[8.1.4e] String Handling

[8.1.4e] String Handling

Strings are sequences of characters such as letters, digits, punctuation, and spaces. In IGCSE programming you regularly need to find the length of a string, take a substring, change case to upper or lower, and understand where positions start from. Mastering these operations helps you validate input, display tidy output, and extract useful information from text.

Strings as sequences

A string behaves like an ordered row of boxes, one box per character. Each box has a position (also called an index). Some languages start counting positions at 0, others at 1. Exam pseudocode for Cambridge uses positions starting at 1 when functions like LENGTH and SUBSTRING are used. Python, which many schools use for practice, starts positions at 0. This difference is important for getting the right characters and avoiding off-by-one errors.

Core string operations

These are the four core operations you must know for this benchmark. Explore them in the tabs below using clear, contrasting examples and edge cases.

• Length() with mixed characters
• Substring selection: start & length
• Changing case: UPPER vs lower
• Indexing schemes: 0-based vs 1-based

// CAIE Pseudocode
DECLARE s : STRING ← "CS 0478!"
OUTPUT LENGTH(s)        // 8

# Python
s = "CS 0478!"
print(len(s))           # 8

// CAIE Pseudocode
DECLARE city : STRING ← "Cambridge"
OUTPUT SUBSTRING(city, 1, 3)     // "Cam"
OUTPUT SUBSTRING(city, 5, 3)     // "rid"

// Python
city = "Cambridge"
print(city[0:3])                 # "Cam"
print(city[4:7])                 # "rid"

// CAIE Pseudocode
DECLARE answer : STRING
INPUT answer
IF UPPER(answer) = "YES" THEN
  OUTPUT "Proceed"
ELSE
  OUTPUT "Cancelled"
ENDIF

# Python
answer = input()
if answer.upper() == "YES":
    print("Proceed")
else:
    print("Cancelled")

// CAIE Pseudocode
DECLARE w : STRING ← "COMPSCI"
OUTPUT SUBSTRING(w, 1, 1)     // "C" first character
OUTPUT SUBSTRING(w, LENGTH(w), 1) // "I" last character

# Python
w = "COMPSCI"
print(w[0])                   # "C" first character (index 0)
print(w[len(w)-1])            # "I" last character

Applying string handling in real tasks

• Input validation: check a password length is at least 8, or a UK mobile number has exactly 11 digits (ignore spaces if specified).
• Formatting names: output names in title case, or create initials like A.B. by taking the first character of each part.
• Searching: convert both the search term and the source text to the same case before comparing.

Deep Dive: Translating between CAIE pseudocode and Python

In assessment you may write CAIE-style pseudocode such as SUBSTRING(text, start, length). When practising in Python, translate using the rule text[start-1 : (start-1)+length]. For example, CAIE SUBSTRING("Cambridge", 5, 3) becomes Python "Cambridge"[4:7]. Remember that Python excludes the end index.

Common pitfalls and how to avoid them

• Counting from the wrong start: write a comment stating whether you are using 0-based or 1-based positions and stick to it.
• Forgetting spaces count: when measuring LENGTH, plan whether to strip spaces first. Only remove spaces if the specification requires it.
• Incorrect substring bounds: double-check that the substring stays within the string. Use LENGTH(text) to compute safe lengths.
• Case-sensitive comparisons: normalise with UPPER or LOWER before comparing user input to fixed words.

Key terminology

• String: an ordered sequence of characters.
• Length: the number of characters in a string.
• Substring: part of a string selected by position and length.
• Upper/Lower case: operations that convert letters to all capitals or all small letters.
• Index: a position used to access characters in a string. May start at 0 or 1 depending on the language or notation.