Python | Find the closest Key in dictionary

The search of keys in dictionary in python has been discussed many times. But sometimes, we may have a problem in which we require to fetch the key which is the nearest one of the given keys. Let’s discuss certain ways in which this task can be performed. Method #1 : Using list comprehension + keys() + lambda The combination of above functions can be used to perform the particular task of finding the closest key in the dictionary. The keys function can be used to access the keys from the dictionary, lambda function can be used to formulate the logic and list comprehension to apply that all to whole list. 

Python3

# Python3 code to demonstrate working of
# Closest key in dictionary
# Using list comprehension + keys() + lambda

# initializing dictionary
test_dict = {13 : 'Hi', 15 : 'Hello',  16 : 'Gfg'}

# initializing nearest key
search_key = 15.6

# printing original dictionary
print("The original dictionary is : " + str(test_dict))

# Using list comprehension + keys() + lambda
# Closest key in dictionary
res = test_dict.get(search_key) or test_dict[
      min(test_dict.keys(), key = lambda key: abs(key-search_key))]

# printing result
print("The value to the closest key : " + str(res))

Time Complexity: O(n)
Space Complexity: O(1)

Output : 

The original dictionary is : {16: 'Gfg', 13: 'Hi', 15: 'Hello'}
The value to the closest key : Gfg

Method #2 : Using bisect_left() + OrderedDict() This method generally uses the binary search method of finding the nearest number. While being fast, it changes the ordering and also returns 2 potential candidates for nearest values, current and the next key’s value in sequence. And just returns position of key. 

Python3

# Python3 code to demonstrate working of
# Closest key in dictionary
# Using bisect_left() + OrderedDict()
import collections
import bisect

# initializing dictionary
test_dict = collections.OrderedDict()
test_dict = {13 : 'Hi', 15 : 'Hello',  16 : 'Gfg'}

# initializing nearest key
search_key = 15.6

# printing original dictionary
print("The original dictionary is : " + str(test_dict))

# Using bisect_left() + OrderedDict()
# Closest key in dictionary
res = bisect.bisect_left(list(test_dict.keys()), 15.6)

# printing result
print("The position of closest key : " + str(res))

Time Complexity: O(log n)

Space Complexity: O(n)

Output : 

The original dictionary is : {16: 'Gfg', 13: 'Hi', 15: 'Hello'}
The position of closest key : 3