Why does list selection in a recursive lambda function throw recursion limit errors?

Question

Consider the two following recursive functions that calculate the factorial of an input that I naively thought would be the same:

fact = lambda x: 1 if x == 0 else x * fact(x-1)
fact = lambda x: [x*fact(x-1),1][x==0]

The first runs fine but the second give me the error RuntimeError: maximum recursion depth exceeded. This is true for for inputs where x==0 and x!=0.

Why can't the lambda function handle the second case?

trincot · Accepted Answer · 2017-11-02 18:47:22Z

2

They are not the same. In the second variant both expressions in the list are evaluated, and only then the appropriate one is picked from it. But this does not stop the recursion from happening even when x == 0. And so the recursion continues into the negative numbers bumping into the memory limit.

... if ... else ... on the other hand will first evaluate the condition following the if keyword, and only evaluate the expression that corresponds to that result.

answered Nov 2, 2017 at 18:47

trincot

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