CODEWITHSUNDEEP
pythonstringlist
Mattjo
Mattjo

Reputation: 157

Adding strings in lists together

I want to transform the list ["A","B","A","A","B"] to the list ["AB","BA","AA","AB"].

I have tried to define a new list in which the first element is deleted and then add the strings of the lists together. After which I plan to delete the last element of the new list to get the result.

lista = sequences
lista.pop(0)
print(lista)

for x in range(sequences):
    mc =sequences[x]+lista[x]

But all I get is

TypeError: 'list' object cannot be interpreted as an integer

Any help is welcome.

Edit : Thank you guys, all your solutions worked perfectly :)

Upvotes: 12

Views: 1377

Answers (4)

U13-Forward
U13-Forward

Reputation: 71560

Best solution, using zip with a list comprehension, cleverest:

>>> l = ["A","B","A","A","B"]
>>> [x + y for x, y in zip(l, l[1:])]
['AB', 'BA', 'AA', 'AB']
>>>

Or use an enumerate with a list comprehension:

>>> l = ["A","B","A","A","B"]
>>> [v + l[i + 1] for i, v in enumerate(l[:-1])]
['AB', 'BA', 'AA', 'AB']
>>>

Upvotes: 14

Toothpick Anemone
Toothpick Anemone

Reputation: 4644

There are several issues in your original code:

sequences = ["A","B","A","A","B"]
lista = sequences
lista.pop(0)
print(lista)

for x in range(sequences):
    mc =sequences[x]+lista[x]

Firstly, the statement lista = sequences does not make a copy of sequences. Instead, lista and sequences become two different names for the same list. What you do using one name also happens to the other. lista.pop(0) is the same as sequences.pop(0). If you want a copy, then import the copy library.

import copy

sequences = ["A","B","A","A","B"]
lista = copy.copy(sequences)
lista.pop(0)
print(lista)

for x in range(sequences):
    mc =sequences[x]+lista[x]

Secondly, your statement range(sequences) is incorrect. The range() function accepts integers as input, not lists. That's what gave you TypeError: 'list' object cannot be interpreted as an integer

# VALID
range(5)
range(3)
range(10)

# INVALID
range(["A","B","A"])
range(["eyes", "nose", "tail"])

sequences is a list. You want range(len(sequences)) notrange(sequences)

In the end, we can modify your original code to work:

import copy

sequences = ["A","B","A","A","B"]
lista = copy.copy(sequences)
lista.pop(0)
print(lista) # prints ["B","A","A","B"]

mc = list()
for x in range(len(lista)):
    mc.append(lista[x] + sequences[x + 1])

Upvotes: 4

Olvin Roght
Olvin Roght

Reputation: 7812

You can use map():

s = list(map(str.__add__, lst[:-1], lst[1:]))

A bit better to use operator.concat() (thanks for advice, @MykolaZotko):

import operator

s = list(map(operator.concat, lst[:-1], lst[1:]))

Upd.

I've decided to do some tests on bigger data.

import operator

lst = [...] # list with 10000 random uppercase letters


def test1():
    return list(map(operator.concat, lst[:-1], lst[1:]))


def test2():
    return [x + y for x, y in zip(lst, lst[1:])]


def test3():
    return [v + lst[i + 1] for i, v in enumerate(lst[:-1])]


def test4():
    s = ''.join(lst)
    return [s[i:i + 2] for i in range(len(s) - 1)]


if __name__ == '__main__':
    import timeit
    print(timeit.timeit("test1()", setup="from __main__ import test1, lst", number=10000))
    print(timeit.timeit("test2()", setup="from __main__ import test2, lst", number=10000))
    print(timeit.timeit("test3()", setup="from __main__ import test3, lst", number=10000))
    print(timeit.timeit("test4()", setup="from __main__ import test4, lst", number=10000))

Results:

  1. Python 2:

    10.447159509
11.529946446
20.962497298000002
20.515838672

  2. Python 3:

    10.370675522
11.429417197
20.836504865999995
20.422865353

On bigger data map() is a bit (~9%) faster, but there's no significant difference between test1() and test2()

Upvotes: 8

RoadRunner
RoadRunner

Reputation: 26315

Use zip():

>>> lst = ["A","B","A","A","B"]
>>> [x + y for x, y in zip(lst, lst[1:])]
['AB', 'BA', 'AA', 'AB']

Upvotes: 13

Related Questions