python list comprehension in nested loops

Question

One basic question in list comprehension as i am starting on that,

can a list comprehension return two arrays?

like I was trying to convert my code to list comprehension

b=[10,2,3]
c=[10,11,12]
d=[]
f=[]
a=10
for i in b:
    if a>i:
        for j in c:
            d.append(j)

print d

I am able to convert the above code using list comprehension as

print [j  for i in b if a>i  for j in c ]

But now I want to add an extra else block to my initial code and looks like

b=[10,2,3]
c=[10,11,12]
d=[]
f=[]
a=10
for i in b:
        if a>i:
            for j in c:
                d.append(j)
        else:
          f.append(i)
print d
print f

d=[10, 11, 12, 10, 11, 12]
f=[10]

is there any way I can add this extra else to my initial list comprehension?

Martijn Pieters · Accepted Answer · 2017-10-19 08:10:46Z

7

You can't use a list comprehension for your second example, because you are not building a single list. List comprehensions build one list object, not two.

You could use two separate list comprehensions:

d = [j for i in b if a > i for j in c]
f = [i for i in b if a <= i]

or you could simplify your loops a little by using list.extend() or += augmented assignments:

for i in b:
    if a > i:
        d.extend(c)
    else:
        f.append(i)

or

for i in b:
    if a > i:
        d += c
    else:
        f.append(i)

edited Oct 19, 2017 at 8:10

answered Oct 19, 2017 at 8:09

Martijn Pieters

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2 Comments

Eric Duminil Over a year ago

You can't use a list comprehension for your second example. Challenge accepted.

Martijn Pieters Over a year ago

@EricDuminil: yes, you can produce multiple sequences and unpack. You can use zip() too. It is never readable, however.

Eric Duminil · Accepted Answer · 2017-10-19 10:14:41Z

2

To answer your question :

Can a list comprehension return two arrays?

Yes, it can:

>>> [[1] * n for n in [3, 5]]
[[1, 1, 1], [1, 1, 1, 1, 1]]
>>> d, f = [[1] * n for n in [3, 5]]
>>> d
[1, 1, 1]
>>> f
[1, 1, 1, 1, 1]

Just for fun, here's a one-liner to define both d and f:

d, f = [[j for i in b if f(i) for j in g(i)] for (f, g) in [(lambda x: x < a, lambda x: c), (lambda x: x >= a, lambda x: [x])]]

It's obviously a bad idea, though, and @MartijnPieters's answer is preferable.

answered Oct 19, 2017 at 10:14

Eric Duminil

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1 Comment

Rookie Over a year ago

You've answered the question, shown code and outputs, and simultaneously shown just because one can does not mean one should. Challenge accomplished!

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