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pythonlistindexingappend
Rayden
Rayden

Reputation: 160

Python List Indexing or Appending?

What is the best way to add values to a List in terms of processing time, memory usage and just generally what is the best programming option. 

list = []

for i in anotherArray:
    list.append(i)

or

list = range(len(anotherArray))

for i in list:
    list[i] = anotherArray[i]

Considering that anotherArray is for example an array of Tuples. (This is just a simple example)

Upvotes: 1

Views: 772

Answers (3)

Nick Humrich
Nick Humrich

Reputation: 15755

It really depends on your use case. There is no generic answer here as it depends on what you are trying to do.

In your example, it looks like you are just trying to create a copy of the array, in which case the best way to do this would be to use copy:

from copy import copy    
list = copy(anotherArray)

If you are trying to transform the array into another array you should use list comprehension.

list =  [i[0] for i in anotherArray]  # get the first item from tuples in anotherArray

If you are trying to use both indexes and objects, you should use enumerate:

for i, j in enumerate(list)

which is much better than your second example.

You can also use generators, lambas, maps, filters, etc. The reason all of these possibilities exist is because they are all "better" for different reasons. The writters of python are pretty big on "one right way", so trust me, if there was one generic way which was always better, that is the only way that would exist in python.

Edit: Ran some results of performance for tuple swap and here are the results:

comprehension: 2.682028295999771
enumerate: 5.359116118001111
for in append: 4.177091988000029
for in indexes: 4.612594166001145

As you can tell, comprehension is usually the best bet. Using enumerate is expensive. Here is the code for the above test:

from timeit import timeit

some_array = [(i, 'a', True) for i in range(0,100000)]

def use_comprehension():
    return [(b, a, i) for i, a, b in some_array]

def use_enumerate():
    lst = []
    for j, k in enumerate(some_array):
        i, a, b = k
        lst.append((b, a, i))
    return lst

def use_for_in_with_append():
    lst = []
    for i in some_array:
        i, a, b = i
        lst.append((b, a, i))
    return lst

def use_for_in_with_indexes():
    lst = [None] * len(some_array)
    for j in range(len(some_array)):
        i, a, b = some_array[j]
        lst[j] = (b, a, i)
    return lst

print('comprehension:', timeit(use_comprehension, number=200))
print('enumerate:', timeit(use_enumerate, number=200))
print('for in append:', timeit(use_for_in_with_append, number=200))
print('for in indexes:', timeit(use_for_in_with_indexes, number=200))

Edit2:

It was pointed out to me the the OP just wanted to know the difference between "indexing" and "appending". Really, those are used for two different use cases as well. Indexing is for replacing objects, whereas appending is for adding. However, in a case where the list starts empty, appending will always be better because the indexing has the overhead of creating the list initially. You can see from the results above that indexing is slightly slower, mostly because you have to create the first list.

Upvotes: 1

Malik Brahimi
Malik Brahimi

Reputation: 16711

I would actually say the best is a combination of index loops and value loops with enumeration:

for i, j in enumerate(list): # i is the index, j is the value, can't go wrong

Upvotes: 0

Kasravnd
Kasravnd

Reputation: 107297

Best way is list comprehension :

my_list=[i for i in anotherArray]

But based on your problem you can use a generator expression (is more efficient than list comprehension when you just want to loop over your items and you don't need to use some list methods like indexing or len or ... ) 

my_list=(i for i in anotherArray)

Upvotes: 1

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