String handling operations in a programming language: string to integer

The idea

Converting a string to an integer takes text that represents a whole number and turns it into an integer value the program can calculate with. For example, int("27") in Python produces the integer 27, which can then be added, subtracted or compared numerically. This matters because input read from a user is usually text, so a program must convert it before doing arithmetic. Learners should recognise when this conversion is needed, predict the resulting type, and understand that the text must contain valid digits; text such as "hello" cannot be converted to an integer and would cause an error.

What earns the marks

• Identifies that converting a string to an integer produces a whole-number value that can be used in arithmetic.
• Recognises that user input is typically text and must be converted before numeric calculation.
• Predicts the data type produced, namely an integer rather than a string.
• Explains that the string must contain valid digit characters, otherwise the conversion fails with an error.
• Distinguishes string-to-integer conversion from string-to-real conversion for decimal values.

Common mistakes and how to put them right

• Adding digit text directly without conversion.

  Put it right: Is that the text must first be converted to an integer so arithmetic is performed on numbers.

• Expecting non-numeric text to convert successfully.

  Put it right: Is that only valid digit strings convert, and invalid text raises an error.

• Confusing integer conversion with real conversion.

  Put it right: Is that integers are whole numbers while reals allow decimal parts.

Exam tips

• Identify whether the value needs to be a whole number or may include decimals before choosing integer or real conversion.
• Remember that input is usually text, so convert before any arithmetic.
• Check that the string contains only valid digits when predicting whether a conversion succeeds.