String handling operations in a programming language: substring

The idea

A substring is a contiguous run of characters taken from a longer string, and extracting one is a core string-handling operation. In most languages you give a start position and a length, or a start and end position, and the language returns the characters in that range. Positions are usually counted from 0 in languages such as Python and from 1 in others, so the same request can select different characters. For example, in Python the expression word[2:5] on the string "COMPUTER" returns "MPU", because index 2 is the third character and the end index is excluded. Learners should be able to read a short code fragment, predict the substring produced, and choose the correct call for a stated goal.

What earns the marks

• Defines a substring as a contiguous sequence of characters taken from within a longer string, preserving their order.
• Identifies that extraction needs a start position and either a length or an end position.
• Recognises that indexing may start at 0 or 1 depending on the language, which changes the characters selected.
• Predicts the exact characters returned by a given substring operation on a supplied string.
• Selects the correct substring call that produces a stated target fragment.

Common mistakes and how to put them right

• Assuming every language counts the first character as position 1.

  Put it right: Check the language, since Python-style slicing starts at 0.

• Treating the end index as inclusive in Python slicing.

  Put it right: The end index is excluded, so word[2:5] gives three characters.

• Thinking a substring can skip characters or reorder them.

  Put it right: A substring is contiguous and keeps the original order.

Exam tips

• Write out the string with index numbers above each character before answering.
• Count characters carefully and check whether the first index is 0 or 1 in the code shown.
• Test your chosen option against the exact string in the question rather than a remembered example.