How to insert extra rows and columns at desired location in a matrix ( python )?

Question

I was running a loop, from where I obtained ( 4 by 4) arrays, and now I want to insert zeros at certain locations in these arrays and turn them into (6 by 6) arrays.

I want to add zeros in third and sixth row , and also 3rd and sixth column

a_obtained = np.array( [[1,     2,   3,    4   ],
                        [5,     6,   7,    8   ],
                        [9,    10,   11,  12   ],
                        [13,   14,   15,   16  ]])

I tried going through np.vstack, np.hstack, but I am not able to place the zeros at specified locations.

a_desired = result = np.array([[1,     2,  0,   3,    4,   0 ],
                               [5,     6,  0,   7,    8,   0 ],
                               [0,     0,  0,   0,    0,   0 ],
                               [9,    10,  0,   11,  12,   0 ],
                               [13,   14,  0,   15,   6,   0 ],
                               [0,     0,  0,   0,    0,   0 ]])

Answer 1

np.insert is relatively easy to use for this task, but internally it uses a variety of methods, depending on the insert specifications. For this case I suspect it does something like the following:

Start with a "blank" array:

In [51]: res = np.zeros((6,6),a_obtained.dtype)

Create a boolean mask of where we want to add rows and columns. Here I can do both rows and columns at once (rather than with 2 insert calls):

In [52]: mask = res.astype(bool)
In [53]: mask[[2,5]] = True
In [54]: mask[:,[2,5]] = True
In [55]: mask
Out[55]: 
array([[False, False,  True, False, False,  True],
       [False, False,  True, False, False,  True],
       [ True,  True,  True,  True,  True,  True],
       [False, False,  True, False, False,  True],
       [False, False,  True, False, False,  True],
       [ True,  True,  True,  True,  True,  True]])

Then just add the original values. Boolean masking selects a "flat" array, so a_obtained has to be flattened as well. Otherwise the insert is straight forward.

In [56]: res[~mask] = a_obtained.ravel()
In [57]: res
Out[57]: 
array([[ 1,  2,  0,  3,  4,  0],
       [ 5,  6,  0,  7,  8,  0],
       [ 0,  0,  0,  0,  0,  0],
       [ 9, 10,  0, 11, 12,  0],
       [13, 14,  0, 15, 16,  0],
       [ 0,  0,  0,  0,  0,  0]])

Answer 2

Using np.insert:

import numpy as np
a_obtained = np.array( [[1,     2,   3,    4   ],
                        [5,     6,   7,    8   ],
                        [9,    10,   11,  12   ],
                        [13,   14,   15,   16  ]])
                        
new_row = np.zeros(4)
a_obtained = np.insert(a_obtained, [3, 4], [new_row], axis= 0)
print(a_obtained)

Answer 3

if you are using multipart/form-data or require a file upload from frontend you will need a multer as middleware for your post/patch requests, otherwise you can set your frontend to send application/json

[edit] this line looks missing from your index.js

app.use(express.urlencoded({extended: true}))

Answer 4

The main reason this does not work is some how the data passed in body is in text format while req.body is expecting json data make sure to double chek the 'Content-Type':'application/json' is set on the request headers