python appending elements to a list from a list

Question

I would like to create a list that adds elements alternately from 2 seperate lists in python . I had the following idea but it doesn't seem to work:

t1 = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
t2 = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun',
'Jul', 'Aug', 'Sept', 'Oct', 'Nov', 'Dec']
t3= [len(t1)+len(t2)]
a = 0

while a < len(t1)+len(t2):
    t3.extend(t1[a])
    t3.extend(t2[a])
    a = a + 1
print t3

So basically I would like ['Jan',31,'Feb',28,'Mar',31, ect...]

score 2 · Answer 1 · answered May 20 '16 at 11:53

2

The shortest solution may be:

list(sum(zip(t2, t1), ()))

answered May 20 '16 at 11:53

Squall

594
4
15

2

In general, [don't use `sum` to flatten a list, it is O(n^2)](http://stackoverflow.com/questions/952914/making-a-flat-list-out-of-list-of-lists-in-python/952952#952952). – kennytm May 20 '16 at 11:56

score 2 · Answer 2 · edited May 23 '17 at 12:16

In Python you don't need to "reserve capacity" for a list. Just write
```
t3 = []
```
In fact, t3 = [len(t1)+len(t2)] doesn't even creates a list with length 24, but creates a list with a single entry [24].
t1[a] and t2[a] are elements you want to add to the list. To add an element, you use the .append method:
```
t3.append(t1[a])
t3.append(t2[a])
```
.extend is used to add a list (in fact, any iterable) to a list, e.g.
```
t3.extend([t1[a], t2[a]])
```
The problem itself can be solved easily using list comprehensions.
```
[a for l in zip(t2, t1) for a in l]
```

There are many other improvements could be made (e.g. use a for loop instead of a while loop). You could take it to http://codereview.stackexchange.com.

(BTW, this code does not handle leap year.)

score 1 · Answer 3 · answered May 20 '16 at 11:51

Just zip the lists and flatten the result.

>>> from itertools import chain
>>> t1 = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
>>> t2 = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun',
... 'Jul', 'Aug', 'Sept', 'Oct', 'Nov', 'Dec']
>>> list(chain(*zip(t2, t1)))
['Jan', 31, 'Feb', 28, 'Mar', 31, 'Apr', 30, 'May', 31, 'Jun', 30, 'Jul', 31, 'Aug', 31, 'Sept', 30, 'Oct', 31, 'Nov', 30, 'Dec', 31]

Without chain:

>>> [x for tup in zip(t2, t1) for x in tup]
['Jan', 31, 'Feb', 28, 'Mar', 31, 'Apr', 30, 'May', 31, 'Jun', 30, 'Jul', 31, 'Aug', 31, 'Sept', 30, 'Oct', 31, 'Nov', 30, 'Dec', 31]

score 1 · Accepted Answer · answered May 20 '16 at 11:52

1

Here you go:

t1 = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
t2 = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun',
      'Jul', 'Aug', 'Sept', 'Oct', 'Nov', 'Dec']
t3 = list()

for i, j in zip(t1, t2):
  t3.append(i)
  t3.append(j)

print(t3)

answered May 20 '16 at 11:52

David Guyon

2,759
1
28
40

score 0 · Answer 5 · answered May 20 '16 at 11:50

you probably have to read more about python lists and their methods. t3= [len(t1)+len(t2)] this is not necessary at all. I guess you have a C background and trying to initialize the list with size. In python you don't have to initialize the list size (its auto increasing). And the items you have in a list are not stored as per the sequence you have entered them in. Please check tuple in python if you want your items to be in the same sequence. Happy Coding

yes I started learning python today. Thank you for your help! — szotyi, May 20 '16 at 11:57

score 0 · Answer 6 · answered May 20 '16 at 11:50

t1 = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
t2 = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun',
      'Jul', 'Aug', 'Sept', 'Oct', 'Nov', 'Dec']

arr = []

for i in range(12):
    arr.append(t2[i])
    arr.append(t1[i])

print(arr)

Output -

['Jan', 31, 'Feb', 28, 'Mar', 31, 'Apr', 30, 'May', 31, 'Jun', 30, 'Jul', 31, 'Aug', 31, 'Sept', 30, 'Oct', 31, 'Nov', 30, 'Dec', 31]

You can alternatively write -

import itertools

arr = list(itertools.chain.from_iterable(zip(t2, t1))

score 0 · Answer 7 · answered May 20 '16 at 11:50

In Python, you can't create lists with a fixed length like you can do with arrays in other languages, so the third line should just be t3 = [].

Also, the extend() function is used to concatenate lists. To add a single new value, you need to use the append() function instead.

score 0 · Answer 8 · answered May 20 '16 at 12:06

Python is dynamic programming language, the type of the identifier is determined when it is assigned value. so basically you can do in this way:

t1 = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
t2 = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun',
'Jul', 'Aug', 'Sept', 'Oct', 'Nov', 'Dec']
t3= []
for a in range(len(t1)):
    append.append(t1[a])
    apppend.append(t2[a])
print t3

python appending elements to a list from a list

8 Answers8