String Manipulation

Working with text

Programs constantly handle text — names, passwords, messages. String manipulation means examining and changing that text. A string is a sequence of characters, each with a position (index), usually starting at 0.

For the string "HELLO": H=0, E=1, L=2, L=3, O=4.

Length

length() gives the number of characters in a string.

• length("HELLO") = 5.
• Spaces and punctuation count too: length("Hi!") = 3.

Getting part of a string (substrings)

You can pull out a section of a string. Exact syntax varies by board/language, but the idea is the same:

• A single character by index: "HELLO"[1] → "E".
• A substring using a start position and length or a range, e.g. substring("HELLO", 1, 3) → "ELL" (3 characters from index 1).

Joining strings (concatenation)

Concatenation joins strings together, usually with +:

first = "Ada"
last = "Lovelace"
full = first + " " + last   // "Ada Lovelace"

Changing case

• upper() / toUpper() converts to CAPITALS: upper("hi") → "HI".
• lower() / toLower() converts to lower case: lower("Hi") → "hi".
• Useful for comparisons (so "YES", "Yes" and "yes" all match).

Converting between types (casting)

Numbers typed in are often read as strings and must be converted before doing maths:

• int("25") → the integer 25 (now you can add to it).
• str(25) → the string "25" (now you can join it to text).
• float("3.5") → the real number 3.5.

Trying to do "25" + 3 may join text or cause an error — convert first.

Worked example

Given word = "Computer", what do these give?

1. length(word) → 8.

2. word[0] → "C".

3. upper(word) → "COMPUTER". ✓

Common mistakes

• Forgetting strings are usually 0-indexed (the first character is index 0).
• Adding a string and a number without converting — causes errors or joins them as text.
• Forgetting spaces/punctuation are counted by length().

Exam tips

• Learn the common operations: length, substring/index, concatenation, upper/lower, and casting (int/str).
• Watch indexing carefully in trace questions — start at 0.
• If input maths goes wrong, suspect a string that needs converting to a number.

Key facts to remember

• Strings are indexed from 0; length() counts every character including spaces.
• Concatenation (+) joins strings; substring/index extracts parts; upper/lower change case.
• Casting converts between strings and numbers (int(), str(), float()) — essential before maths on input.