CODEWITHSUNDEEP
python
memyself
memyself

Reputation: 12628

iterating quickly through list of tuples

I wonder whether there's a quicker and less time consuming way to iterate over a list of tuples, finding the right match. What I do is:

# this is a very long list.
my_list = [ (old1, new1), (old2, new2), (old3, new3), ... (oldN, newN)]

# go through entire list and look for match
for j in my_list:
    if j[0] == VALUE:
        PAIR_FOUND = True
        MATCHING_VALUE = j[1]
        break

this code can take quite some time to execute, depending on the number of items in the list. I'm sure there's a better way of doing this.

Upvotes: 58

Views: 176069

Answers (5)

Vaibhav Singh
Vaibhav Singh

Reputation: 139

The question is dead but still knowing one more way doesn't hurt:

my_list = [ (old1, new1), (old2, new2), (old3, new3), ... (oldN, newN)]

for first,*args in my_list:
    if first == Value:
        PAIR_FOUND = True
        MATCHING_VALUE = args
        break

Upvotes: 5

Peter Tsung
Peter Tsung

Reputation: 945

I wonder whether the below method is what you want.

You can use defaultdict.

>>> from collections import defaultdict
>>> s = [('red',1), ('blue',2), ('red',3), ('blue',4), ('red',1), ('blue',4)]
>>> d = defaultdict(list)
>>> for k, v in s:
       d[k].append(v)    
>>> sorted(d.items())
[('blue', [2, 4, 4]), ('red', [1, 3, 1])]

Upvotes: 1

Sanjay T. Sharma
Sanjay T. Sharma

Reputation: 23218

Assuming a bit more memory usage is not a problem and if the first item of your tuple is hashable, you can create a dict out of your list of tuples and then looking up the value is as simple as looking up a key from the dict. Something like:

dct = dict(tuples)
val = dct.get(key) # None if item not found else the corresponding value

EDIT: To create a reverse mapping, use something like:

revDct = dict((val, key) for (key, val) in tuples)

Upvotes: 30

tom
tom

Reputation: 19163

The code can be cleaned up, but if you are using a list to store your tuples, any such lookup will be O(N).

If lookup speed is important, you should use a dict to store your tuples. The key should be the 0th element of your tuples, since that's what you're searching on. You can easily create a dict from your list:

my_dict = dict(my_list)

Then, (VALUE, my_dict[VALUE]) will give you your matching tuple (assuming VALUE exists).

Upvotes: 2

Eric
Eric

Reputation: 19873

I think that you can use 

for j,k in my_list:
  [ ... stuff ... ]

Upvotes: 84

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