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
– Sven Marnach, Commented Nov 21, 2011 at 15:54

cel106 · Accepted Answer · 2011-11-21 15:59:38Z

30

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, 2011 at 15:59

cel106

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questionto42 Over a year ago

The question was about array, not list.

pajton · Accepted Answer · 2011-11-21 15:54:45Z

22

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

answered Nov 21, 2011 at 15:54

pajton

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Comments

fortran · Accepted Answer · 2011-11-22 12:31:18Z

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, 2011 at 12:31

answered Nov 21, 2011 at 15:58

fortran

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1 Comment

questionto42 Over a year ago

The += will add the values, it does not affect the structure.

questionto42 · Accepted Answer · 2021-07-15 22:54:25Z

0

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.

answered Jul 15, 2021 at 22:54

questionto42

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