Discover Lucky Numbers in a List with a One‑Line Python Trick

Problem Definition

Given a list of integers, a "lucky number" is an element whose value equals its frequency in the list.

Step‑by‑step solution

1. Remove duplicates

Use a set to obtain the unique elements.

arr = [3,5,2,7,3,8,1,2,4,8,9,3]
unique = set(arr)  # {1,2,3,4,5,7,8,9}

2. Count occurrences

The list method count() returns the frequency of an element.

pairs = [(x, arr.count(x)) for x in unique]
# [(1,1), (2,2), (3,3), (4,1), (5,1), (7,1), (8,2), (9,1)]

3. Filter lucky numbers

Replace the explicit loop with map, zip and filter, then sort.

m = map(arr.count, unique)
z = zip(unique, m)
f = filter(lambda x: x[0] == x[1], z)
s = sorted(f, key=lambda x: x[0])
print('Lucky numbers are:', [item[0] for item in s])
# Lucky numbers are: [1, 2, 3]

4. One‑liner version

The whole logic can be expressed in a single statement.

print('Lucky numbers are:', [x for x in sorted(filter(lambda x: x[0]==x[1], zip(set(arr), map(arr.count, set(arr)))), key=lambda x:x[0])])