how to create a multidimensional array on the fly using python?

Question

I have a loop which generates a value_list each time it runs, at the end of each iteration i want to append all the lists into a one multi dimensional array

I have:

value_list = [1,2,3,4] in 1st iteration
value_list = [5,6,7,8] in 2nd iteration
value list = [9,10,11,12] in 3rd iteration
etc...

At the end of each iteration I want one multi dimensional array like

value_list_copy = [[1,2,3,4]] in the 1st iteration
value_list_copy = [[1,2,3,4],[5,6,7,8]] in the 2nd iteration
value_list_copy = [[1,2,3,4],[5,6,7,8],[9,10,11,12]]
etc...

How could I achieve this?

Thanks

I rolled back your edit because after that edit the question altogether became a new question. If you have to respect the time the users put in to answer the question you posted. All of a sudden you can't just edit and ask a new question. You can always ask another question. To ask new question click here — Ch3steR
– Ch3steR, Commented Apr 9, 2020 at 9:45
Don't be... BTW Welcome to Stack Overflow. I just wanted to tell how things work here. ;) — Ch3steR
– Ch3steR, Commented Apr 9, 2020 at 9:55
@PriyaRamakrishnan You can mark any of the answers as your solution to thank the people taking time answering your question (and it will also gives you some additional reputation). Cheers and Welcome to the best community in the world~! — Jean-Francois T.
– Jean-Francois T., Commented Apr 10, 2020 at 4:10

user2390182 · Accepted Answer · 2020-04-09 07:21:48Z

3

You can use a nested comprehension and itertools.count:

from itertools import count, islice

cols = 4
rows = 5

c = count(1)
matrix = [[next(c) for _ in range(cols)] for _ in range(rows)]
# [[1, 2, 3, 4], 
#  [5, 6, 7, 8], 
#  [9, 10, 11, 12], 
#  [13, 14, 15, 16], 
#  [17, 18, 19, 20]]

The cool kids might also want to zip the count iterator with itself:

list(islice(zip(*[c]*cols), rows))
# [(1, 2, 3, 4), 
#  (5, 6, 7, 8), 
#  (9, 10, 11, 12), 
#  (13, 14, 15, 16), 
#  (17, 18, 19, 20)]

edited Apr 9, 2020 at 7:21

answered Apr 9, 2020 at 6:26

user2390182

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Ch3steR · Accepted Answer · 2020-04-09 06:43:36Z

1

If you are using Python3.8 then use Walrus assignment(:=).

For Syntax and semantic.

count=0
rows=5
cols=4
[[(count:=count+1) for _ in range(cols)] for _ in range(rows)]

Output:

[[1, 2, 3, 4],
 [5, 6, 7, 8],
 [9, 10, 11, 12],
 [13, 14, 15, 16],
 [17, 18, 19, 20]]

Without using :=.

rows=5
cols=4
[list(range(i,i+cols)) for i in range(1,rows*cols,cols)]

edited Apr 9, 2020 at 6:43

answered Apr 9, 2020 at 6:35

Ch3steR

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1 Comment

Ch3steR Over a year ago

@Jean-FrancoisT. IMO the most standout new feature in python3.8

Ketan Krishna Patil · Accepted Answer · 2020-04-09 07:02:28Z

1

Try this:

limit = 10
length_of_elements_in_each_list = 4
[range(i, i+length_of_elements_in_each_list) for i in range(1, limit)]

You can set a limit and length_of_elements_in_each_list according to your need.

answered Apr 9, 2020 at 7:02

Ketan Krishna Patil

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2 Comments

Phillyclause89 Over a year ago

This creates a list of range objects not a list of list objects. Even if you do [list(range(i, i+length_of_elements_in_each_list)) for i in range(1, limit)] to make it a list of list objects, the nested lists don't match the pattern described in the question.

Ketan Krishna Patil Over a year ago

@Phillyclause89 I forgot to add [/*]. We can get required result by changing the last line to: print([[*range(i, i+length_of_elements_in_each_list)] for i in range(1, limit)])

Jean-Francois T. · Accepted Answer · 2020-04-10 04:06:41Z

0

Try this below :

value_list_copy = []

for i in range(n): # ----------> Assuming n is the number of times your loop is running
    value_list_copy.append(value_list)  # ------ Append your value list in value_list_copy in every iteration

Here you will get an array of arrays.

print(value_list_copy)

edited Apr 10, 2020 at 4:06

Jean-Francois T.

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answered Apr 9, 2020 at 6:25

Abhishek Kulkarni

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Comments

Phillyclause89 · Accepted Answer · 2020-04-09 07:10:30Z

0

Here are two other possible solutions:

Double for loop approach

rows, cols, start = 3, 4, 1

value_list_copy = []
for j in range(rows):
    value_list = []
    for i in range(start, cols + start):
        value_list.append((j*cols)+i)
    value_list_copy.append(value_list)
    print(
        f'value_list = {value_list}\n'
        f'value_list_copy = {value_list_copy}\n'
    )

List comp method

rows, cols, start = 3, 4, 1

value_list_copy_2 = [
    [
        (j*cols)+i for i in range(start, cols + start)
    ] for j in range(rows)
]
print(f'value_list_copy_2 = {value_list_copy_2}')

Python Tutor Link to example code

answered Apr 9, 2020 at 7:10

Phillyclause89

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Anonymous · Accepted Answer · 2024-02-01 08:28:09Z

0

(lambda x,y: np.arange(1,1+x*y).reshape((x,y)).tolist())(3,4)

answered Feb 1, 2024 at 8:28

Anonymous

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