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Binary Search

A binary search algorithm takes the data and keeps dividing it in half until it finds the item it is looking for.

How to code a Binary search - step by step

A step by step explanation of how to code a Binary search in python using a relevant example: searching a friend list (e.g. facebook) for a friend!

Challenge

#1 Write a Python program to search using the binary search method

#2 Comment each line of the python solution below to show your understanding of the algorithm

Solution

```def binary_search(item_list,item):
first = 0
last = len(item_list)-1
found = False
mid = (first + last)//2
if item_list[mid] == item :
found = True
else:
if item < item_list[mid]:
last = mid - 1
else:
first = mid + 1
return found

print(binary_search([1,2,3,5,8], 6))
print(binary_search([1,2,3,5,8], 5))
```

Explanation with 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. (Source: BBC Bitesize)

Above example represented as a flow chart

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```

General Flow Chart

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