You're forgetting to append the empty list beforehand. Thus why you get a, IndexError when you try to do arr[i].
arr = []
for i in range(10):
arr.append([])
for j in range(5):
arr[i].append(i*j)
Top answer 1 of 4
8
You're forgetting to append the empty list beforehand. Thus why you get a, IndexError when you try to do arr[i].
arr = []
for i in range(10):
arr.append([])
for j in range(5):
arr[i].append(i*j)
2 of 4
7
You need to define your initial array in the following way: arr=[[] for i in range(10)], as you cannot append a value to a nonexistent array (which is what happens when i>=1). So the code should look like:
arr=[[] for i in range(10)]
for i in range(10):
for j in range(5):
arr[i].append(i*j)
print(i,i*j)
print(arr)
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:',...
python - Append data in 2 dimensional array through for loop - Stack Overflow
I have a two dimensional array. I have created two nested loops to append data to this array, but I am getting error. The error is ''numpy.float64' object has no attribute 'append''. Any help to ov... More on stackoverflow.com
Two dimensional array in python - Stack Overflow
But when I try to do, arr[1].append("bb1"), I get the following error: IndexError: list index out of range. Am I doing anything silly in trying to declare the 2-D array? Edit: but I do not know the number of elements in the array (both rows and columns). ... You do not "declare" arrays or anything else in python... More on stackoverflow.com
python - Numpy append 2D array in for loop over rows - Stack Overflow
I want to append a 2D array created within a for-loop vertically. I tried append method, but this won't stack vertically (I wan't to avoid reshaping the result later), and I tried the vstack() func... More on stackoverflow.com
python - Initialize 2D array within loop - Stack Overflow
Here np.append is sample code where I need to concatenate/append/hstack arrays together... (np.append didn't work.) How to populate 2D array within the loop? More on stackoverflow.com
01:23
How to Append a 2D Array as a Column of Arrays in NumPy - YouTube
How to append data to a 2D list @MyComputerScienceTeacher
03:05
How Does Append Work With 2d Lists In Python - YouTube
03:13
Python NumPy | 2D Arrays - YouTube
03:21
Python NumPy | Append - YouTube
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
Stack Overflow
stackoverflow.com › questions › 62939358 › append-data-in-2-dimensional-array-through-for-loop
python - Append data in 2 dimensional array through for loop - Stack Overflow
July 16, 2020 - arr1[i][j] is selecting a single value, which you are then trying to append to, throwing the error. Try print(arr1[i][j]) to see this in action · G. Anderson – G. Anderson · 2020-07-16 16:39:18 +00:00 Commented Jul 16, 2020 at 16:39 · Use arr1[i][j] = x, if you want to assign to a position. ... @Austin, yes it workes but the values get added from 1st row. So 0th row is "NaN". Any suggestions for that ? ... The function call is not represented as a member function of numpy array.
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']]
Top answer 1 of 2
4
Use np.empty to initialize an empty array and define the axis you want to append across:
import numpy as np
mat = np.empty((0,2))
for i in np.arange(3):
val = np.random.rand(2, 2)
mat = np.append(mat,val, axis=0)
print(mat)
Output:
[[0.08527627 0.40567273]
[0.39701354 0.72642426]
[0.17540761 0.02579183]
[0.76271521 0.83032347]
[0.08105248 0.67986726]
[0.48079453 0.37454798]]
However, as stated in my comment, if you need to append a lot of times you should look into initializing an array of the correct size then assigning values over using np.append() or appending to a list instead (if you do not know the size of the array) and then creating a numpy array after
2 of 2
1
Another way to do it is using the * operator. Notice you can do this even if you are not working with Numpy
mat = []
for i in np.arange(3):
if len(mat) == 0:
mat = np.random.rand(2, 2)
else:
mat = [*mat, *np.random.rand(2, 2)]
# And in case you need it back as a 2D list and not numpy arrays
mat = [list(element) for element in mat]
GeeksforGeeks
geeksforgeeks.org › python › appending-to-2d-list-in-python
Appending to 2D List in Python - GeeksforGeeks
July 23, 2025 - A 2D list in Python is a list of lists where each sublist can hold elements. Appending to a 2D list involves adding either a new sublist or elements to an existing sublist.
Delft Stack
delftstack.com › "delft stack" › "howto" › "python tutorial: how-to guides for everyday python problems" › "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.
