String handling operations in a programming language: position

The idea

This row names one string handling operation in AQA 8525 section 3.2.8: finding the position of a character or substring within a string. The skill is to know that a position (often called an index) locates where a piece of text occurs, and to use the operation your language provides. In Python, "computer".find("put") returns 3, because counting starts at 0: c is 0, o is 1, m is 2, p is 3. Many languages count from 0, but some, such as those using 1-based indexing, start at 1, so always check your language. If the substring is absent, the operation usually returns a sentinel such as -1 in Python. A typical use is checking whether a word appears in a sentence before acting on it. Practise predicting the returned value, then confirm it by running the code.

What earns the marks

• States that the position operation locates a character or substring within a string and returns an index.
• Recognises that indexing commonly starts at 0 in languages such as Python, so the first character is at position 0.
• Applies the operation correctly to a given string, for example finding the position of a substring and returning the expected index.
• Explains that a not-found result is signalled by a special value, such as -1 in Python, rather than an error.
• Notes that indexing conventions differ between languages, so the language in the question must be checked.

Common mistakes and how to put them right

• Counting the first character as position 1 in a 0-based language.

  Put it right: Start counting at 0 and re-check the returned index.

• Confusing the position of a substring with its length.

  Put it right: Separate the two operations and test each one.

• Assuming a missing substring causes a crash.

  Put it right: Test a search for text that is absent and observe the sentinel value returned.

• Forgetting that spaces count as characters.

  Put it right: Count every character, including spaces, when predicting an index.

Exam tips

• Write out the string with an index number under each character before you answer, so you do not miscount.
• Check whether the question's language uses 0-based or 1-based indexing before giving a position.
• If a substring is not present, state the value the operation returns in that language rather than saying it errors.