You do not "declare" arrays or anything else in python. You simply assign to a (new) variable. If you want a multidimensional array, simply add a new array as an array element.
arr = []
arr.append([])
arr[0].append('aa1')
arr[0].append('aa2')
or
arr = []
arr.append(['aa1', 'aa2'])
Top answer 1 of 12
91
You do not "declare" arrays or anything else in python. You simply assign to a (new) variable. If you want a multidimensional array, simply add a new array as an array element.
arr = []
arr.append([])
arr[0].append('aa1')
arr[0].append('aa2')
or
arr = []
arr.append(['aa1', 'aa2'])
2 of 12
82
There aren't multidimensional arrays as such in Python, what you have is a list containing other lists.
>>> arr = [[]]
>>> len(arr)
1
What you have done is declare a list containing a single list. So arr[0] contains a list but arr[1] is not defined.
You can define a list containing two lists as follows:
arr = [[],[]]
Or to define a longer list you could use:
>>> arr = [[] for _ in range(5)]
>>> arr
[[], [], [], [], []]
What you shouldn't do is this:
arr = [[]] * 3
As this puts the same list in all three places in the container list:
>>> arr[0].append('test')
>>> arr
[['test'], ['test'], ['test']]
GeeksforGeeks
geeksforgeeks.org › python › appending-to-2d-list-in-python
Appending to 2D List in Python - GeeksforGeeks
July 23, 2025 - This modifies the contents of the sublist while keeping the 2D structure intact. += operator allows you to extend a sublist with multiple elements in a concise manner. ... This is equivalent to using extend(). If we want to append multiple sublists dynamically list comprehension is a concise option. ... List comprehension generates sublists dynamically and appends them to the original list.
Append a 1d array to a 2d array in Numpy Python - Stack Overflow
I have a numpy 2D array [[1,2,3]]. I need to append a numpy 1D array,( say [4,5,6]) to it, so that it becomes [[1,2,3], [4,5,6]] More on stackoverflow.com
python - Using np.array to append into a 2D np.array - Stack Overflow
V = np.array([]) T = np.array([]) ... T = np.append(T, np.array(temperature[idx])) print(V) # [630.40002441 605.29998779 611.79998779 ... 461.79998779 460.1000061 466.8999939 ] print(T) # [132000. 216000. 194000. ... 22100. 19300. 22200.] # truncated because of large data set · I then try to combine these two arrays into a 2D np.array that ... More on stackoverflow.com
python - How to use append for 2D arrays? - Stack Overflow
You can check the difference in ... Top comment says that np.append uses np.concatenate 2019-02-25T08:18:35.197Z+00:00 ... K.Maj your answer does not answer the question. The question was about: Why x does not change the value? 2019-02-25T11:03:50.27Z+00:00 ... Thanks. But it seems it changes x from a 2D array to 1D ... More on stackoverflow.com
python - Append element to 2D array - Stack Overflow
How can I append an element to first or second element of a numpy array in python? My code does not work. I did like this: my_num = np.array([[],[]]) my_num[0] = np.append(my_num[0],6) print(my_num... More on stackoverflow.com
Reddit
reddit.com › r/learnpython › do you guys know how to add append an array in a 2d array?
r/learnpython on Reddit: do you guys know how to add append an array in a 2d array?
February 19, 2024 -
say i have array [[1,2,3,4,5]], how can i duplicate it 5 times to [[1,2,3,4,5],[1,2,3,4,5],[1,2,3,4,5],[1,2,3,4,5],[1,2,3,4,5]] ?
Top answer 1 of 4
1
my_list = [[1, 2, 3, 4, 5]] new_list = my_list * 5 print(new_list) Prints: [[1, 2, 3, 4, 5], [1, 2, 3, 4, 5], [1, 2, 3, 4, 5], [1, 2, 3, 4, 5], [1, 2, 3, 4, 5]] But be aware that this only creates multiple labels to the same [1, 2, 3, 4, 5] list object. Mutating any one of the inner lists mutates all of the inner lists as there is only one inner list object: new_list[0][0] += 40 print(new_list) print(my_list) # The original list object Prints: [[41, 2, 3, 4, 5], [41, 2, 3, 4, 5], [41, 2, 3, 4, 5], [41, 2, 3, 4, 5], [41, 2, 3, 4, 5]] [[41, 2, 3, 4, 5]] but you can reassign different objects within new_list: new_list[1] = [7,8,9] print(new_list) Prints: [[41, 2, 3, 4, 5], [7, 8, 9], [41, 2, 3, 4, 5], [41, 2, 3, 4, 5], [41, 2, 3, 4, 5]] If you actually want deep copies of the original inner list, you can use a list comprehension: my_list = [[1, 2, 3, 4, 5]] new_list = [my_list[0][:] for _ in range(5)] new_list[0][0] += 40 print(new_list) Prints: [[41, 2, 3, 4, 5], [1, 2, 3, 4, 5], [1, 2, 3, 4, 5], [1, 2, 3, 4, 5], [1, 2, 3, 4, 5]]
2 of 4
1
Easy way: x = your_list[0] for i in range(len(5)): your_list.append(x) Might be a more optimized way to do it but at least it should work
Delft Stack
delftstack.com › "delft stack" › "howto" › "python how-to's" › "how to append 2d array in python"
How to Append 2D Array in Python | Delft Stack
February 22, 2025 - Python’s built-in lists can represent 2D arrays as lists of lists. The simplest way to append values is using the append() method.
