Adding a duplicate item on to a list

Question

I'm trying to add the last item of a list onto the list my code is:

def dup_last(data):
    data.append([-1])
    return data

and calling for the function is:

item = dup_last([1,2,3])
print(item)

but I want my output to be within only one set of brackets like:

[1, 2, 3, 3]

It's good to also include the output you're getting, so we can see why it's wrong :). — Cyphase, Aug 07 '15 at 06:15
@tomknight, did one of the answers solve your problem? If so, please accept it :). — Cyphase, Aug 10 '15 at 06:52

score 3 · Answer 1 · answered Aug 07 '15 at 06:01

3

data.append([-1])

Here you are appending [-1], a list with an element of -1, change it to:

data.append(data[-1])

answered Aug 07 '15 at 06:01

Yu Hao

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Thanks Yu Hao. I didn't realize you had to do that, one more lesson learnt. – tom knight Aug 07 '15 at 06:19

score 2 · Answer 2 · edited May 23 '17 at 10:26

In addition to other answers, I would also suggest to use slicing notation [:] when dealing with lists to prevent getting list index out of range errors in case there is no item:

def dup_last(data):
    data.append(data[-1])
    return data

The above function will raise IndexError if data is empty list:

>>> print dup_last([])
----> 2     data.append(data[-1])
      3     return data
      4
IndexError: list index out of range

When you update your function as follows, you no longer get that kind of error:

def dup_last(data):
    data.extend(data[-1:])
    return data

>>> print dup_last([])
[]
>>> print dup_last([1])
[1, 1]
>>> print dup_last([1, 2])
[1, 2, 2]

There is a good explanation in this SO question about how slicing works in Python.

Cyphase · Answer 3 · 2015-08-07T06:14:24.807

You need to do data.append(data[-1]); data.append([-1]) appends a value which is a list containing only -1, so your result will be [1, 2, 3, [-1]].

Note that this will modify the list in-place, so whichever list you pass in will also have the last element duplicated, not just the list you get out (though they could be the same list).

I wouldn't use a function for this; just do data.append(data[-1]) instead of data = dup_last(data), or even dup_last(data). Also, it's probably better to just add the duplicate manually if you're working with a list literal; data = [1, 2, 3, 3] vs data = dup_last([1, 2, 3]) or similar.

Adding a duplicate item on to a list

3 Answers3