# Basic binary search in python

I just started learning to code during quarantine and whilst learning python, I created this program for a binary search. This is really basic so I assume that there is a way easier method. If anyone knows how I can make this way simpler, your help would be appreciated.

list = [1,15,37,53,29,22,31,90,14,6,37,40]
finished = False
target = 37
list.sort()
print(list)
while finished == False:
n = len(list)
print(n)
if target < list[int(n/2)]:
for items in range(int(n / 2), n):
list.pop()
print('Item is too large.')
elif target == list[int(n/2)]:
finished = True
print('The item has been found.')
else:
list.reverse()
for items in range(int(n / 2), n):
list.pop()
list.reverse()
print('Item is too small.')
print(list)

• i'm no expert but have studied computer science. You are going to keep reversing the entire list? seriously? how can that possibly be efficient. No binary search i've seen even reverses it once. And you should know there are already binary search algorithms written, you can look up any of them and none of them do anything like what you are doing. Binary search is a fairly standard algorithm, not something you make up as you go along May 15, 2020 at 0:26
• bisect is what I'd imagine reading Basic binary search: Please introduce your code with a description what it is to achieve, and how the implementation is a basic binary search. May 15, 2020 at 1:53

• Avoid using list as the variable name, because it is a function in Python.
• You don't need to manipulate the list, such as list.pop() and list.reverse(). It is inefficient. You can determine the updated search range with index.
• When target is assigned a value not within list, there will be IndexError: list index out of range. It means you didn't handle the case well.

Modified code:

search_list = [1,15,37,53,29,22,31,90,14,6,37,40]
target = 37
start = 0
end = len(search_list)-1
search_list.sort()
print('The sorted list:',search_list)
while start<=end:
n = int((end + start)/2)
if target < search_list[n]: # the target is small
end = n-1
elif target > search_list[n]: # the target is large
start = n+1
elif target == search_list[n]: # the target is found
print('The item {} has been found at the index {} in the sorted list.'.format(target,n))
break
else:


In addition to Chris Tang's answer, I strongly suggest that you encapsulate your binary search algorithm in a function.

Currently, your code only works for the list defined at the beginning of the program. But what if you want to reuse it with another list later on? Or reuse this snippet in your next program? Encapsulating your code in a function would allow you to do that much more easily.

I understand that, as a beginner, you just wanted to see if you could implement that algorithm, and don't actually reuse it, but it is a good habit to take on early. Including docstrings to document your code to document what it does is also a good things.

Here is a slightly modified version of Chris Tang's code, encapsulating the logic:

def binary_search(search_list, target):
"""Search a sorted list for the target value
Returns the target's index, or None if the target is not in the list"""

start = 0
end = len(search_list) - 1

while start<=end:
n = int((end + start) / 2)
if target < search_list[n]: # the target is small
end = n-1
elif target > search_list[n]: # the target is large
start = n+1
else: # the target is found
return n
return None

if __name__ == '__main__':
my_list = [1,15,37,53,29,22,31,90,14,6,37,40]
my_list.sort()
target = 37
target_index = binary_search(my_list, target)
if target is not None:
print(f'{target} was found at index {target_index}')
else:
print(f'{target} was not found in the list')


Some other remarks about my code:

• The function returns a value, instead of printing it. This means, should you reuse the function in some other code, you can decide what to do with the value.
• I include the execution code in a if __name__ == '__main__':. This means that, should you import your script in another piece of code, it will not run the code included in that if statement, but it will be executed if you run your script directly. You will probably never include this particular script, but I believe this is a good habit to take.
• I use f-strings instead of string.format() as it works mostly the same and I believe it is more readable. It is a matter of preference, I suppose.