strings together using the + operator. The strings are joined end-to-end in the order given. You can concatenate string variables, string literals, and the results of string functions.

OCR ERL — CONCATENATION

first ← "Hello" second ← " World" result ← first + second print(result) ← outputs: Hello World // Concatenate with a literal: name ← "Alice" print("Hello, " + name + "!") ← outputs: Hello, Alice!

CANNOT MIX TYPES WITH +

"Age: " + 16 causes an error — you cannot use + to join a string and an integer. Cast the integer first: "Age: " + str(16)

String Operations

String length and character indexing

LENGTH

myString.length (OCR ERL) or len(myString) returns the total number of characters in the string — including spaces and symbols.

OCR ERL — LENGTH

word ← "Computer" print(word.length) ← outputs 8 print(len(word)) ← also outputs 8 sentence ← "Hi there" print(sentence.length) ← outputs 8 (space counts!)

PYTHON — LENGTH

word = "Computer" print(len(word)) # outputs 8

CHARACTER INDEXING — 0-BASED

Access a single character using its index in square brackets. The first character is always at index 0. The last character is at index length − 1.

"Computer" — index diagram

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word ← "Computer" print(word[0]) ← "C" print(word[3]) ← "p" print(word[7]) ← "r" (last char)

⚡ The last character index is always length − 1, not length. For "Computer" (length 8), the last character "r" is at index 7. This is a classic off-by-one error in the exam.

String Operations

Extracting a substring

substring() — OCR ERL

myString.substring(start, length) returns a portion of a string. start is the 0-based index of the first character to extract. length is how many characters to return.

OCR ERL — SUBSTRING

word ← "Computer" // First 4 characters: print(word.substring(0, 4)) ← "Comp" // 3 characters from index 3: print(word.substring(3, 3)) ← "put" // Last character only: print(word.substring(7, 1)) ← "r"

"Computer" — substring visualised

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substring(0, 4) → "Comp"

PYTHON — SLICING

word = "Computer" # Slicing syntax: string[start:end] # 'end' is EXCLUSIVE — not included print(word[0:4]) # "Comp" (0,1,2,3) print(word[3:6]) # "put" (3,4,5) print(word[:4]) # "Comp" (from start) print(word[4:]) # "uter" (to end)

OCR ERL vs PYTHON — KEY DIFFERENCE

OCR ERL substring(start, length) — second argument is how many characters.
Python [start:end] — second number is the stop index (exclusive), not the length.

USING substring() WITH length

word ← "Computer" // Get the last 3 characters: last3 ← word.substring(word.length - 3, 3) print(last3) ← "ter" // Get first character: initial ← word.substring(0, 1) print(initial) ← "C"

String Operations

Case conversion — upper() and lower()

upper() AND lower()

myString.upper() returns a new string with all letters converted to UPPERCASE. myString.lower() returns a new string with all letters converted to lowercase. The original string is not changed — a new string is returned.

OCR ERL — CASE CONVERSION

word ← "Hello World" print(word.upper()) ← "HELLO WORLD" print(word.lower()) ← "hello world" print(word) ← "Hello World" (unchanged!)

PYTHON EQUIVALENT

word = "Hello World" print(word.upper()) # "HELLO WORLD" print(word.lower()) # "hello world"

REAL-WORLD USE — CASE-INSENSITIVE COMPARISON

// Without case conversion — fragile: answer ← input("Continue? (yes/no): ") IF answer == "yes" THEN ← "Yes" fails! print("Continuing...") ENDIF // With lower() — robust: answer ← input("Continue? (yes/no): ") IF answer.lower() == "yes" THEN print("Continuing...") ENDIF ← "Yes", "YES", "yEs" all work

WHY THIS MATTERS IN THE EXAM

Converting user input to lower() or upper() before comparing is a common exam technique tied to defensive design (Topic 2.3.1). It handles unexpected capitalisation from the user — a mark-winning detail in "write a program" questions.

String Operations

Type conversion — str(), int(), float()

WHY TYPE CONVERSION IS NEEDED

User input always arrives as a string — even if the user types a number. To do arithmetic, you must cast it to an integer or float. To output numbers in a sentence, you must cast them back to a string.

