mirror of
https://github.com/TheAlgorithms/Python.git
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130 lines
3.4 KiB
Python
130 lines
3.4 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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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 __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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# For python 2.x and 3.x compatibility: 3.x has not raw_input builtin
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# otherwise 2.x's input builtin function is too "smart"
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if sys.version_info.major < 3:
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input_function = raw_input
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else:
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input_function = input
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user_input = input_function('Enter numbers separated by coma:\n')
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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 = input_function(
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'Enter a single number to be found in the list:\n'
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)
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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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