CODEWITHSUNDEEP
pythonlist
tdc
tdc

Reputation: 8597

python replace list values using a tuple

If I have a list:

my_list = [3,2,2,3,4,1,3,4]

and a tuple

my_tuple = (3,5)

What's the best way of replacing elements in my_list using the tuple:

result = [5,2,2,5,4,1,5,4]

e.g.

for item in my_list:
    if(item == my_tuple[0]):
        item = my_tuple[1]

More generally, I would have a list of lists, and a list of tuples, and I would like to apply each of the tuples to each of the lists within the list of lists.

Upvotes: 3

Views: 3550

Answers (4)

Rik Poggi
Rik Poggi

Reputation: 29302

If you are trying to replace every 3 in your list with 5, this will do:

[x == my_tuple[0] and my_tuple[1] or x for x in my_list]

If you want to do this, with more than one "translational" tuple, then I really suggest to use a dictionary instead:

trans = {3: 5, 4: 6}
[trans.get(x,x) for x in my_list]

And in the more general case where you have more than one list:

ll = [[3, 2, 3, 4], [5, 4, 3, 4]]
trans = {3: 5, 4: 6}
for i in range(len(ll)):
    ll[i] = [trans.get(x,x) for x in ll[i]]

Supposing that you want to replace every old list in ll with the new one.

Upvotes: 1

ben author
ben author

Reputation: 2955

Per @Wooble's comment, your code will work if you enumerate.

list_of_lists = [[3,2,2,3,4,1,3,4], [1,3,5,3,4,6,3]]
list_of_tuples = [(3,5), (1,9)]

def tup_replace(mylist, mytuple):
    for i, item in enumerate(mylist):
        if item == mytuple[0]:
            mylist[i] = mytuple[1]
    return mylist

then you can just nest that some more to work on a list of list and list of tuples.

for mylist in list_of_lists:
    for mytuple in list_of_tuples:
        mylist = tup_replace(mylist, mytuple)
    print mylist

That said, the dictionary approach is probably better.

Upvotes: 3

KobeJohn
KobeJohn

Reputation: 7545

Using if item == my_tuple[0], ... in a general case sounds like you are making a switch statement that you want to apply to each item in your list. Use a dictionary instead if you can. (Why isn't there a switch or case statement in python?)

Convert your list of tuples to a lookup dictionary (python's switch statement):

replacements = dict(my_tuples) #thanks to @julio

Then for a single list, reproduce the list with a comprehension, but replace each value with the new value from replacements if it exists:

replaced_list = [replacements.get(original,original) for original in my_list]

I guess there is a more efficient way to do it, but that's for a single list with a list of tuples. You say you also need to do it for a list of lists? Just nest that?

Could you explain more about where you are getting this data and why you need to do it?

Upvotes: 1

DSM
DSM

Reputation: 353019

The more natural data structure for my_tuple is a dictionary. Consider something like this and use the .get() method:

>>> my_lists = [[3,2,2,3,4,1,3,4], [1,2,3,4,5,6]]
>>> my_tuple_list = [(3,5), (6, 7)]
>>> my_dict = dict(my_tuple_list)
>>> my_dict
{3: 5, 6: 7}
>>> my_lists = [[my_dict.get(x,x) for x in somelist] for somelist in my_lists]
>>> my_lists
[[5, 2, 2, 5, 4, 1, 5, 4], [1, 2, 5, 4, 5, 7]]

Upvotes: 11

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