On 22/10/14 19:48, Clayton Kirkwood wrote:
Regarding the index out of range, I know what it meant, I was just kind of
surprised that Python didn't automatically create the node and stuff a value
in.
But what should Python do in this case
aNewList[1000000] = 42
Should Python create a 1 million element list with only one value?
That would be a slow operation. What should the other values be
set to? None, presumably?
!description.append() = something
!
!
!from. There's nothing in Python that I've ever seen that suggests that
!would work, and the error message should be clear:
freshfruit = [' banana', ' loganberry ', 'passion fruit ']
[weapon.strip() for weapon in freshfruit]
['banana', 'loganberry', 'passion fruit']
> From 5.1.3 in the Python3 tutorial.
> Although not an = assignment, some value
> is being stored in weapon after being strip of whitespace.
> I realize they are different, maybe just not sure how different:<)
This is a list comprehension with a very specific syntax.
It is equivalent to
aList = []
for weapon in freshfruit:
aList.append(weapon.strip())
It is very different from assigning a value to a function call.
In this case, somehow
Python seems to keep track of the location so that the modified
value(dropping spaces) replaces the original location.
No it does not replace the original location it is appended
to the list.
So two questions remain. Why can't codes.append(code) just replace the code
in the previous line and descriptions.append(description) replace the
description in the previous line.
Because the append() adds it at the end it doesn't replace anything.
value.strip() = some value . Second question, why can't a numeric index be
used to make assignment to a specific location like a[1] = "some value"?
It can if that entry already exists.
listA = [0,1,2,3,4,5] #initialize with elements
listA[3] = 9
print(listA) # -> [0,1,2,9,4,5]
listB = []
listB[3] = 9 # error because listB has no elements yet.
listB += [0,1,2,3,4] # add some elements
listB[3] = 9 # OK now.
It is clear that once the array is created,
Its not an array, its a list. Most specifically it is not a C style
block of memory that is just filled in with a set of bytes. It is a
dynamic sequence of objects of potentially mixed(and changing) type.
# create a list of 2-tuples like (number, square)
[(x, x**2) for x in range(6)]
[(0, 0), (1, 1), (2, 4), (3, 9), (4, 16), (5, 25)]
Why does this not need the .append or .insert? Square brackets around the
whole line?
Because the list comprehension implicitly does an append
for you (see above)
--
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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