# List comprehension for loops Python

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I use a lot of N dimensional arrays and it gets a pain to have to write such indented code and I know some codes can be replaced with list comprehensions and inline statements. For example:

``````for x in (0,1,2,3):
for y in (0,1,2,3):
if x < y:
print (x, y, x*y)
``````

can be replaced with:

``````print [(x, y, x * y) for x in (0,1,2,3) for y in (0,1,2,3) if x < y]
``````

But how could I change the action instead of print to do something else like:

``````total = x+y
``````

So what I want to do is something like:

``````[(total+=x+y) for x in (0,1,2,3) for y in (0,1,2,3) if x < y]
``````

However this doesn’t work

Is there a smart way to do this rather than:

``````for x in (0,1,2,3):
for y in (0,1,2,3):
if x < y:
total+=x+y
``````

`sum` works here:

``````total = sum(x+y for x in (0,1,2,3) for y in (0,1,2,3) if x < y)
``````

As an alternative to writing loops N levels deep, you could use `itertools.product()`:

``````In : import itertools as it

In : for x, y in it.product((0,1,2,3),(0,1,2,3)):
...:     if x < y:
...:         print x, y, x*y

0 1 0
0 2 0
0 3 0
1 2 2
1 3 3
2 3 6
``````

This extends naturally to N dimensions.

Use numpy. This lets you use arrays that add up like vectors:

``````x = numpy.arange(3)
y = numpy.arange(3)
total = x + y
``````

With the modified question, add a call to `sum` as well

``````total = numpy.sum(x+y)
``````

Reduce function directly reduces collective items to single item. You can read more about them here, but this should work for you:

``````total=reduce(lambda x,y:x+y,range(4))
``````

or

``````total=reduce(lambda x,y:x+y,(0,1,2,3))
``````

Another possibility is:

``````for x,y in ((x,y) for x in (0,1,2,3) for y in (0,1,2,3) if x < y):
print (x, y, x * y)
``````

In this way you can iterate over anything you’d use in a list comprehension without actually creating the comprehended list (if you get my meaning 😉 If comprehended list is big, maybe so big it saturates or even doesn’t fit in memory, that’s quite handy.. The answers/resolutions are collected from stackoverflow, are licensed under cc by-sa 2.5 , cc by-sa 3.0 and cc by-sa 4.0 .