[SOLVED] List comprehension works but not generator expression

List comprehension works but not generator expression

I am doing this problem:

Given an array of integers arr, a lucky integer is an integer which has a frequency in the array equal to its value. Return a lucky integer in the array. If there are multiple lucky integers return the largest of them. If there is no lucky integer return -1.

I am using this code, which works and passed all tests:

class Solution:
    def findLucky(self, arr: List[int]) -> int:
        values = [n for n in arr if n if arr.count(n) == n]
        return max(values) if values else -1

However, when I try to change the list comprehension to a generator expression like so:

class Solution:
    def findLucky(self, arr: List[int]) -> int:
        values = (n for n in arr if n if arr.count(n) == n)
        return max(values) if values else -1

I get this error:

Traceback (most recent call last):
  File "...", line 10, in <module>
    print(Solution().findLucky([2, 3, 4]))
  File "...", line 7, in findLucky
    return max(values) if values else -1
ValueError: max() arg is an empty sequence

Why does a list comprehension work whereas a generator expression fails?

I've tried converting the generator expression to a list in the comprehension, but that won't work because then the generator is exhuased.

Solution

Every generator is true, so you always call max(values) even if the generator is "empty". The right way to do it is to tell max what to do then:

return max(values, default=-1)