On 21/01/14 06:18, Adriansanchez wrote:
Hello everyone,
I am newbie to Python and programming in general. My question is, given a list:
X=['washington','adams','jefferson','madison','monroe']
And a string:
Y='washington,adams,jefferson,madison,monroe'
How would I print washington and monroe using [:]?
How would I print every element but those two names?
The [:] syntax is used for selecting a range of values
from a starting point to a finish. Its not appropriate
for selecting arbitrary items out of the list.
If you know which items you want you can use a simple
index to access them (remember the first item is index 0)
So to print the first item and the fourth item:
print(X[0],X[3])
In your case it's the first and last so we can do
a similar thing:
print(X[0], X[4])
But for the last element we can alternatively use
a shortcut to save counting the indexes; that's use
an index of -1:
print(X[0],X[-1])
Printing every element except those two is harder.
The simplest approach is to use a loop to process
the list and test each value:
for name in X:
if name not in (X[0], X[-1]):
print name
For the special case of excluding the first and
last names you could use the [:] notation like
this:
print X[1:-1]
But that only works where you want *all* the
names in a sequence between two end points.
Finally there is a more advanced way of filtering
out items from a list called a list comprehension:
print ( [name for name in X if name not in (X[0],X[-1])] )
Which is pretty much our 'for' loop above, written in
a shorthand single line form.
hth
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.flickr.com/photos/alangauldphotos
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