[Tutor] List Comprehension Syntax

[Mitya Sirenef]

On 12/23/2012 02:48 AM, Mario Cacciatore wrote:
> Hey everyone,
 >
 > I am having a very hard time understanding the list comprehension 
syntax. I've followed the docs and could use some guidance from the fine 
folks here to supplement my findings. If someone wouldn't mind replying 
back with an example or two, with some explanation of each part I'd 
appreciate it.


Hi Mario, here are some examples (using python3 but very similar in py2.7):

>>> L = range(20)
 >>> [x for x in L]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]

>>> [x for x in L if  x<=10]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

>>> [(x,x) for x in L]
[(0, 0), (1, 1), (2, 2), (3, 3), (4, 4), (5, 5), (6, 6), (7, 7), (8, 8), 
(9, 9), (10, 10), (11, 11), (12, 12), (13, 13), (14, 14), (15, 15), (16, 
16), (17, 17), (18, 18), (19, 19)]

>>> [x*2 for x in L]
[0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38]

>>> [(x, x*3) for x in L if  x<=10]
[(0, 0), (1, 3), (2, 6), (3, 9), (4, 12), (5, 15), (6, 18), (7, 21), (8, 
24), (9, 27), (10, 30)]

>>> def strmul(x): return  str(x), x*2
...

>>> [strmul(x) for x in L  if x<=10]
[('0', 0), ('1', 2), ('2', 4), ('3', 6), ('4', 8), ('5', 10), ('6', 12), 
('7', 14), ('8', 16), ('9', 18), ('10', 20)]



Hope this helps!


-- 
Lark's Tongue Guide to Python: http://lightbird.net/larks/