Top answer 1 of 2
2
Start with an empty list:
output_arr = []
Append within the loop:
for _ in range(5):
output_arr.append([1, 1])
Outside the loop, just call np.array:
X = np.array(output_arr)
2 of 2
0
If you really just want to construct with numpy, construct an np.empty array at the beginning, and then if you need to operate on the currently filled part of the array, use output[:i+1]
import numpy as np
output_arr = np.empty((5,2))
for i in range(0,4):
temp_arr = np.ones(2)
print temp_arr.shape
output_arr[i] = temp_arr
print output_arr[:i + 1].shape
print output_arr.shape
Snakify
snakify.org › two-dimensional lists (arrays)
Two-dimensional lists (arrays) - Learn Python 3 - Snakify
To process 2-dimensional array, you typically use nested loops. The first loop iterates through the row number, the second loop runs through the elements inside of a row.
Top answer 1 of 7
2
A simple list comprehension and zip will be sufficient, Use:
c = [[x + y for x, y in zip(s1, s2)] for s1, s2 in zip(a, b)]
Result:
#print(c)
[[6, 6, 9, 9], [9, 11, 12, 10]]
2 of 7
2
What you are seeking is essentially matrix addition:
import numpy as np
a = np.array([[5, 4, 5, 4], [4, 5, 6, 8]])
b = np.array([[1, 2, 4, 5], [5, 6, 6, 2]])
c = a + b
Where "array" is numpy's vector and matrix object, so when you return "c", you should see
>>> c
array ([[6, 6, 9, 9],
[9, 11, 12, 10]])
CodeRivers
coderivers.org › blog › append-to-a-2d-array-python
Appending to a 2D Array in Python — CodeRivers
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.
AskPython
askpython.com › python › two-dimensional-array-in-python
Two Dimensional Array in Python - AskPython
December 27, 2019 - 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....
Dot Net Perls
dotnetperls.com › 2d-python
Python - 2D List Examples - Dot Net Perls
Step 2 We access each sub-list and call append on it. This appends to the inner lists. Step 3 We display the element at indexes 0, 0 and this value is 1. With the print() method we can display the entire list contents. Step 4 This loop can iterate rows and columns in the 2D list.
w3resource
w3resource.com › numpy › manipulation › append.php
Numpy: numpy.append() function - w3resource
April 25, 2026 - In the above code, the np.append() function is used to append arrays in NumPy. The first argument passed to the function is a one-dimensional NumPy array and the second argument is a two-dimensional NumPy array.
DataCamp
datacamp.com › doc › numpy › append
NumPy append()
The `append()` function is typically used when you want to add more data to an array without altering the original array.
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]] ...
Reddit
reddit.com › r/learnpython › preserving 2d list in for loops
r/learnpython on Reddit: preserving 2d list in for loops
April 25, 2023 -
Very early into my newest hobby, learning to code so forgive my poor etiquette
Trying to figure out when iterating through a 2d list with a for loop
This structure doesn't preserve the 2d list
new_list = []
for temp_var1 in two_d_lst:
for temp_var2 in temp_var1:
new_list.append(temp_var2.method())But this structure does
new_list = []
for temp_var1 in two_d_lst:
unpacking_lst = []
for temp_var2 in temp_var1:
unpacking_lst.append(temp_var2.method())
new_list.append(unpacking_lst)I'm not sure I understand why...
Greatly appreciate sarcastic remarks, especially if it helps me understand this.
Top answer 1 of 3
5
In your first example, you append each item to a list (new_list), but since those items are just individually appended to the list, you end up with a 1-D list. A 2-D list is just a list of lists, so to retain that structure, you have to append lists to your variable new_list. I do notice a typo in your second example. You append temp_var.method() when you should append temp_var2.method()
2 of 3
2
I know this isn't exactly your example, it's just a similar one to illustrate the logic. two_d_lst = [ [1, 2, 3], [4, 5, 6], [7, 8, 9] ] new_lst = [] for row in two_d_lst: # To preserve the original structure, we need to create a new row # when we iterate over a new row from the original list new_row = [] # Iterate over the values in the row and append them to the new row for value in row: new_row.append(value) # Add the new row to the new list new_lst.append(new_row) print(new_lst) The issue in your first example is that you were not creating a new row when you intended to make one. I hope the in-line commentary and different variable names makes the structure more clear of what is happening at each step. Also, to format code for Reddit, indent your code by 4 spaces.