mirror of
https://github.com/TheAlgorithms/Python.git
synced 2024-11-24 05:21:09 +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
"""
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This is pure python implementation of binary search algorithm
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For doctests run following command:
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python -m doctest -v binary_search.py
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or
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python3 -m doctest -v binary_search.py
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For manual testing run:
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python binary_search.py
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"""
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from __future__ import print_function
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import bisect
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try:
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raw_input # Python 2
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except NameError:
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raw_input = input # Python 3
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def binary_search(sorted_collection, item):
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"""Pure implementation of binary search algorithm in Python
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Be careful collection must be sorted, otherwise result will be
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unpredictable
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:param sorted_collection: some sorted collection with comparable items
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:param item: item value to search
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:return: index of found item or None if item is not found
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Examples:
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>>> binary_search([0, 5, 7, 10, 15], 0)
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0
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>>> binary_search([0, 5, 7, 10, 15], 15)
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4
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>>> binary_search([0, 5, 7, 10, 15], 5)
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1
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>>> binary_search([0, 5, 7, 10, 15], 6)
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"""
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left = 0
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right = len(sorted_collection) - 1
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while left <= right:
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midpoint = (left + right) // 2
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current_item = sorted_collection[midpoint]
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if current_item == item:
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return midpoint
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else:
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if item < current_item:
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right = midpoint - 1
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else:
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left = midpoint + 1
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return None
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def binary_search_std_lib(sorted_collection, item):
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"""Pure implementation of binary search algorithm in Python using stdlib
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Be careful collection must be sorted, otherwise result will be
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unpredictable
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:param sorted_collection: some sorted collection with comparable items
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:param item: item value to search
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:return: index of found item or None if item is not found
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Examples:
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>>> binary_search_std_lib([0, 5, 7, 10, 15], 0)
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0
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>>> binary_search_std_lib([0, 5, 7, 10, 15], 15)
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4
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>>> binary_search_std_lib([0, 5, 7, 10, 15], 5)
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1
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>>> binary_search_std_lib([0, 5, 7, 10, 15], 6)
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"""
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index = bisect.bisect_left(sorted_collection, item)
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if index != len(sorted_collection) and sorted_collection[index] == item:
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return index
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return None
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def binary_search_by_recursion(sorted_collection, item, left, right):
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"""Pure implementation of binary search algorithm in Python by recursion
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Be careful collection must be sorted, otherwise result will be
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unpredictable
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First recursion should be started with left=0 and right=(len(sorted_collection)-1)
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:param sorted_collection: some sorted collection with comparable items
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:param item: item value to search
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:return: index of found item or None if item is not found
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Examples:
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>>> binary_search_std_lib([0, 5, 7, 10, 15], 0)
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0
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>>> binary_search_std_lib([0, 5, 7, 10, 15], 15)
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4
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>>> binary_search_std_lib([0, 5, 7, 10, 15], 5)
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1
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>>> binary_search_std_lib([0, 5, 7, 10, 15], 6)
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"""
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if (right < left):
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return None
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midpoint = left + (right - left) // 2
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if sorted_collection[midpoint] == item:
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return midpoint
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elif sorted_collection[midpoint] > item:
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return binary_search_by_recursion(sorted_collection, item, left, midpoint-1)
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else:
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return binary_search_by_recursion(sorted_collection, item, midpoint+1, right)
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def __assert_sorted(collection):
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"""Check if collection is sorted, if not - raises :py:class:`ValueError`
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:param collection: collection
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:return: True if collection is sorted
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:raise: :py:class:`ValueError` if collection is not sorted
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Examples:
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>>> __assert_sorted([0, 1, 2, 4])
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True
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>>> __assert_sorted([10, -1, 5])
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Traceback (most recent call last):
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...
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ValueError: Collection must be sorted
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"""
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if collection != sorted(collection):
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raise ValueError('Collection must be sorted')
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return True
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if __name__ == '__main__':
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import sys
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user_input = raw_input('Enter numbers separated by comma:\n').strip()
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collection = [int(item) for item in user_input.split(',')]
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try:
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__assert_sorted(collection)
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except ValueError:
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sys.exit('Sequence must be sorted to apply binary search')
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target_input = raw_input('Enter a single number to be found in the list:\n')
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target = int(target_input)
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result = binary_search(collection, target)
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if result is not None:
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print('{} found at positions: {}'.format(target, result))
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else:
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print('Not found')
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