mirror of
https://github.com/TheAlgorithms/Python.git
synced 2024-12-25 12:40:14 +00:00
53681f199c
In binary_search_by_recursion, if you search array for a value that does not exist, you will get this error:
RecursionError: maximum recursion depth exceeded in comparison
To fix this, first check to make sure that the value is between left and right points like this:
if (right < left):
return None
A similar construct has already been applied to binary_search, but did not exist in the recursive alternative.
162 lines
4.4 KiB
Python
162 lines
4.4 KiB
Python
"""
|
|
This is pure python implementation of binary search algorithm
|
|
|
|
For doctests run following command:
|
|
python -m doctest -v binary_search.py
|
|
or
|
|
python3 -m doctest -v binary_search.py
|
|
|
|
For manual testing run:
|
|
python binary_search.py
|
|
"""
|
|
from __future__ import print_function
|
|
import bisect
|
|
|
|
try:
|
|
raw_input # Python 2
|
|
except NameError:
|
|
raw_input = input # Python 3
|
|
|
|
|
|
def binary_search(sorted_collection, item):
|
|
"""Pure implementation of binary search algorithm in Python
|
|
|
|
Be careful collection must be sorted, otherwise result will be
|
|
unpredictable
|
|
|
|
:param sorted_collection: some sorted collection with comparable items
|
|
:param item: item value to search
|
|
:return: index of found item or None if item is not found
|
|
|
|
Examples:
|
|
>>> binary_search([0, 5, 7, 10, 15], 0)
|
|
0
|
|
|
|
>>> binary_search([0, 5, 7, 10, 15], 15)
|
|
4
|
|
|
|
>>> binary_search([0, 5, 7, 10, 15], 5)
|
|
1
|
|
|
|
>>> binary_search([0, 5, 7, 10, 15], 6)
|
|
|
|
"""
|
|
left = 0
|
|
right = len(sorted_collection) - 1
|
|
|
|
while left <= right:
|
|
midpoint = (left + right) // 2
|
|
current_item = sorted_collection[midpoint]
|
|
if current_item == item:
|
|
return midpoint
|
|
else:
|
|
if item < current_item:
|
|
right = midpoint - 1
|
|
else:
|
|
left = midpoint + 1
|
|
return None
|
|
|
|
|
|
def binary_search_std_lib(sorted_collection, item):
|
|
"""Pure implementation of binary search algorithm in Python using stdlib
|
|
|
|
Be careful collection must be sorted, otherwise result will be
|
|
unpredictable
|
|
|
|
:param sorted_collection: some sorted collection with comparable items
|
|
:param item: item value to search
|
|
:return: index of found item or None if item is not found
|
|
|
|
Examples:
|
|
>>> binary_search_std_lib([0, 5, 7, 10, 15], 0)
|
|
0
|
|
|
|
>>> binary_search_std_lib([0, 5, 7, 10, 15], 15)
|
|
4
|
|
|
|
>>> binary_search_std_lib([0, 5, 7, 10, 15], 5)
|
|
1
|
|
|
|
>>> binary_search_std_lib([0, 5, 7, 10, 15], 6)
|
|
|
|
"""
|
|
index = bisect.bisect_left(sorted_collection, item)
|
|
if index != len(sorted_collection) and sorted_collection[index] == item:
|
|
return index
|
|
return None
|
|
|
|
def binary_search_by_recursion(sorted_collection, item, left, right):
|
|
|
|
"""Pure implementation of binary search algorithm in Python by recursion
|
|
|
|
Be careful collection must be sorted, otherwise result will be
|
|
unpredictable
|
|
First recursion should be started with left=0 and right=(len(sorted_collection)-1)
|
|
|
|
:param sorted_collection: some sorted collection with comparable items
|
|
:param item: item value to search
|
|
:return: index of found item or None if item is not found
|
|
|
|
Examples:
|
|
>>> binary_search_std_lib([0, 5, 7, 10, 15], 0)
|
|
0
|
|
|
|
>>> binary_search_std_lib([0, 5, 7, 10, 15], 15)
|
|
4
|
|
|
|
>>> binary_search_std_lib([0, 5, 7, 10, 15], 5)
|
|
1
|
|
|
|
>>> binary_search_std_lib([0, 5, 7, 10, 15], 6)
|
|
|
|
"""
|
|
if (right < left):
|
|
return None
|
|
|
|
midpoint = left + (right - left) // 2
|
|
|
|
if sorted_collection[midpoint] == item:
|
|
return midpoint
|
|
elif sorted_collection[midpoint] > item:
|
|
return binary_search_by_recursion(sorted_collection, item, left, midpoint-1)
|
|
else:
|
|
return binary_search_by_recursion(sorted_collection, item, midpoint+1, right)
|
|
|
|
def __assert_sorted(collection):
|
|
"""Check if collection is sorted, if not - raises :py:class:`ValueError`
|
|
|
|
:param collection: collection
|
|
:return: True if collection is sorted
|
|
:raise: :py:class:`ValueError` if collection is not sorted
|
|
|
|
Examples:
|
|
>>> __assert_sorted([0, 1, 2, 4])
|
|
True
|
|
|
|
>>> __assert_sorted([10, -1, 5])
|
|
Traceback (most recent call last):
|
|
...
|
|
ValueError: Collection must be sorted
|
|
"""
|
|
if collection != sorted(collection):
|
|
raise ValueError('Collection must be sorted')
|
|
return True
|
|
|
|
|
|
if __name__ == '__main__':
|
|
import sys
|
|
user_input = raw_input('Enter numbers separated by comma:\n').strip()
|
|
collection = [int(item) for item in user_input.split(',')]
|
|
try:
|
|
__assert_sorted(collection)
|
|
except ValueError:
|
|
sys.exit('Sequence must be sorted to apply binary search')
|
|
|
|
target_input = raw_input('Enter a single number to be found in the list:\n')
|
|
target = int(target_input)
|
|
result = binary_search(collection, target)
|
|
if result is not None:
|
|
print('{} found at positions: {}'.format(target, result))
|
|
else:
|
|
print('Not found')
|