[Tutor] Novice Python Question
alan.gauld at btinternet.com
Tue Jul 1 14:38:22 CEST 2008
"S Potter" <f8lcoder at hotmail.com> wrote
> How do I filter my first list based upon the selected position
> or a variable equal to the value of the selected position from
> my second list?
The most common way of filtering a list is to use a list comprehension
filteredList = [item <for expression> if <filter expression> ]
where <filter expression> can be any expression or function that you
and <for expression> is like a normal for loop construct.
So if you write a function that compares an item from the original
with the items in your control list you can get what you want
>>> L = [5,6,7,8,9]
>>> [n for i,n in enumerate(L) if i % 2] # equivalent to getting all
>>> odd indexed items
There is also the older filter() function which some people prefer.
You will find examples of both in the Functional Programming topic
of my tutor.
> Question 2.) If I assign a value to a variable x = "MyVlaue"
> How do I put the value of x into a list?
You can use append or insert or use slicing to replace a series of
with a new value.
>>> x = 42
>>> myList = [x] # new list containing x
>>> myList.append(66) # add 66 to the list -> [42,66]
>>> myList.insert(1,7) # insert 7 at index 1 -> [42,7,66]
> I would think it would be something like:
That would convert x to a string and then try to use it to
index a list called list (which is a bad name because it would
hide the function for converting things to a list!)
Which tutorial are you using to learn? Most tutorials will cover this
sort of stuff. Look in the Raw Materials topic of my tutor for more
info on using lists.
> This would probably be considered macro-substitution in other
> languages but I cannot find reference to this in python.
There is no real concept of macro substitution in Python and frankly
I don't see what you describe as being much like what I understand
macro substitution to be in languages like C, Assembler or Lisp...
Author of the Learn to Program web site
More information about the Tutor