I'm stuck at higher-order functions in python. I need to write a repeat function repeat
that applies the function f
n
times on a given argument x
.
For example, repeat(f, 3, x)
is f(f(f(x)))
.
This is what I have:
def repeat(f,n,x):
if n==0:
return f(x)
else:
return repeat(f,n-1,x)
When I try to assert the following line:
plus = lambda x,y: repeat(lambda z:z+1,x,y)
assert plus(2,2) == 4
It gives me an AssertionError
. I read about How to repeat a function n times but I need to have it done in this way and I can't figure it out...
You have two problems:
n == 1
, the function should be called once); and f
on the returned value from the recursive call, so the function is only ever applied once. Try:
def repeat(f, n, x):
if n == 1: # note 1, not 0
return f(x)
else:
return f(repeat(f, n-1, x)) # call f with returned value
or, alternatively:
def repeat(f, n, x):
if n == 0:
return x # note x, not f(x)
else:
return f(repeat(f, n-1, x)) # call f with returned value
(thanks to @Kevin for the latter, which supports n == 0
).
Example:
>>> repeat(lambda z: z + 1, 2, 2)
4
>>> assert repeat(lambda z: z * 2, 4, 3) == 3 * 2 * 2 * 2 * 2
>>>
You've got a very simple error there, in the else block you are just passing x along without doing anything to it. Also you are applying x when n == 0, don't do that.
def repeat(f,n,x):
"""
>>> repeat(lambda x: x+1, 2, 0)
2
"""
return repeat(f, n-1, f(x)) if n > 0 else x
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