Multidimensional array by user input in python

Question

I used Jupyter notebook, I am new to Python, I try to fetch value from user in multidimensional array how I do that? I write a little code, after put first value I get error that I don't understand

Error:

Traceback (most recent call last)
   <ipython-input-64-4d8986a5e412> in <module>
          5 for i in range(lengthrow):
          6     for j in range(lengthcol):
    ----> 7         arr[i][j]=int(input("enter value"))
          8 print(arr)

IndexError: index 0 is out of bounds for axis 0 with size 0

code:

from numpy import*
arr = array([[],[]])
lengthrow=int(input("enter array row length"))
lengthcol=int(input("enter array col length"))
for i in range(lengthrow):
    for j in range(lengthcol):
        arr[i][j]=int(input("enter value"))
print(arr)

You made arr a (2,0) shape array. arr[0] is a (0,) shape array. Trying to index that with [0] produces the error. — hpaulj
– hpaulj, Commented Feb 12, 2019 at 17:09

Darius · Accepted Answer · 2019-02-14 19:04:37Z

1

I took @Austin great answer and made some little changes:

import numpy as np

n_rows = int(input("Enter number of rows: "))
n_cols = int(input("Enter number of columns: "))

arr = [[int(input("Enter value for {}. row and {}. column: ".format(r + 1, c + 1))) for c in range(n_cols)] for r in range(n_rows)]

print(np.array(arr))

The output is:

Enter number of rows: 2
Enter number of columns: 3
Enter value for 1. row and 1. column: 1
Enter value for 1. row and 2. column: 2
Enter value for 1. row and 3. column: 3
Enter value for 2. row and 1. column: 4
Enter value for 2. row and 2. column: 5
Enter value for 2. row and 3. column: 6
[[1 2 3]
 [4 5 6]]

You got an exception, because you initialized an empty array and used invalid indices. With this answer you generate the array after you have entered the users input.

Here is the long version of the one-liner (arr = [[...) which gives you the same result:

outer_arr = []
for r in range(n_rows):
    inner_arr = []
    for c in range(n_cols):
        num = int(input("Enter value for {}. row and {}. column: ".format(r + 1, c + 1)))
        inner_arr.append(num)
    outer_arr.append(inner_arr)

print(np.array(outer_arr))

edited Feb 14, 2019 at 19:04

answered Feb 12, 2019 at 16:13

Darius

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5 Comments

DevWebTk Over a year ago

Thanks Darius Morawec, this method as also great actually I need this output thanks a lot.Can you suggest some reference to learn two dimensional array I need to deeply learn about it.Thank you :)

Darius Over a year ago

Sure, multidimensional arrays are matrices in linear algebra, which allows you do to powerful operations. Here are some introductions: #1, #2, #3 and #4.

DevWebTk Over a year ago

Again Thanks a lot @Darius Morawiec :)

DevWebTk Over a year ago

Can you explain this line [[int(input("Enter value for {}. row and {}. column: ".format(r + 1, c + 1))) for c in range(n_cols)] for r in range(n_rows)] i don't really understand how flow of for loop

Darius Over a year ago

@ToufikKhan Sure, I edited the answer and added a longer version of the same code. We use two for loops to create a list of lists which we convert to a two-dimensional array.

Austin · Accepted Answer · 2019-02-12 16:13:26Z

1

You can use a list-comprehension here as the final result is a list of lists:

lengthrow = int(input("enter array row length: "))
lengthcol = int(input("enter array col length: "))

arr = [[int(input("enter value: ")) for _ in range(lengthcol)] for _ in range(lengthrow)]

print(arr)

Problem with your code:

arr = array([[],[]])
print(arr.shape)
# (2, 0)

That means you have a size 0 column in array, so when you do arr[0][0], for example, it throws error.

edited Feb 12, 2019 at 16:13

answered Feb 12, 2019 at 16:02

Austin

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Comments

hpaulj · Accepted Answer · 2019-02-12 18:38:04Z

0

The problem is the shape of the initial array:

In [1]: arr = np.array([[], []])
In [2]: arr
Out[2]: array([], shape=(2, 0), dtype=float64)
In [3]: arr[0]
Out[3]: array([], dtype=float64)
In [4]: arr[0][0]
... 
IndexError: index 0 is out of bounds for axis 0 with size 0

Think of this array as 2 rows, 0 columns. You can't reference a non-existent column. And, in contrast to some other languages, you can't grow an array simply by referencing a new index. Once created the size is fixed.

While others have shown nice list comprehension approaches, I'll show how to use your approach correctly:

In [8]: lengthrow, lengthcol = 2,3    # or user input
In [9]: arr = np.zeros((lengthrow, lengthcol), dtype=int)  # large enough array
In [10]: arr
Out[10]: 
array([[0, 0, 0],
       [0, 0, 0]])
In [11]: for i in range(2):
    ...:     for j in range(3):
    ...:         arr[i,j] = int(input(f'{i},{j}:'))
    ...:         
0,0:0
0,1:1
0,2:2
1,0:3
1,1:4
1,2:5
In [12]: arr
Out[12]: 
array([[0, 1, 2],
       [3, 4, 5]])

edited Feb 12, 2019 at 18:38

answered Feb 12, 2019 at 18:23

hpaulj

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2 Comments

DevWebTk Over a year ago

Thanks for your valuable replay, this salutation is really good. once again thank you :)

DevWebTk Over a year ago

np.array() function convert the list to numpy array

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