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?
This question is related to
python
loops
if-statement
for-loop
As per The Zen of Python (if you are wondering whether your code is "Pythonic", that's the place to go):
The Pythonic way of getting the sorted
intersection
of two set
s is:
>>> sorted(set(a).intersection(xyz))
[0, 4, 6, 7, 9]
Or those elements that are xyz
but not in a
:
>>> sorted(set(xyz).difference(a))
[12, 242]
But for a more complicated loop you may want to flatten it by iterating over a well-named generator expression and/or calling out to a well-named function. Trying to fit everything on one line is rarely "Pythonic".
I'm not sure what you are trying to do with enumerate
, but if a
is a dictionary, you probably want to use the keys, like this:
>>> a = {
... 2: 'Turtle Doves',
... 3: 'French Hens',
... 4: 'Colly Birds',
... 5: 'Gold Rings',
... 6: 'Geese-a-Laying',
... 7: 'Swans-a-Swimming',
... 8: 'Maids-a-Milking',
... 9: 'Ladies Dancing',
... 0: 'Camel Books',
... }
>>>
>>> xyz = [0, 12, 4, 6, 242, 7, 9]
>>>
>>> known_things = sorted(set(a.iterkeys()).intersection(xyz))
>>> unknown_things = sorted(set(xyz).difference(a.iterkeys()))
>>>
>>> for thing in known_things:
... print 'I know about', a[thing]
...
I know about Camel Books
I know about Colly Birds
I know about Geese-a-Laying
I know about Swans-a-Swimming
I know about Ladies Dancing
>>> print '...but...'
...but...
>>>
>>> for thing in unknown_things:
... print "I don't know what happened on the {0}th day of Christmas".format(thing)
...
I don't know what happened on the 12th day of Christmas
I don't know what happened on the 242th day of Christmas
a = [2,3,4,5,6,7,8,9,0]
xyz = [0,12,4,6,242,7,9]
set(a) & set(xyz)
set([0, 9, 4, 6, 7])
You can use generators too, if generator expressions become too involved or complex:
def gen():
for x in xyz:
if x in a:
yield x
for x in gen():
print x
I personally think this is the prettiest version:
a = [2,3,4,5,6,7,8,9,0]
xyz = [0,12,4,6,242,7,9]
for x in filter(lambda w: w in a, xyz):
print x
if you are very keen on avoiding to use lambda you can use partial function application and use the operator module (that provides functions of most operators).
https://docs.python.org/2/library/operator.html#module-operator
from operator import contains
from functools import partial
print(list(filter(partial(contains, a), xyz)))
The following is a simplification/one liner from the accepted answer:
a = [2,3,4,5,6,7,8,9,0]
xyz = [0,12,4,6,242,7,9]
for x in (x for x in xyz if x not in a):
print(x)
12
242
Notice that the generator
was kept inline. This was tested on python2.7
and python3.6
(notice the parens in the print
;) )
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)
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)
I would probably use:
for x in xyz:
if x not in a:
print x...
Use intersection
or intersection_update
intersection :
a = [2,3,4,5,6,7,8,9,0]
xyz = [0,12,4,6,242,7,9]
ans = sorted(set(a).intersection(set(xyz)))
intersection_update:
a = [2,3,4,5,6,7,8,9,0]
xyz = [0,12,4,6,242,7,9]
b = set(a)
b.intersection_update(xyz)
then b
is your answer
Source: Stackoverflow.com