Need Help on using map for functions in python

Question

So I have written a function f(a, b). The structure is:

def f(a, b):
    global result

    def g(a, b):
        global result
        return result

    deg h(a):
        return a  # an updated a

    g(a, b)
    a = h(a)
    g(a, b)
    a = h(a)
    g(a, b)
    a = h(a)
    g(a, b)
    return result

You can see that I have two sub-functions g(a, b) and h(a) in f(a, b).
I need to run g(a, b) to get the partial result
Then I updated the a
So that I can run g(a, b) again, to update the result.
After 4 runs of g(a, b), I got the full piece of result and return the value.

There must be a way to structure the process here that looks simpler and clearer.

I also tried:

g(a, b)
g(h(a), b)
g(h(h(a), b)
g(h(h(h(a), b)

And that looks hideous as well.

I need some suggestion to simply to structure here, maybe to use the map() or other high order functions? But I just couldn't figure it out.

Thanks!

map works on lists - it has nothing to do with high-order functions — Nir Alfasi
– Nir Alfasi, Commented Nov 10, 2017 at 17:09
What's the purpose of the global result? And why aren't you saving the value returned by the g function? And why doesn't g do anything with the arguments you pass it? — PM 2Ring
– PM 2Ring, Commented Nov 10, 2017 at 17:09
Have you considered using function returns instead of a global variable? Then a simple for loop would let you do it 4 times. — Kenny Ostrom
– Kenny Ostrom, Commented Nov 10, 2017 at 17:10
@PM2Ring I guess I was not giving enough details. The reason to do global result is because the g(a, b) only returns part of the result. So i need to update the a, and run g() again, the another part of the result. I have to run g() 4 times to get the full result. But again, I am a beginner here, if you can show me how to structure it without using global I will appreciate it. I understand that the use of global is often avoided. — jxie0755
– jxie0755, Commented Nov 10, 2017 at 18:26

sapht · Accepted Answer · 2017-11-11 17:13:49Z

First off you should use arguments and return values, and not side effects (i.e. returning or modifying global variables). I find that the use of global result makes your code unreadable, but I think you are asking about dynamic function composition, which can be expressed like this:

def compose(*fv):
    return reduce(lambda f, g: lambda x: f(g(x)), fv)

And then used like this (note that the evaluation order is right-to-left, which is common):

def add1(x): return x + 1
def mul2(x): return x * 2

>>> compose(add1, add1, add1)(1)
4
>>> compose(mul2, add1, add1, add1)(1)
8

This is common in functional languages, but I find that when you want this pattern in python, you're not making things simple enough.

Nir Alfasi · Accepted Answer · 2017-11-10 22:10:19Z

1

If what you want to do is something like that:

g(a, b)
a = h(a)
g(a, b)
a = h(a)
g(a, b)
a = h(a)
g(a, b)

You can do:

for i in range(4):
    g(a, b)
    a = h(a)
g(a, b)

a bit more compact

edited Nov 10, 2017 at 22:10

answered Nov 10, 2017 at 17:11

Nir Alfasi

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1 Comment

Enrico Borba Over a year ago

range isn't a statement, should have parenthesis.

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