np.append only giving last value appended?

Question

In the code below I tried to use np.append but it only gives me the last value appended in the loop.

I have two nested for loops (one runs with index i , the other one with k ). Now I want to exercise a certain function for different indices and I want the outcomes to be summarized in one array. So that each line in the array contains the outcome for one index.

Here a minimal reproducible example:

import numpy as np

a = np.arange(0,2,1)
b = np.arange(3,5,1)
c = np.array([])

for i in range(0,2,1):
    for k in range(0,2,1):
        c = np.append(a[i],b[k])
print(c)

The outcome is [1 4] . But I want one single vector c containing ([0 3][0 4][1 3][1 4])

Answer 1

You are confused with syntax for np.append()

Use below for correct implementation:

c = np.append(c,[a[i],b[k]])

Also for that you will get the output printed as:

[0. 3. 0. 4. 1. 3. 1. 4.]

I'm confused for the vector you are talking about: ([0 3][0 4][1 3][1 4])

The representation stated by you is not a vector but seems as a matrix with dimension 4x2 , where .reshape() might help you.

If you want that just write: c.reshape((4,2)) which will get you:

[[0. 3.]
 [0. 4.]
 [1. 3.]
 [1. 4.]]

Answer 2

You are creating 1d array instead of 2d. and in np.append you have to pass current array and appending elements and it will return a new array.

a = np.arange(0,2,1)
b = np.arange(3,5,1)
c = np.empty(shape=[0, 2])

for i in range(0,2,1):
    for k in range(0,2,1):
        c = np.append(c,[[a[i],b[k]]],axis=0)
        
print(c)

Outptut:

[[0. 3.]
 [0. 4.]
 [1. 3.]
 [1. 4.]]