On 29/10/14 04:33, Clayton Kirkwood wrote:
!If we'd wanted the new list to contain double the original values we'd
!write:
!
! [ pair*2 for pair in values if key == pair[0] ]
Ok, I am somewhat confused again. In the original example up above, it
appears that the pair list or tuple gets overridden.
The point Cameron is making is that the first expression is completely
arbitrary it does not even need to contain the variable used in the for
loop. Here is an example of how to create a random number for each
matching pair:
from random import random
...
random_list = [random() for pair in values if pair[0] == 2]
Notice the resultant list will contain one random number
for each pair found that has 2 as its first element.
Again the equivalent code:
random_list = []
for pair in values:
if pair[0] == 2:
random_list.append(random())
Another example, this time using numbers and a more complex
expression. Let's say we want the square root of all the
even numbers in a list:
from math import sqrt
roots = [sqrt(n) for n in [1,3,5,4,6,9,7,8,34,67,22] if n % 2 == 0]
yields:
[2.0, 2.449, 2.828, 5.830, 4.690]
So the expression that is put in the result is not necessarily the same
as the value in the initial for loop, and in fact may be completely
unrelated. It just happens to be a very common usage that we want
to extract a sub set of the values of a list.
--
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.flickr.com/photos/alangauldphotos
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