EyeHunts
tutorial.eyehunts.com › home › python 2d array append
Python 2D array append
June 27, 2023 - # Initialize a 2D array array_2d = [[1, 2, 3], [4, 5, 6]] # Append a new row to the 2D array new_row = [7, 8, 9] array_2d.append(new_row) # Append elements to an existing row in the 2D array existing_row_index = 1 array_2d[existing_row_index].append(7) array_2d[existing_row_index].append(8) # Print the modified 2D array for row in array_2d: print(row)
AskPython
askpython.com › python › two-dimensional-array-in-python
Two Dimensional Array in Python - AskPython
August 6, 2022 - ... size = int(input()) array_input ... ... Elements in a 2D array can be inserted using the insert() function specifying the index/position of the element to be inserted....
IQCode
iqcode.com › code › python › python-2d-array-append
python 2d array append Code Example
temp_list = [[], [], [], []] temp_list[0].append("a1") temp_list[1].append("a2") temp_list[2].append("a3") tem...
Python Forum
python-forum.io › thread-16863.html
2D arrays and appending values using a loop
Hi, I need some help with the appending of the values into the 2D array using a FOR loop. I am ok with printing the values, its the append that I need help with: Array=[[100,20],[90,29],[102,89],[2,2]] for row in Array: print ('Row Values:',...
CodeRivers
coderivers.org › blog › append-to-a-2d-array-python
Appending to a 2D Array in Python - CodeRivers
February 22, 2026 - For example: ... Here, two_d_array is a 2D array with 3 rows and 3 columns. Each inner list represents a row, and the elements within the inner list are the columns of that row. The simplest way to append to a 2D array in Python is by using the built-in append method of lists.
Top answer 1 of 2
20
You want vstack:
In [45]: a = np.array([[1,2,3]])
In [46]: l = [4,5,6]
In [47]: np.vstack([a,l])
Out[47]:
array([[1, 2, 3],
[4, 5, 6]])
You can stack multiple rows on the condition that The arrays must have the same shape along all but the first axis.
In [53]: np.vstack([a,[[4,5,6], [7,8,9]]])
Out[53]:
array([[1, 2, 3],
[4, 5, 6],
[4, 5, 6],
[7, 8, 9]])
2 of 2
1
Try this:
np.concatenate(([a],[b]),axis=0)
when
a = np.array([1,2,3])
b = np.array([4,5,6])
then result should be:
array([[1, 2, 3], [4, 5, 6]])
Stack Overflow
stackoverflow.com › questions › 71366005
python - Using np.array to append into a 2D np.array - Stack Overflow
V = np.array([]) T = np.array([]) for ii in range(len(velocity)): if (velocity[ii] !=-9.999999848243207e+30 ) and (velocity[ii] != -9.999999848243207e+30): idx.append(ii) # create array with integer x_axis instead of time variable V = np.append(V, np.array(velocity[idx])) T = np.append(T, np.array(temperature[idx])) print(V) # [630.40002441 605.29998779 611.79998779 ... 461.79998779 460.1000061 466.8999939 ] print(T) # [132000. 216000. 194000. ... 22100. 19300. 22200.] # truncated because of large data set · I then try to combine these two arrays into a 2D np.array that looks like np.array([x1,y1],[x2,y2],....):
DataCamp
datacamp.com › doc › numpy › append
NumPy append()
Note that if values have a different data type than arr, the resulting array will have a data type determined by NumPy’s type promotion rules. axis: The axis along which values are appended. If None, values are flattened before use. When axis is specified, values must have the same shape as arr except for the dimension corresponding to the specified axis.
Stack Overflow
stackoverflow.com › questions › 74565257 › append-element-to-2d-array
python - Append element to 2D array - Stack Overflow
How can I append an element to first or second element of a numpy array in python? My code does not work. I did like this: my_num = np.array([[],[]]) my_num[0] = np.append(my_num[0],6) print(my_num...
iO Flood
ioflood.com › blog › numpy-append
Numpy Append Function: From Basics to Advanced Usage
January 31, 2024 - In Numpy, the ‘axis’ parameter refers to the dimension of the array. For a 1D array, there’s only one axis (axis=0). For a 2D array, there are two axes: axis=0 (rows) and axis=1 (columns). When you use the ‘numpy.append()’ function, the ‘axis’ parameter defines along which axis the arrays will be appended.
NumPy
numpy.org › doc › 2.2 › reference › generated › numpy.append.html
numpy.append — NumPy v2.2 Manual
A copy of arr with values appended to axis. Note that append does not occur in-place: a new array is allocated and filled.
Note.nkmk.me
note.nkmk.me › home › python › numpy
NumPy: append() to add values to an array | note.nkmk.me
February 4, 2024 - By default, when the first argument is a two-dimensional array, it is flattened to one dimension, and then the values from the second argument are appended to it. a_2d = np.arange(6).reshape(2, 3) print(a_2d) # [[0 1 2] # [3 4 5]] ...
Top answer 1 of 2
1
The problem is the way you created your fixed-length array. Do it like this:
CCList = [[] for _ in range(3)]
CCList[0].append([2,4])
CCList[1].append([3,4])
CCList[1].append([4,5])
print(CCList)
#[[[2, 4]], [[3, 4], [4, 5]], []]
2 of 2
0
I have found another solution, as well:
you just have to make sure it is populated before doing append
if not CCList[1]:
CCList[1] = [[3,4]]
else:
CCList[1].append([2,3])
Top answer 1 of 8
2
You can use numpy. Perform indexing and concatenate at the end:
import numpy as np
b = np.array([[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12]])
print(np.concatenate((b[:,:2], b[:,2:])))
# [[ 1 2]
# [ 5 6]
# [ 9 10]
# [ 3 4]
# [ 7 8]
# [11 12]]
2 of 8
1
data =[]
data.append(['a',1])
data.append(['b',2])
data.append(['c',3])
data.append(['d',4])
print(data)
output
[['a', 1], ['b', 2], ['c', 3], ['d', 4]]