python 2d list append row

How to append new rows to 2d array python

stackoverflow.com › questions › 57808549 › how-to-append-new-rows-to-2d-array-python

try that:

l = [['a','b','c'],['d','e','f']]
l.append([0,0,0])

Answer from Mario Khoury on Stack Overflow

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 - The append() method is useful when you want to expand a 2D list dynamically by adding more data over time. However, note that all rows may need to have the same length for structured data processing.

GeeksforGeeks

geeksforgeeks.org › python › appending-to-2d-list-in-python

Appending to 2D List in Python - GeeksforGeeks

July 23, 2025 - += 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.

Discussions

Appending to 2D lists in Python - Stack Overflow

this is the most bizarre functionality ... in Python. seems almost like a bug. 2021-11-22T22:26:19.01Z+00:00 ... Save this answer. ... Show activity on this post. ... In other words, all three list references refer to the same list instance. ... Save this answer. ... Show activity on this post. Came here to see how to append an item to a 2D array, but ... More on stackoverflow.com

stackoverflow.com

Two dimensional array in python - Stack Overflow

Shouldn't this be arr.append(...) instead of arr[0] = ..., to avoid IndexError: list assignment index out of range? 2011-11-18T13:37:52.21Z+00:00 ... There aren't multidimensional arrays as such in Python, what you have is a list containing other lists. More on stackoverflow.com

stackoverflow.com

preserving 2d list in for loops

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() More on reddit.com

r/learnpython

April 25, 2023

python - How to append items into a 2D list? - Stack Overflow

I have a 2D list trends. I open a folder and go through every file in the folder (mostly CSV Files). For every file, I want to put the data into a new slot on the first dimension of the list. For e... More on stackoverflow.com

stackoverflow.com

June 4, 2021

Top answer

1 of 1

try that:

l = [['a','b','c'],['d','e','f']]
l.append([0,0,0])

Append a 1d array to a 2d array in Numpy Python

python 3.x - adding rows in a 2d list

arrays - Initializing 2D list in python with variable rows and columns

python - Initialize 2D array within loop

python - quick way to add values to a 2d array

More results from stackoverflow.com

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.

Python Pool

pythonpool.com › home › tutorials › python 2d lists: create, index, copy, flatten, and loop

Python 2d List: From Basic to Advance

July 14, 2026 - Copy safely: Shallow row copies, aliasing, list multiplication, and deep copies. When the number of rows will grow over time, start with an empty outer list and append a new row for each record.

Stack Overflow

stackoverflow.com › questions › 7745562 › appending-to-2d-lists-in-python

Appending to 2D lists in Python - Stack Overflow

Top answer

1 of 3

You haven't created three different empty lists. You've created one empty list, and then created a new list with three references to that same empty list. To fix the problem use this code instead:

listy = [[] for i in range(3)]

Running your example code now gives the result you probably expected:

>>> listy = [[] for i in range(3)]
>>> listy[1] = [1,2]
>>> listy
[[], [1, 2], []]
>>> listy[1].append(3)
>>> listy
[[], [1, 2, 3], []]
>>> listy[2].append(1)
>>> listy
[[], [1, 2, 3], [1]]

2 of 3

[[]]*3 is not the same as [[], [], []].

It's as if you'd said

a = []
listy = [a, a, a]

In other words, all three list references refer to the same list instance.

Find elsewhere

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Snakify

snakify.org › two-dimensional lists (arrays)

Two-dimensional lists (arrays) - Learn Python 3 - Snakify

Another (but similar) way: create an empty list and then append a new element to it n times (this element should be a list of length m): ... But the easiest way is to use generator, creating a list of n elements, each of which is a list of m zeros: ... In this case each element is created independently from the others. The list [0] * m is n times consructed as the new one, and no copying of references occurs. Say, a program takes on input two-dimensional array in the form of n rows, each of which contains m numbers separated by spaces.

AdamSmith.haus

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How to append to a 2D list in Python - Adam Smith

Python answers, examples, and documentation

Statistics Globe

statisticsglobe.com › home › python programming language for statistics & data science › append to 2d list in python (3 examples)

Extend 2D List in Python (3 Examples) | Append New Elements

April 5, 2023 - How to add a new element to a 2D list in Python - Create demo 2D list - Add new element to 2D list using append() method

Iditect

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Appending to 2D lists in Python

matrix = [[1, 2, 3], [4, 5, 6]] new_row = [7, 8, 9] matrix.append(new_row) "Python extend 2D list" Description: This query involves extending a 2D list in Python with additional elements.

