How do you create an empty two dimensional array in python?
Asked 11 years, 2 months ago Show Viewed 2.8m times I want to define a two-dimensional array without an initialized length like this:
But this gives an error:
Mateen Ulhaq 22.2k16 gold badges86 silver badges127 bronze badges asked Jul 12, 2011 at 15:54
Masoud AbasianMasoud Abasian 10.2k6 gold badges22 silver badges22 bronze badges 3 You're technically trying to index an uninitialized array. You have to first initialize the outer list with lists before adding items; Python calls this "list comprehension".
#You can now add items to the list:
Note that the matrix is "y" address major, in other words, the "y index" comes before the "x index".
Although you can name them as you wish, I look at it this way to avoid some confusion that could arise with the indexing, if you use "x" for both the inner and outer lists, and want a non-square Matrix.
Saeed 2,9315 gold badges31 silver badges48 bronze badges answered Jul 12, 2011 at 15:59
14 If you really want a matrix, you might be better off using
Here are some other ways to create 2-d arrays and matrices (with output removed for compactness):
answered Jul 12, 2011 at 16:04
senderlesenderle 140k35 gold badges206 silver badges231 bronze badges 6 Here is a shorter notation for initializing a list of lists:
Unfortunately shortening this to something like
answered Jul 12, 2011 at 16:17
Andrew ClarkAndrew Clark 195k33 gold badges264 silver badges297 bronze badges 11 If you want to create an empty matrix, the correct syntax is
And if you want to generate a matrix of size 5 filled with 0,
answered Jul 12, 2011 at 16:00
mripardmripard 2,1882 gold badges13 silver badges11 bronze badges 3 If all you want is a two dimensional container to hold some elements, you could conveniently use a dictionary instead:
Then you can do:
This works because As indicated by osa and Josap Valls, you can also use Vatsal further points that this method is probably not very efficient for large matrices and should only be used in non performance-critical parts of the code.
answered May 29, 2014 at 7:23
enobayramenobayram 4,55822 silver badges35 bronze badges 8 In Python you will be creating a list of lists. You do not have to declare the dimensions ahead of time, but you can. For example:
Now matrix[0][0] == 2 and matrix[1][0] == 3. You can also use the list comprehension syntax. This example uses it twice over to build a "two-dimensional list":
answered Jul 12, 2011 at 16:04
wberrywberry 17.7k8 gold badges51 silver badges82 bronze badges 1
Why such a long code, that too in Long back when I was not comfortable with Python, I saw the single line answers for writing 2D matrix and told myself I am not going to use 2-D matrix in Python again. (Those single lines were pretty scary and It didn't give me any information on what Python was doing. Also note that I am not aware of these shorthands.) Anyways, here's the code for a beginner whose coming from C, CPP and Java background Note to Python Lovers and Experts: Please do not down vote just because I wrote a detailed code. answered Jul 6, 2018 at 21:06
unknownerrorunknownerror 2,0552 gold badges22 silver badges24 bronze badges 0 The accepted answer is good and correct, but it took me a while to understand that I could also use it to create a completely empty array.
results in
answered Dec 4, 2015 at 14:13
FabianFabian 5,3364 gold badges33 silver badges45 bronze badges 0 You should make a list of lists, and the best way is to use nested comprehensions:
On your
answered Jul 12, 2011 at 16:00
utdemirutdemir 25.8k10 gold badges61 silver badges81 bronze badges 1 Use:
The *5 for the first dimension works because at this level the data is immutable.
answered Aug 5, 2015 at 1:10
innov8innov8 1,9252 gold badges20 silver badges29 bronze badges 1 This is how I usually create 2D arrays in python.
I find this syntax easy to remember compared to using two for loops in a list comprehension. answered Jun 3, 2018 at 15:32
MichaelMichael 68510 silver badges25 bronze badges A rewrite for easy reading:
answered Jul 2, 2016 at 11:33
0 To declare a matrix of zeros (ones):
e.g.
or numpy.ones((x, y)) e.g.
