Key Index in Dictionary - Python

We are given a dictionary and a specific key, our task is to find the index of this key when the dictionary’s keys are considered in order. For example, in {'a': 10, 'b': 20, 'c': 30}, the index of 'b' is 1.

Using dictionary comprehension and get()

This method builds a dictionary using dictionary comprehension that maps each key to its index, allowing O(1) lookups. It is efficient when multiple lookups are required.

d = {'a': 10, 'b': 20, 'c': 30}
k = 'b'

# Precompute key-to-index mapping
idx_map = {key: i for i, key in enumerate(d)}

# O(1) lookup
print(idx_map.get(k)) 

1

Explanation:

• {key: i for i, key in enumerate(d)} creates a mapping of keys to their positions.
• .get(k) retrieves the index in O(1) time returning None if the key is not found.

Iterative Search with enumerate

This method iterates through the dictionary using enumerate, checking each key until a match is found. It avoids extra memory usage and exits early when the key is located.

d = {'a': 10, 'b': 20, 'c': 30}
k = 'b'

for i, key in enumerate(d):
    if key == k:
        print(i) 
        break
else:
    print("Key not found")

1

Explanation:

• enumerate(d) provides both the index and key.
• The loop checks each key and exits early when a match is found, if no match is found then the else block runs printing "Key not found".

Convert Keys to a List and Use .index()

This method converts the dictionary keys into a list and finds the index using .index(). It provides a concise solution but is less efficient due to list creation.

d = {'a': 10, 'b': 20, 'c': 30}
k = 'b'

try:
    i = list(d).index(k)
    print(i)  
except ValueError:
    print("Key not found")

1

Explanation:

• list(d) creates a list of dictionary keys and .index(k) finds the index of k in the list.
• try-except handles cases where k is not in the dictionary.