What is the syntax for writing txt file with multiple numpy arrays+scalars and how to read it in again?

I have 2 numpy arrays of same length lets call them A and B and 2 scalar values named C and D. I want to store these values into a single txt file. I thought of the following structure:

It doesnt have to have this format I just thought its convenient and clear. I know how to write a the numpy arrays into a txt file and read them out again, but I struggle how to write the txt file as a combination of arrays and scalar values and how to read them out again from txt to numpy.

A = np.array([1, 2, 3, 4, 5])
B = np.array([5, 4, 3, 2, 1])
C = [6]
D = [7]
np.savetxt('file.txt', (A, B))
A_B_load = np.loadtxt('file.txt')
A_load = A_B_load[0,:]
B_load= A_B_load[1,:]

This doesnt give me the same column structure that I proposed but stores the arrays in rows but that doesnt really matter.

I found one solution which is a bit unhandy since I have to fill up the scalar values with 0 for them to become of the same length like the arrays A and B there must be a smarter solution.

    A = np.array([1, 2, 3, 4, 5])
    B = np.array([5, 4, 3, 2, 1])
    C = [6]
    D = [7]
    fill = np.zeros(len(A)-1)
    C = np.concatenate((C,fill))
    D = np.concatenate((D, fill))
    np.savetxt('file.txt', (A,B,C,D))
    A_B_load = np.loadtxt('file.txt')
    A_load = A_B_load[0,:]
    B_load = A_B_load[1,:]
    C_load = A_B_load[2,0]
    D_load = A_B_load[3,0]

In [123]: A = np.array([1, 2, 3, 4, 5])
     ...: B = np.array([5, 4, 3, 2, 1])
     ...: C = [6]
     ...: D = [7]

savetxt is designed to write a 2d array in a consistent csv form - a neat table with the same number of columns in each row.

In [124]: arr = np.stack((A,B), axis=1)
In [125]: arr
Out[125]: 
array([[1, 5],
       [2, 4],
       [3, 3],
       [4, 2],
       [5, 1]])

Here's one possible write format:

In [126]: np.savetxt('foo.txt', arr, fmt='%d', header=f'{C} {D}', delimiter=',')
     ...: 
In [127]: cat foo.txt
# [6] [7]
1,5
2,4
3,3
4,2
5,1

I put the scalars in a header line, since they don't match with the arrays.

loadtxt can recreate that arr array:

In [129]: data = np.loadtxt('foo.txt', dtype=int, skiprows=1, delimiter=',')
In [130]: data
Out[130]: 
array([[1, 5],
       [2, 4],
       [3, 3],
       [4, 2],
       [5, 1]])

The header line can be read with:

In [138]: with open('foo.txt') as f:
     ...:     header = f.readline().strip()
     ...:     line = header[1:]
     ...: 
In [139]: line
Out[139]: ' [6] [7]'

I should have saved it as something that's simpler to parse, like '# 6,7'

Your accepted answer creates a dataframe with nan values and blanks in the csv

In [143]: import pandas as pd
In [144]: df = pd.concat([pd.DataFrame(arr) for arr in [A,B,C,D]], axis=1)
     ...: df.to_csv("test.txt", na_rep="", sep=" ", header=False, index=False)
In [145]: df
Out[145]: 
   0  0    0    0
0  1  5  6.0  7.0
1  2  4  NaN  NaN
2  3  3  NaN  NaN
3  4  2  NaN  NaN
4  5  1  NaN  NaN
In [146]: cat test.txt
1 5 6.0 7.0
2 4  
3 3  
4 2  
5 1 

Note that np.nan is a float, so some of the columns are float as a result. loadtxt can't handle those "blank" columns; np.genfromtxt is better at that, but it needs a delimiter like , to mark them.

Writing and reading the full length arrays is easy. But mixing types gets messy.

Here's a format that would be easier to write and read:

In [149]: arr = np.zeros((5,4),int)
     ...: for i,var in enumerate([A,B,C,D]):
     ...:     arr[:,i] = var
     ...: 
In [150]: arr
Out[150]: 
array([[1, 5, 6, 7],
       [2, 4, 6, 7],
       [3, 3, 6, 7],
       [4, 2, 6, 7],
       [5, 1, 6, 7]])
In [151]: np.savetxt('foo.txt', arr, fmt='%d', delimiter=',')
In [152]: cat foo.txt
1,5,6,7
2,4,6,7
3,3,6,7
4,2,6,7
5,1,6,7
In [153]: np.loadtxt('foo.txt', delimiter=',', dtype=int)
Out[153]: 
array([[1, 5, 6, 7],
       [2, 4, 6, 7],
       [3, 3, 6, 7],
       [4, 2, 6, 7],
       [5, 1, 6, 7]])