Pythonic way to combine for-loop and if-statement

user_413143 0
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I know how to use both for loops and if statements on separate lines, such as:

>>> a = [2,3,4,5,6,7,8,9,0]
... xyz = [0,12,4,6,242,7,9]
... for x in xyz:
...     if x in a:
...         print(x)
0,4,6,7,9

And I know I can use a list comprehension to combine these when the statements are simple, such as:

print([x for x in xyz if x in a])

But what I can't find is a good example anywhere (to copy and learn from) demonstrating a complex set of commands (not just "print x") that occur following a combination of a for loop and some if statements. Something that I would expect looks like:

for x in xyz if x not in a:
    print(x...)

Is this just not the way python is supposed to work?

if-statement for-loop python loops
11 Answers

I liked Alex's answer, because a filter is exactly an if applied to a list, so if you want to explore a subset of a list given a condition, this seems to be the most natural way

mylist = [1,2,3,4,5]
another_list = [2,3,4]

wanted = lambda x:x in another_list

for x in filter(wanted, mylist):
    print(x)

this method is useful for the separation of concerns, if the condition function changes, the only code to fiddle with is the function itself

mylist = [1,2,3,4,5]

wanted = lambda x:(x**0.5) > 10**0.3

for x in filter(wanted, mylist):
    print(x)

The generator method seems better when you don't want members of the list, but a modification of said members, which seems more fit to a generator

mylist = [1,2,3,4,5]

wanted = lambda x:(x**0.5) > 10**0.3

generator = (x**0.5 for x in mylist if wanted(x))

for x in generator:
    print(x)

Also, filters work with generators, although in this case it isn't efficient

mylist = [1,2,3,4,5]

wanted = lambda x:(x**0.5) > 10**0.3

generator = (x**0.9 for x in mylist)

for x in filter(wanted, generator):
    print(x)

But of course, it would still be nice to write like this:

mylist = [1,2,3,4,5]

wanted = lambda x:(x**0.5) > 10**0.3

# for x in filter(wanted, mylist):
for x in mylist if wanted(x):
    print(x)
user_47014180

A simple way to find unique common elements of lists a and b:

a = [1,2,3]
b = [3,6,2]
for both in set(a) & set(b):
    print(both)
user_18802880

based on the article here: https://towardsdatascience.com/a-comprehensive-hands-on-guide-to-transfer-learning-with-real-world-applications-in-deep-learning-212bf3b2f27a I used the following code for the same reason and it worked just fine:

an_array = [x for x in xyz if x not in a]

This line is a part of the program! this means that XYZ is an array which is to be defined and assigned previously, and also the variable a

Using generator expressions (which is recommended in the selected answer) makes some difficulties because the result is not an array

user_26761970
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