list insertion question

[Paul Rubin]

eight02645999 at yahoo.com writes:

> hi
> i have a list (after reading from a file), say
> data = [ 'a','b','c','d','a','b','e','d']
> 
> I wanted to insert a word after every 'a', and before every 'd'. so i
> use enumerate this list:
> for num,item in enumerate(data):
>     if "a" in item:
> 	data.insert(num+1,"aword")
>     if "d" in item:
>         data.insert(num-1,"dword") #this fails
> but the above only inserts after 'a' but not before 'd'.  What am i
> doing wrong? is there better way?thanks

As others have said, you're mutating the list while iterating through
it, which can give whacked results.  Also, even if you operate on a
copy of the list, that algorithm uses quadratic time because of all
the insertions into the list.  These days I like to write in the style

    def g():
       for w in data:
         if 'd' in w: yield 'dword'
         yield w
         if 'a' in w: yield 'aword'
    data = list(g(data))

instead of using list.append as someone else suggested.  The iterator
approach is probably a bit slower but can be seen as a bit cleaner,
depending on your stylistic preferences.