CODEWITHSUNDEEP
pythonpython-3.x
Commissar Vasili Karlovic
Commissar Vasili Karlovic

Reputation: 399

More elegant way to write a function composition procedure?

I tried to solve the following problem:

my_function_composition(f,g)

input: two functions f and g, represented as dictionaries, such that g ◦ f exists.

output: dictionary that represents the function g ◦ f.

example: given f = {0:’a’, 1:’b’} and g = {’a’:’apple’, ’b’:’banana’}, return {0:’apple’, 1:’banana’}.

I have found a solution, but it feels pretty messy to me:

def my_function_composition(f,g): return {list(f.keys())[i]: list(g.values())[i] for i in f if list(f.values()) == list(g.keys())}

# example/test
w = {0:'a', 1:'b', 2: 'c'}
v = {'a': 'apple', 'b':'banana', 'c': 'carrot'}

my_function_composition(w,v)

According to the book, the next section includes loop problems, thus I assume there's an elegant one-line solution to this one.

Note: I would highly appreciate if you could provide comments to your solution, because I'm new with python

Upvotes: 0

Views: 182

Answers (2)

blhsing
blhsing

Reputation: 106523

You can use dict comprehension:

d = {k: g[v] for k, v in f.items()}

d becomes:

{0: 'apple', 1: 'banana'}

Upvotes: 2

smac89
smac89

Reputation: 43078

I would do it like this:

return {fk:g[fv] for fk, fv in f.items() if fv in g}

fk is a key in f and fv is the value of the key in f.

All I do is create a dictionary where the keys are made up of the keys of f and the values are made up of looking up the values of f in g (if they exist).

Just to be pedantic, I added the if fv in g, which I don't think is a requirement of this exercise.

Example:

>>> def fcomp(f, g): return {fk:g[fv] for fk, fv in f.items() if fv in g}
... 
>>> fcomp({0:'a', 1:'b', 2: 'c'}, {'a': 'apple', 'b':'banana', 'c': 'carrot'})
{0: 'apple', 1: 'banana', 2: 'carrot'}
>>>

Upvotes: 1

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