OCR ERL — TYPE CONVERSION

// String → integer: ageStr ← input("Enter age: ") age ← int(ageStr) print(age + 1) ← numeric addition // String → float: price ← float(input("Price: ")) // Integer → string: score ← 95 print("Score: " + str(score))

PYTHON EQUIVALENT

age = int(input("Enter age: ")) price = float(input("Price: ")) score = 95 print("Score: " + str(score))

CONVERSION FUNCTIONS — QUICK REFERENCE

Function	Converts	Example
int()	str/float → integer	int("5") → 5
float()	str/int → float	float("3.14") → 3.14
str()	int/float → string	str(42) → "42"

NOTE

int("3.7") raises an error — you cannot convert a decimal string directly to int. Cast to float first: int(float("3.7")) → 3

⚡ A very common pattern in exam programs: age ← int(input("Age: ")) — combines input and conversion in one line. Both OCR ERL and Python accept this style.

String Operations

ASCII codes — ASC() and CHR()

WHAT IS ASCII?

ASCII (American Standard Code for Information Interchange) assigns a unique number to each character. Computers store characters as numbers. Every letter, digit, and symbol has an ASCII code — 'A' = 65, 'a' = 97, '0' = 48.

OCR ERL — ASC() AND CHR()

// ASC() → returns ASCII code of a char: print(ASC("A")) ← 65 print(ASC("a")) ← 97 print(ASC("0")) ← 48 // CHR() → returns char from ASCII code: print(CHR(65)) ← "A" print(CHR(98)) ← "b"

PYTHON — ord() AND chr()

print(ord("A")) # 65 print(chr(65)) # "A" print(ord("a")) # 97

KEY ASCII VALUES TO KNOW

Char	Code	Notes
'0' – '9'	48 – 57	Digit chars
'A' – 'Z'	65 – 90	Uppercase
'a' – 'z'	97 – 122	Lowercase

USING ASC() FOR CIPHER PROBLEMS

// Shift letter by 1 (Caesar cipher style): ch ← "A" shifted ← CHR(ASC(ch) + 1) print(shifted) ← "B"

⚡ The difference between 'A' (65) and 'a' (97) is exactly 32. Lowercase letters always have higher ASCII codes than their uppercase equivalents. In OCR ERL, ASC() takes a single character — not a full string.

String Operations

Useful string operations — comparison and search

STRING COMPARISON

Strings can be compared using ==, !=, and even < / > (alphabetical order based on ASCII values). Comparison is case-sensitive — "Hello" ≠ "hello".

word1 ← "apple" word2 ← "Apple" IF word1 == word2 THEN print("Same") ELSE print("Different") ← this runs ENDIF // Case-insensitive compare: IF word1.lower() == word2.lower() THEN print("Same") ← this runs ENDIF

CHECKING FIRST CHARACTER

// Check if string starts with "A": name ← "Alice" IF name[0] == "A" THEN print("Starts with A") ENDIF

PYTHON — ADDITIONAL STRING METHODS

text = "Hello World" # Check if substring present: print("World" in text) # True print("world" in text) # False (case-sensitive) # Find position of substring: print(text.find("World")) # 6 # Replace a substring: print(text.replace("World", "Alice")) # "Hello Alice" # Strip whitespace: print(" hi ".strip()) # "hi"

IN THE EXAM

For OCR ERL pseudocode, the examiners expect: length, substring(), upper(), lower(), ASC(), CHR(), and indexing with [i]. Python methods like find() and replace() only appear in Python-specific questions.

Worked Example

String manipulation — exam-style problem

PROBLEM

Write OCR ERL pseudocode that inputs a user's first name and surname separately, then outputs their full name (first + space + surname), the total number of characters (including the space), and their initials (first character of each name).

OCR ERL SOLUTION

first ← input("First name: ") surname ← input("Surname: ") // Build full name: fullName ← first + " " + surname print("Full name: " + fullName) // Total character count: total ← fullName.length print("Characters: " + str(total)) // Initials: initials ← first[0] + "." + surname[0] + "." print("Initials: " + initials)

PYTHON SOLUTION

first = input("First name: ") surname = input("Surname: ") # Full name: fullName = first + " " + surname print("Full name: " + fullName) # Character count: print("Characters: " + str(len(fullName))) # Initials: initials = first[0] + "." + surname[0] + "." print("Initials: " + initials)

OPERATIONS USED IN THIS EXAMPLE

Concatenation — joining first, " ", and surname with +

length — total characters in the full name including the space

Indexing [0] — extract the first character of each name

str() — cast the integer total to string for concatenation

Exam Practice

String manipulation — exam questions

Question 1 — 1 mark

A variable is declared as: word ← "Science"
What is the value of word[2]?

