How to implement a recursive function using lambda expressions

Question

My task is to implement the factorial function using just a lambda expression. Here's what I have tried

fact = lambda n: if n == 0 return 1 else ...

I'm stuck!

Edit: fix if statement syntax error

fact = lambda n: 1 if n == 0 else ...

I'm stuck again..

How to do it?

The title and your question are not the same thing. Do you need to implement factorial with a lambda or do you need to do it with recursion using a lambda? — pault
– pault, Commented Jan 13, 2019 at 20:03
You can implement factorial using recursion. There is not a difference in what you're saying. — moctarjallo
– moctarjallo, Commented Jan 13, 2019 at 20:53
You can also implement it without recursion. An iterative approach is much better for this. For example: fact = lambda n: reduce(operator.mul, range(1,n+1)) — pault
– pault, Commented Jan 13, 2019 at 21:37
That's why I did title it "recursive" because I want the recursive implementation to showcase the use of lambda expressions in that case, but not the iterative one. — moctarjallo
– moctarjallo, Commented Jan 13, 2019 at 21:40

cdlane · Accepted Answer · 2019-01-13 20:02:48Z

4

A simple approach is to use the name of the variable you assign the lambda into as the way to call the code recursively:

>>> fact = lambda n: 1 if n <= 0 else n * fact(n - 1)
>>> fact(10)
3628800
>>>

There are more complex solutions involving passing lambda expressions to lambda expressions or Y-combinators. Your approach was already doomed by the use of an if ... else ... statement. A lambda expression can only contain other expressions, not statements, so you'd need to use the ... if ... else ... expression syntax instead.

edited Jan 13, 2019 at 20:02

answered Jan 13, 2019 at 19:55

cdlane

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1 Comment

moctarjallo Over a year ago

Yeah actually the "if statement I did" was a mistake i meant what you did. Thanks for your solution; I didn't know one can access the variable before it's finished being defined.

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