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Binary search is a faster method for searching for an item that is in an ordered list.

An ordered list is one where the sequence of items in the list is important. An ordered list does not necessarily contain a sequence of numbers (eg 1, 2, 3, 4) or characters (eg A, B, C, D). It might also contain, eg a list of names in alphabetical order, a list of files from smallest to largest or a list of records from earliest to most recent.

Example

Imagine that you have a database of customers and want to search for the customer John Smith. We first need the database to be ordered into alphabetical order by surname. We then search for the record ‘John Smith’ by surname.

The binary search will split the database in half, and compare the midpoint (the middle name) to the name ‘John Smith’. It will see whether the midpoint comes before or after ‘Smith’ and will throw away the set of records that doesn’t contain ‘Smith’ as a surname. It will keep dividing the records in that way until it reaches two records left, one of which is ‘John Smith’. It will then throw away the other record and output John Smith’s details. Binary search effectively divides the data in half and throws away, or ‘bins’ the half that does not contain the search term.

In pseudocode this would look like:

OUTPUT "Which customer do you want to find?"
INPUT user inputs John Smith
STORE the user's input in the customer_name variable
customer_found = False
(we need to create a flag that identifies if the customer is found)
WHILE customer_found = False:
	Find midpoint of list
	IF customer_name = record at midpoint of list THEN
		customer_found = True
	ELSE IF customer comes before the midpoint THEN
		throw away the second half of the list
	ELSE 
		throw away the first part of the list
OUTPUT customer details

As a flowchart, this would look like: