Python/searches/binary_search.py

163 lines
4.5 KiB
Python
Raw Normal View History

2016-07-29 13:03:20 +00:00
"""
This is pure python implementation of binary search algorithm
For doctests run following command:
python -m doctest -v binary_search.py
2016-07-29 13:03:20 +00:00
or
python3 -m doctest -v binary_search.py
2016-07-29 13:03:20 +00:00
For manual testing run:
python binary_search.py
"""
from __future__ import print_function
2016-07-29 14:12:51 +00:00
import bisect
2016-07-29 13:03:20 +00:00
def binary_search(sorted_collection, item):
2016-07-29 14:12:51 +00:00
"""Pure implementation of binary search algorithm in Python
2016-07-29 13:03:20 +00:00
Be careful collection must be sorted, otherwise result will be
unpredictable
2016-07-29 13:03:20 +00:00
: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:
2016-07-29 13:03:20 +00:00
if item < current_item:
right = midpoint - 1
else:
2016-07-29 13:03:20 +00:00
left = midpoint + 1
return None
2016-07-29 14:12:51 +00:00
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
2016-07-29 14:12:51 +00:00
: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
2017-07-05 08:40:18 +00:00
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)
"""
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)
2017-07-05 08:40:18 +00:00
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
2016-07-29 13:03:20 +00:00
if __name__ == '__main__':
import sys
2017-10-07 13:47:50 +00:00
# For python 2.x and 3.x compatibility: 3.x has no raw_input builtin
2016-07-29 13:03:20 +00:00
# otherwise 2.x's input builtin function is too "smart"
if sys.version_info.major < 3:
input_function = raw_input
else:
input_function = input
user_input = input_function('Enter numbers separated by coma:\n')
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')
2016-07-29 13:03:20 +00:00
target_input = input_function(
'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')