Thedeveloperblog

thedeveloperblog.com › 2d-list-python

Python 2D List Examples

elements = [] # Append empty lists in first two indexes. elements.append([]) elements.append([]) # Add elements to empty lists. elements[0].append(1) elements[0].append(2) elements[1].append(3) elements[1].append(4) # Display top-left element. print(elements[0][0]) # Display entire list. print(elements) # Loop over rows. for row in elements: # Loop over columns. for column in row: print(column, end="") print(end="\n") Output 1 [[1, 2], [3, 4]] 12 34 · Jagged lists. The term "jagged" implies that sub lists have uneven lengths. Here we create a list of two lists—one of length 2, the other of length 5. We display their lengths. ... Python program that uses jagged lists # A jagged list.

YouTube

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How Does Append Work With 2d Lists In Python - YouTube

03:05

In this python tutorial we are talking about lists in python! Specifically, we are answering the question of how does append work with 2d lists in python! Le...

Published: January 4, 2023

Stack Overflow

stackoverflow.com › questions › 8183146 › two-dimensional-array-in-python

Two dimensional array in python - Stack Overflow

Top answer

1 of 12

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')

arr = []
arr.append(['aa1', 'aa2'])

2 of 12

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']]

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...

Kosbie

kosbie.net › cmu › fall-11 › 15-112 › handouts › notes-2d-lists.html

2d Lists

Wrong: Cannot use * (Shallow Copy) # Try, and FAIL, to create a variable-sized 2d list rows = 3 cols = 2 a = [ [0] * cols ] * rows # Error: creates shallow copy # Creates one unique row, the rest are aliases! print "This SEEMS ok. At first:" print " a =", a a[0][0] = 42 print "But see what happens after a[0][0]=42" print " a =", a · Right: Append Each Row # Create a variable-sized 2d list rows = 3 cols = 2 a=[] for row in xrange(rows): a += [[0]*cols] print "This IS ok.

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

2 of 3

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.

Stack Overflow

stackoverflow.com › questions › 67837449 › how-to-append-items-into-a-2d-list

python - How to append items into a 2D list? - Stack Overflow

Top answer

1 of 1

I would suggest first creating the inner array, and then appending it to outer array as a whole, like this:

trends = []
for file in files:
    file_result = []
    with open(file) as csv_file:
        csv_reader = csv.reader(csv_file, delimiter=',') 
        next(csv_reader) # To skip the header
        for row in csv_reader:
            file_result.append(row[1])
    trends.append(file_result)

This way you don't need to keep the counters at all and your code will be easier to read.

You also don't need to close the file if you are using context manager (with statement)

Stack Overflow

stackoverflow.com › questions › 63622574 › python-appending-numerous-2d-list-to-one-2d-list

Python: Appending numerous 2D list to one 2D list - Stack Overflow

Top answer

1 of 2

You can iterate through the items of the lists and then add them to your masterlist:

a = [[1,2,3],
     [1,2,3]]   
b = [[4,5,6],
     [4,5,6]]

masterlist = []

for aa,bb in zip(a,b):                 # loop over lists
    for itema, itemb in zip(aa,bb):    # loop over items in list
        masterlist = masterlist + [[itema, itemb]]

output:

[[1, 4], [2, 5], [3, 6], [1, 4], [2, 5], [3, 6]]

2 of 2

If you use numpy,this is really easy

import numpy as np
a = np.array([[1,2,3],
     [1,2,3]])
    
b = np.array([[4,5,6],
     [4,5,6]])

fl = np.vstack(np.dstack((a,b)))

output

array([[1, 4],
       [2, 5],
       [3, 6],
       [1, 4],
       [2, 5],
       [3, 6]])

Kosbie

kosbie.net › cmu › fall-10 › 15-110 › koz › notes-2d-lists.html

Two-Dimensional Lists

Wrong: Cannot use * (Shallow Copy) # Try, and FAIL, to create a variable-sized 2d list rows = 3 cols = 2 a = [ [0] * cols ] * rows # Error: creates shallow copy print "This SEEMS ok. At first:" print " a =", a a[0][0] = 42 print "But see what happens after a[0]=42" print " a =", a · Right: Append Each Row # Create a variable-sized 2d list rows = 3 cols = 2 a=[] for row in range(rows): a += [[0]*cols] print "This IS ok.