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String Manipulation

FoundationHigherAQA

Revise String Manipulation for GCSE Computer Science with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA GCSE Computer Science (8525). String manipulation extracts, joins and changes parts of text, such as length, substrings and case.

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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA specification. Every worksheet comes with a full mark scheme.

Foundation

WorksheetFoundation · PDFMark SchemeFoundation · PDF

Higher

WorksheetHigher · PDFMark SchemeHigher · PDF

Topic overview

String manipulation extracts, combines and examines parts of text. Several operations must be known.

Length gives the number of characters. Substring extracts part of a string, given a starting position and a length. Concatenation joins strings together. Position finds where a character or substring occurs. Conversion changes case, or converts between a string and a number.

The detail that causes most errors is indexing. String positions usually start at 0, not 1, so the first character of "HELLO" is at position 0 and the last is at position 4. Assuming positions start at 1 gives an answer one out every time.

Revision notes

The main operations

LEN(string) gives the number of characters.

SUBSTRING extracts part of a string from a starting position for a given length. Concatenation joins strings, often with +. POSITION finds where a character occurs.

Zero-based indexing

String positions usually start at 0, not 1.

In "HELLO", H is at position 0, E at 1, and O at 4. The length is 5 but the last position is 4. Assuming positions start at 1 produces an off-by-one error every time.

Type conversion

Converting between strings and numbers is often needed.

A number entered by a user arrives as a string and must be converted before arithmetic. Converting the other way allows a number to be joined into a message with concatenation.

Key points

  • LEN gives the number of characters.
  • SUBSTRING extracts part of a string.
  • Concatenation joins strings together.
  • String positions usually start at 0.
  • The last position is the length minus 1.
  • Numbers from input arrive as strings.

Worked examples

Example 1

State the length of the string "COMPUTER" and the position of the first character. [2 marks]

Working

The length is 8, as there are 8 characterscount the characters
The first character C is at position 0, because indexing starts at 0state the starting position

Example 2

Explain why the last character of a string of length 6 is at position 5. [2 marks]

Working

Positions start at 0 rather than 1state the indexing rule
so positions run from 0 to 5, making the last position one less than the lengthexplain the consequence

Example 3

A user enters a number which is stored as a string. Explain what must happen before it is used in a calculation. [2 marks]

Working

The string must be converted to a numeric data type such as an integerstate what is needed
otherwise the + operator would concatenate the values rather than adding themexplain why

Common mistakes

  • Assuming positions start at 1.

    They usually start at 0, making the last position length minus 1.

  • Forgetting to convert input before arithmetic.

    Input arrives as a string and would be concatenated, not added.

  • Confusing length with last position.

    Length is one more than the last position.

  • Miscounting substring length.

    Substring takes a start position and a number of characters.

Exam tips

  • Remember indexing starts at 0.
  • The last position is the length minus 1.
  • Convert input before doing arithmetic.
  • Count characters carefully when finding a length.

Key terms

String
A sequence of characters.
Substring
Part of a string extracted from it.
Concatenation
Joining strings together.
Index
The position of a character, starting at 0.

Related topics

Written and reviewed against the current AQA specification. Spotted an error? Let us know.