Students should know the advantages and disadvantages of both algorithms.

The idea

Learners must be able to judge when each search suits a situation. Linear search advantages: it works on unordered data, needs no pre-processing, and is simple to implement; it is fine for small lists. Disadvantages: on large lists it may examine many or all items, so it can be slow. Binary search advantages: because it halves the remaining search space each step, it is fast on large sorted data. Disadvantages: the data must be sorted first (which itself costs time), and it only works where items can be ordered and compared. A multiple-choice question may ask which statement is an advantage or disadvantage, or which algorithm suits a given scenario such as a small unsorted list. Decide by checking the data requirement and the likely number of comparisons.

What earns the marks

• Linear search advantage: works on unordered data without prior sorting.
• Linear search disadvantage: may need to examine every item, so is slow on large lists.
• Binary search advantage: halves the search space each step, so is efficient on large sorted data.
• Binary search disadvantage: requires the data to be sorted before searching.
• Selects the algorithm suited to a scenario by weighing list size and whether data is sorted.

Common mistakes and how to put them right

• Choosing binary search for an unsorted list.

  Put it right by recognising it requires sorted data.

• Calling linear search inefficient in all cases.

  Put it right by noting it suits small or unsorted lists.

• Ignoring the cost of sorting before a binary search.

  Put it right by counting sorting as a disadvantage.

• Confusing an advantage with a disadvantage.

  Put it right by checking whether the statement helps or hinders the search.

Exam tips

• Read the scenario for two clues: list size and whether the data is sorted.
• Match each option to a specific advantage or disadvantage rather than a vague impression.
• Eliminate options that ignore the sorting requirement of binary search.