# Binary Search Algorithm in Python

Today, we will learn a very fast searching algorithm – the binary search algorithm in Python. We will see its logic, how to write it in Python and what makes it so fast.

## The Binary Search Algorithm

There is one thing to note before starting, the algorithm requires that the given list should be sorted. This is because we can find if a number is after or before a certain another number in a list based on the list’s sorting.

Recall how we find words in a dictionary or page numbers in a book. We simply go to a point in the sequence and check if what we need to find is after or before that point, we make guesses like this until we have found the item.

Similarly, in Binary Search, we start by looking at the centre of the list. Either we will find the item there, in which case the algorithm is over, or we will know whether the item is after or before the middle item based on how the list is sorted.

After this, we will simply ignore the half which is not supposed to have the item we need. And we repeat this process by going to the middle of the other half.

Eventually, we will either find the item or there will be no more halves to eliminate, which will end the algorithm either successfully or unsuccessfully.

Notice that we are dividing the list into two halves and then eliminating one half, because of this behaviour of the algorithm, it is aptly named Binary Search.

Merriam-Webster Dictionary’s meaning of “Binary”: a division into two groups or classes that are considered diametrically opposite.

Recommended read: Binary search tree algorithm in Python

## Theoretical Example of the Binary Search Algorithm

Let us take an example to understand it better:

Given List: 11, 23, 36, 47, 51, 66, 73, 83, 92
To find: 23

• The list has 9 items, so the center one must be in position 5, which is 51.
• 51 is not equal to 23, but it is more than 23. So if 23 is there in the list, it has to be before 51. So we eliminate 51 and all items after it.
• Remaining List: 11, 23, 36, 47
• Now we have 4 items in the list, and depending on how you calculate the center index, it will either tell us that 2 is the center position or 3 is the center position.
• For simplicity, we will calculate the mean of the start and end positions to get the center.
• Here, start = 1 and end = 4, so the mean is 2 (integer part of 2.5).
• So, in position 2, we have 23, which is the item we needed to find. And the algorithm will end and give us the target’s position.

Now let us see how the binary search algorithm is coded in Python.

## Binary Search in Python

def binary_search(lst, target):
start = 0
end = len(lst) - 1
while(start <= end):
mid = (start + end) // 2
if(lst[mid] > target):
end = mid - 1
elif(lst[mid] < target):
start = mid + 1
else:
return mid
return None

Let us go through the algorithm,

• We create a function that takes two arguments, the first one is the list and the second one is the target that we need to find.
• We declare two variables start and end that point to the start (0) and end (length – 1) of the list respectively.
• These two variables are responsible for eliminating items from the search because the algorithm won’t consider items outside this range.
• The next loop will continue to find and eliminate items as long as the start is less than or equal to the end because the only way the start becomes greater than the end is if the item is not on the list.
• Inside the loop, we find the integer value of the mean of start and end, and consider that as the middle item of the list.

Now, if the middle item is more than the target, it means that the target can only be present before the middle item. So we set the end of the list as the index before the middle, this way, all indexes after mid, including mid, are eliminated from consideration.

Similarly, if the middle item is less than the target, it means that the target can only be present after the middle item, and in order to eliminate the index mid and all indexes before mid, we set the start variable as the index after mid.

If none of the above two cases is true, i.e. if the item at the middle is neither greater nor smaller than the target, then it must be the target. So we simply return the index of this middle item and end the algorithm.

If the loop finishes, then that means that the target was not found, this means that the target was not in the list and the function simply returns None.

Let us see the code perform and check its output.

The Output

We can see that 23 was present in the list numbers, so the function returned its index, which is 2, but 70 was not present in the list, and so the function returned None.

## What Makes Binary Search Fast?

Consider a simple searching algorithm like linear search where we have to go through each item till we find what we are looking for. This means that for larger input sizes, the time it takes to find an item increases by as much amount as the input size increases. Quantifiably, its time complexity is O(n).

Time complexity is a way of quantifying how fast or efficient an algorithm is. In the case of Binary Search, its time complexity is “O(log2n)“, which means that if we double the size of the input list, the algorithm will perform just one extra iteration.

Similarly, if the input size is multiplied by a thousand, then the loop will just have to run 10 more times.

Recall that in every iteration, half of the list is eliminated, so it doesn’t take very long to eliminate the entire list.

## Conclusion

In this tutorial, we studied what Binary Search is, how it got its name, what it exactly does to find items, and how is it so fast. We discussed its efficiency in terms of time complexity, and we saw how to code it in Python.

Binary Search is one of many search algorithms, and it is one of the fastest. I hope you enjoyed learning about Binary Search and see you in the next tutorial.