question about a list of lists

"shawn bright" <nephishgmail.com> wrote

> new_list = []
> for item in lists: # item will look something like
[var1, var2, 
> var3]
>    if item[0] in new_list

> basicly, i want to know if item[0]   is one of the
items[0] in my 
> new_list

Your pseudo code is pretty much exactly right.
What more are you looking for?

Alan G. 


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shawn bright wrote:
> lo there all.
> 
> i have a list of lists that i want to build, only if an
item is not in 
> the list already.
> 
> kinda like this
> new_list = []
> for item in lists: # item will look something like
[var1, var2, var3]
>     if item[0] in new_list ( only the first element of
each list ) like 
> new_list[0][0]
> 
> basicly, i want to know if item[0]   is one of the
items[0] in my new_list
> 
> whats a good pythonic way to do this? i mean, i have a
couple of way to 
> do this, but they are ugly.

One way to do this is to keep a helper set that contains the
first 
elements of each list. Something like
new_list = []
firsts = set()
for item in lists:
   if item[0] not in firsts:
     new_list.append(item)
     firsts.add(item[0]

If you don't care about the order of the result, and if two
lists have 
duplicate first items you are happy to use the first, then
you could use 
a dict mapping first item to list:

new_list = dict((item[0], item) for item in lists).values()

Kent

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shawn bright wrote:
> Thanks Kent,
>    i am going with option A, the helper set, because i
also need to 
> count the occurances and this seems to be the easiest
solution.

If you need the number of unique items that is just the
length of the 
final list. If you need the number of occurrences of each
initial item 
then a dict mapping initial item to count (or maybe a list
of lists) 
would be the way to go.

Kent

> 
> thanks for your help.
> 
> shawn
> 
> On 1/28/07, *Kent Johnson* <kent37tds.net
<mailto:kent37tds.net>> wrote:
> 
>     shawn bright wrote:
>      > lo there all.
>      >
>      > i have a list of lists that i want to build,
only if an item is
>     not in
>      > the list already.
>      >
>      > kinda like this
>      > new_list = []
>      > for item in lists: # item will look something
like [var1, var2,
>     var3]
>      >     if item[0] in new_list ( only the first
element of each list
>     ) like
>      > new_list[0][0]
>      >
>      > basicly, i want to know if item[0]   is one
of the items[0] in my
>     new_list
>      >
>      > whats a good pythonic way to do this? i mean,
i have a couple of
>     way to
>      > do this, but they are ugly.
> 
>     One way to do this is to keep a helper set that
contains the first
>     elements of each list. Something like
>     new_list = []
>     firsts = set()
>     for item in lists:
>        if item[0] not in firsts:
>          new_list.append(item)
>          firsts.add(item[0]
> 
>     If you don't care about the order of the result,
and if two lists have
>     duplicate first items you are happy to use the
first, then you could use
>     a dict mapping first item to list:
> 
>     new_list = dict((item[0], item) for item in
lists).values()
> 
>     Kent
> 
> 


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