The binary search is an excellent algorithm to find an item in a *sorted* list, being **O(Log n)** it is very efficient. This is an implementation in Python:

```
def binary_search(items, required_item):
low = 0
high = len(items) - 1
while low <= high:
index = (low + high) // 2
item = items[index]
if item == required_item:
return index
if item > required_item:
high = index - 1
else:
low = index + 1
```

Essentially it works like this:

- start with low set to the first item, high to the last, index to the middle
- if the item at that position is a match, we are done, return the index
- if the item is greater, half the list by ignoring the current and following items
- otherwise half the list by ignoring the current and previous items
- repeat until there are no more items to check

For example, consider this :

```
animals = ["cat", "dog", "elephant", "lion", "sparrow", "tiger", "wombat"]
print('index of tiger : ', binary_search(animals, "tiger") or "not found")
print('index of dog : ', binary_search(animals, "dog") or "not found")
print('index of turtle: ', binary_search(animals, "turtle") or "not found")
#
```**outputs**:
index of tiger : 5
index of dog : 1
index of turtle: not found

These are the steps involved in finding *tiger*:

- Our initial search range is between the very first and last items, hence
**low**is**0**and**high**is**6**. We calculate the**middle**to be**3**, so we set the**index**accordingly.

**Initial Step**

0 cat1 dog 2 elephant 3 lion<-- low4 sparrow 5 tiger 6 wombat<-- index<-- high

*lion*is less than*tiger*, so we now set**low**to**index + 1**, thus excluding lion and the items before it from our next search. We calculate the new**middle**to be**5**, and set the**index**.

**Next Step**

0 cat 1 dog 2 elephant 3 lion 4 sparrow<5 tiger-- low6 wombat<-- index<-- high

- tiger is indeed what we are looking for, so we have finished our search.

That’s the binary search in a nutshell.