"markscottwright" <[EMAIL PROTECTED]> wrote in message
news:[EMAIL PROTECTED]
> Just for the hell of it, I've been going through the old Scheme-based
> textbook "Structure and Interpretation of Computer Programs" and seeing
> what I can and can't do with python. I'm trying to create a function
> that returns the function (not the results of the function, but a
> function object) that results from applying function f to it's (single)
> argument N times. For example, if you have "def sq(x): return x*x",
> then repeated(sq, 2)(2) = 16, repeated(sq, 3)(2) = 256, etc.
>
> I can do it recursively, like this:
>
> def repeated(f, count):
> if count == 1:
> return f
> else:
> return lambda x: f(repeated(f, count - 1)(x)
>
> But when I try to do it iteratively, it just hangs when I try to
> evaluate the results (for count > 1):
>
> def repeated2(f, count):
> newfun = f
> for i in range(count-1):
> newfun = lambda x: newfun(f(x))
> return newfun
The trouble is that Python's scoping rules are subtly different from
Schemes, so you're binding to the wrong instance of newfun. You should do
this:
def repeated2(f, count):
newfun = f
for i in range(count-1):
newfun = lambda x, g=newfun: g(f(x))
return newfun
Alternatively, you could do it this way:
def repeated3(f, count):
def g(x):
for i in range(count):
x = f(x)
return x
return g
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