What is the expected output of the following Python program?
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What is the expected output of the following Python program?
List = [3.1416, 1.618, 2.7183, 0.57721]
func = lambda x: (func(x[1:]) + x[:1]) if x else []
print(func(List))
Show answer & explanation
Answer: A. [0.57721, 2.7183, 1.618, 3.1416]
The lambda function recursively reverses the list by taking the rest of the list (x[1:]), recursing on it, and appending the first element (x[:1]) to the end. Lambda functions can call themselves via variable reference. The recursion unwinds from the empty base case, building the reversed list from the last element backward.
Step-by-step Derivation:
Execution trace:
- func([3.1416, 1.618, 2.7183, 0.57721])
- → func([1.618, 2.7183, 0.57721]) + [3.1416]
- → (func([2.7183, 0.57721]) + [1.618]) + [3.1416]
- → ((func([0.57721]) + [2.7183]) + [1.618]) + [3.1416]
- → (((func([]) + [0.57721]) + [2.7183]) + [1.618]) + [3.1416]
- → ((([] + [0.57721]) + [2.7183]) + [1.618]) + [3.1416]
- → [[0.57721] + [2.7183] + [1.618] + [3.1416]]
- → [0.57721, 2.7183, 1.618, 3.1416]
Note: Lambda functions can reference themselves via their assigned variable name, so recursion is supported.