Understand and explain how the linear search algorithm works.

The idea

A linear search checks items one at a time, in order, from the start of the list until the target is found or the list ends. To explain it, describe the method: examine the first item and compare it with the target; if it matches, stop and report the position; if not, move to the next item and repeat. If the end of the list is reached without a match, report that the target is not present. For example, searching for 7 in the list 3, 9, 7, 1 compares 3 (no match), then 9 (no match), then 7 (match at the third position) and stops. It works on unsorted as well as sorted data, and in the worst case it examines every item.

What earns the marks

• States that a linear search examines items one at a time, in sequence, starting from the first item.
• Describes comparing each item with the target value and moving to the next item when there is no match.
• States that the search stops and reports the position when a match is found.
• States that if the end of the list is reached with no match, the target is reported as not found.
• Recognises that a linear search can be used on unsorted data and may need to check every item in the worst case.

Common mistakes and how to put them right

• Claiming a linear search requires the data to be sorted first.

  Put it right: It works on unsorted data because it simply checks items in order.

• Saying the search continues through the whole list even after a match is found.

  Put it right: It stops as soon as the target is located and reports that position.

• Confusing linear search with binary search by describing repeated halving of the list.

  Put it right: Linear search moves through items one at a time and does not split the list.

Exam tips

• Use a short worked example list and state each comparison in order, showing where the search stops.
• Include both outcomes: a successful search that reports a position, and an unsuccessful search that reports the target is absent.
• Use precise wording such as 'compares each item in turn with the target' rather than vague phrases like 'looks through the list'.