Answer — Q1

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word[2] = "i" — strings are 0-indexed, so index 2 is the third character. (1 mark)

Question 2 — 2 marks

Write OCR ERL to output the first 3 characters of a string stored in a variable called word, then output its full length.

Answer — Q2

print(word.substring(0, 3)) ← [1] print(str(word.length)) ← [1]

Mark 1: correct substring(0, 3). Mark 2: word.length or len(word) output.

Question 3 — 4 marks

Write OCR ERL pseudocode for a program that:
• inputs a word from the user
• outputs the word in uppercase
• outputs the first character of the word
• outputs the length of the word

Exam Answers

Question 3 — answer and mark scheme

Q3 MARK SCHEME

word ← input("Enter a word: ") ← [1] print(word.upper()) ← [1] print(word[0]) ← [1] print(str(word.length)) ← [1]

Mark 1: correct input into a variable.
Mark 2: word.upper() used in print.
Mark 3: word[0] to get first character.
Mark 4: word.length or len(word) correctly output.

COMMON MARK LOSSES ON THIS Q

• Using word[1] instead of word[0] — classic off-by-one
• Forgetting str() when concatenating length with a string
• Writing word.upper without the parentheses — in OCR ERL, functions need ()

PYTHON EQUIVALENT

word = input("Enter a word: ") print(word.upper()) print(word[0]) print(len(word))

OCR ERL STRING FUNCTIONS — REFERENCE

Operation	OCR ERL	Python
Length	s.length	len(s)
Index	s[i]	s[i]
Substring	s.substring(a,n)	s[a:a+n]
Uppercase	s.upper()	s.upper()
Lowercase	s.lower()	s.lower()
ASCII	ASC() / CHR()	ord() / chr()

⚡ In a 4-mark string program question: 1 mark input, 1 mark per string operation applied correctly. Show the operation name clearly and apply it to the right variable — those two things are what the mark scheme checks.

Common Mistakes

Common mistakes — avoid these in the exam

MISTAKE 1 — Off-by-one indexing

Using word[1] to get the first character. Strings are 0-indexed — the first character is always at index 0. Using index 1 returns the second character

✓ First character: word[0] — last character: word[word.length - 1]

MISTAKE 2 — Confusing substring() arguments

Writing word.substring(0, 4) expecting characters 0 to 4 — but the second argument is length, not an end index. substring(0, 4) returns 4 characters starting from 0

✓ OCR ERL: substring(start, length). Python slicing: [start:end] — these are NOT the same

MISTAKE 3 — Mixing types in concatenation

Writing "Length: " + word.length — this causes an error because length returns an integer and you cannot concatenate a string with an integer using +

✓ Always cast: "Length: " + str(word.length)

MISTAKE 4 — Thinking upper() changes the original

word.upper() does NOT change word — it returns a new string. After calling word.upper(), the variable word is still in its original case

✓ Store the result: upper ← word.upper() or use it directly in print

Summary

Key points — 2.2.1e

A string is a sequence of characters. Characters are stored at positions starting from index 0. Access with string[i]. Join strings using the + operator — this is concatenation

length — use myString.length or len(myString). The last character is always at index length − 1, not length. Remember: 0-indexed means one less than you might expect

substring(start, length) in OCR ERL extracts part of a string — second argument is how many characters. Python uses slicing [start:end] where end is exclusive (not included)

upper() and lower() return a new converted string — the original is unchanged. Use str(), int(), float() to convert between types. You cannot concatenate a string and an integer without casting

ASC() returns the ASCII code of a character — CHR() returns the character from a code. In Python: ord() and chr(). Key values: 'A'=65, 'a'=97, '0'=48

⚡ Next topic: 2.2.1f — Subroutines: Functions and Procedures. How to write and call reusable code blocks.

2.2.1e Complete

String Manipulation
Length · Index · Substring · Case

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Next Up

2.2.1f — Subroutines: Functions & Procedures

Programming TechniquesString Manipulation

By the end of this video you will be able to...

What is a string — and concatenation

String length and character indexing

Extracting a substring

Case conversion — upper() and lower()

Type conversion — str(), int(), float()

ASCII codes — ASC() and CHR()

Useful string operations — comparison and search

String manipulation — exam-style problem

String manipulation — exam questions

Question 3 — answer and mark scheme

Common mistakes — avoid these in the exam

Key points — 2.2.1e

String ManipulationLength · Index · Substring · Case

Programming Techniques
String Manipulation

String Manipulation
Length · Index · Substring · Case