Appending two arrays together in Python

Question

I've been working in Python with an array which contains a one-dimensional list of values. I have until now been using the array.append(value) function to add values to the array one at a time.

Now, I would like to add all the values from another array to the main array instead. In other words I don't want to add single values one at a time. The secondary array collects ten values, and when these are collected, they are all transfered to the main array. The problem is, I can't simply use the code 'array.append(other_array)', as I get the following error:

unsupported operand type(s) for +: 'int' and 'list'

Where am I going wrong?

`array.append(other_array)` will never lead to the cited error message. Please show your real code. — Sven Marnach, Nov 21 '11 at 15:54

score 29 · Answer 1 · answered Nov 21 '11 at 15:59

29

Lists can be added together:

>>> a = [1,2,3,4]
>>> b = [5,6,7,8]
>>> a+b
[1, 2, 3, 4, 5, 6, 7, 8]

and one can be easily added to the end of another:

>>> a += b
>>> a
[1, 2, 3, 4, 5, 6, 7, 8]

answered Nov 21 '11 at 15:59

cel106

291
2
2

1

The question was about array, not list. – questionto42 Jul 15 '21 at 22:45

score 22 · Accepted Answer · answered Nov 21 '11 at 15:54

22

You are looking for array.extend() method. append() only appends a single element to the array.

answered Nov 21 '11 at 15:54

pajton

15,828
8
54
65

fortran · Answer 3 · 2011-11-22T12:31:18.990

2

Array (as in numpy.array or array module) or list? Because given your error message, it seems the later.

Anyway, you can use the += operator, that should be overridden for most container types, but the operands must be of the same (compound) type.

edited Nov 22 '11 at 12:31

answered Nov 21 '11 at 15:58

fortran

74,053
25
135
175

The += will add the values, it does not affect the structure. – questionto42 Jul 15 '21 at 22:46

score 0 · Answer 4 · answered Jul 15 '21 at 22:54

Usually, if you want to expand a structure to the right (axis=1) or at the bottom (axis=0), you should have a look at the numpy.concatenate() function, see Concatenate a NumPy array to another NumPy array.

np.concatenate(arr1, arr2, axis=0)

is probably what is needed here, adding a new row in a nested array.

Appending two arrays together in Python

4 Answers4