Even three dimensions are possible. (http://www.astro.ufl.edu/~warner/prog/python.html see --> Multi-dimensional arrays) answered Dec 7, 2013 at 20:45
khazkhaz 3023 silver badges11 bronze badges I'm on my first Python script, and I was a little confused by the square matrix example so I hope the below example will help you save some time:
so that
answered Mar 28, 2014 at 10:14
user110954user110954 7318 silver badges8 bronze badges You can create an empty two dimensional list by nesting two or more square bracing or third bracket (
Now suppose you want to append 1 to
Now, type Matrix and hit Enter. The output will be:
If you entered the following statement instead
then the Matrix would be
answered Aug 3, 2019 at 9:42
Meraj al MaksudMeraj al Maksud 1,4522 gold badges22 silver badges35 bronze badges Using NumPy you can initialize empty matrix like this:
And later append data like this:
answered Sep 15, 2017 at 10:54
1 I read in comma separated files like this:
The list "data" is then a list of lists with index data[row][col] answered Sep 4, 2013 at 19:40
wsanderswsanders 1984 silver badges8 bronze badges 0 That's what dictionary is made for!
You can define keys and values in two ways:
or
Result:
answered Jan 16, 2017 at 6:38
Use:
I do think NumPy is the way to go. The above is a generic one if you don't want to use NumPy.
answered Nov 1, 2015 at 7:48
pterodragonpterodragon 4219 silver badges16 bronze badges 1 If you want to be able to think it as a 2D array rather than being forced to think in term of a list of lists (much more natural in my opinion), you can do the following:
The result is a list (not a NumPy array), and you can overwrite the individual positions with numbers, strings, whatever.
answered Jul 14, 2016 at 8:55
alessadnroalessadnro 711 silver badge2 bronze badges 1
Will be faster than:
answered Nov 18, 2018 at 14:02
Harsh SharmaHarsh Sharma 10.4k2 gold badges16 silver badges27 bronze badges 1 by using list :
by using dict: you can also store this info in the hash table for fast searching like
matrix['1'] will give you result in O(1) time *nb: you need to deal with a collision in the hash table answered Feb 5, 2018 at 4:27
If you don't have size information before start then create two one-dimensional lists.
Store the entire row in the 1st list. Once done, append list 1 into list 2:
Output:
answered Aug 5, 2017 at 11:55
Nagendra NigadeNagendra Nigade 8162 gold badges11 silver badges27 bronze badges
Be careful about this short expression, see full explanation down in @F.J's answer
gongzhitaao 6,2593 gold badges35 silver badges44 bronze badges answered Feb 8, 2014 at 10:24
和風信使和風信使 1351 silver badge2 bronze badges 6 Here is the code snippet for creating a matrix in python:
Please suggest if I have missed something. answered Dec 9, 2019 at 12:54
Chandra ShekharChandra Shekhar 5761 gold badge7 silver badges23 bronze badges Usually, the go-to module is NumPy:
answered Dec 7, 2021 at 20:35
dejanualexdejanualex 3,3316 gold badges22 silver badges33 bronze badges Try this:
answered Dec 28, 2018 at 8:45
Ankit SharmaAnkit Sharma 1,4911 gold badge13 silver badges18 bronze badges In case if you need a matrix with predefined numbers you can use the following code:
answered Jan 21, 2019 at 17:15
Vlad BezdenVlad Bezden 76k23 gold badges236 silver badges175 bronze badges User Define function to input Matrix and print
marc_s 713k171 gold badges1314 silver badges1433 bronze badges answered May 24, 2021 at 11:35
If you want to create a 2d matrix which dimension is defined by two variables and initialise it with a default value for all its elements. You can use this simple syntax
answered Jun 18 at 11:26
MarioMario 293 bronze badges How do you create an empty 2D numpy array?Create an empty 2D Numpy array using numpy.empty()
To create an empty 2D Numpy array we can pass the shape of the 2D array ( i.e. row & column count) as a tuple to the empty() function. It returned an empty 2D Numpy Array of 5 rows and 3 columns but all values in this 2D numpy array were not initialized.
How do you create a two dimensional array in python?Insert.py. # Write a program to insert the element into the 2D (two dimensional) array of Python.. from array import * # import all package related to the array.. arr1 = [[1, 2, 3, 4], [8, 9, 10, 12]] # initialize the array elements.. print("Before inserting the array elements: "). print(arr1) # print the arr1 elements.. How do you create an empty array in python?Create empty array Python
In python, we don't have built-in support for the array, but python lists can be used. Create a list [0] and multiply it by number and then we will get an empty array.
How do you create an array of two dimensional?We can declare a two-dimensional integer array say 'x' of size 10,20 as: int x[10][20]; Elements in two-dimensional arrays are commonly referred to by x[i][j] where i is the row number and 'j' is the column number.
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