Python 3.4: adding value to list if condition exists

Question

i have a scenario like this one:

mainList = [[9,5],[17,3],[23,1],[9,2]]
secondaryList = [9,12,28,23,1,6,95]
myNewList = []

myNewList.append([[a,b] for a,b in mainList if a in secondaryList])

this, return me to me:

myNewList = [[9,5],[23,1],[9,2]]

but I need only the first occourance of "a". In other words I need to obtain:

myNewList = [[9,5],[23,1]]

How can I achieve this?

Solution 1

First of all:

myNewList = []
myNewList.append([[a,b] for a,b in mainList if a in secondaryList])

simply is the same as

myNewList = [[a,b] for a,b in mainList if a in secondaryList]

Then: What you're building is functionally a python dictionary. Your two-element lists in mainList are the same as dict.items()!

So what you'd do is build a dict out of mainList (reversing it, because usually, you'd just save the last, not the first occurence):

mainDict = dict([reversed(mainList)])

Then you just make your new list:

myNewList = [ (key, mainDict[key]) for key in secondaryList ]

Solution 2

You can use a set to store the first elements and then check for existing the first element before adding the sub-lists :

>>> seen=set()
>>> l=[]
>>> for i,j in mainList:
...    if i in secondaryList and i not in seen:
...        seen.add(i)
...        l.append([i,j])
... 
>>> l
[[9, 5], [23, 1]]

Or you can use collections.defaultdict and deque with specifying its maxlen.But note that you need to loop over your list from end to start if you want the first occourance of a because deque will keep the last insert value :

>>> from collections import defaultdict
>>> from functools import partial

>>> d=defaultdict(partial(deque, maxlen=1))
>>> for i,j in mainList[::-1]:
...    if i in secondaryList:
...       d[i].append(j)
... 
>>> d
defaultdict(<functools.partial object at 0x7ff672706e68>, {9: deque([5], maxlen=1), 23: deque([1], maxlen=1)})

Comments

[jul]

Does not work for me: mainDict = dict([reversed(secondaryList)]) ValueError: dictionary update sequence element #0 has length 7; 2 is required