Concatenate arrays in Python - output error

Question

I am writing a program to concatenate two numpy arrays and I want the program to print a message for each possible outcome (horizontal, vertical, or no concatenation). I have the following code and I don´t understand why even when the first condition (np.hstack) is met it continues evaluating the subsequent if and else statements and wrongly prints that there is both a horizontal concatenation (first condition is met) and that a concatenation is not possible.

import numpy as np
def fun1(a,b):
    if a.shape[0] == b.shape[0]:
        print("The horizontal concatenation is:", np.hstack((a,b)))
    if a.shape[1] == b.shape[1]:
        print("The vertical concatenation is:", np.vstack((a,b)))
    else:
        print("These arrays cannot be concatenated.")

a = np.floor(10*np.random.random((3,2)))
b = np.floor(10*np.random.random((3,4)))
fun1(a,b)

Output:

The horizontal concatenation is: [[5. 0. 1. 1. 3. 7.]
                                  [4. 1. 8. 3. 1. 9.]
                                  [9. 1. 5. 7. 0. 0.]]
These arrays cannot be concatenated.

Your second if should probably be an elif. As it reads now you have two separate if statements. — Roger Lindsjö
– Roger Lindsjö, Commented Feb 14, 2019 at 21:54
The if/else structure is insufficient. To avoid the wrong message you would need if/elif/else as written. But this does not cover the case when both concatenations are possible. — mkiever
– mkiever, Commented Feb 14, 2019 at 21:55
@mkiver thanks, I corrected it with an elif instead of the second if and now it works. However, I had not considered the case when there is a "perfect" concatenation on both axes - is there a convention that says which axis takes priority then? — Madrid_datalady
– Madrid_datalady, Commented Feb 14, 2019 at 22:00
@Madrid_datalady See my answer. With 3 if statements both horizontal and vertical concatenation would be printed. — mkiever
– mkiever, Commented Feb 14, 2019 at 22:06

mkiever · Accepted Answer · 2019-02-14 22:08:45Z

1

Instead of the else part you need a third if statement with this condition:

if a.shape[0] == b.shape[0]:
    print("The horizontal concatenation is:", np.hstack((a,b)))
if a.shape[1] == b.shape[1]:
    print("The vertical concatenation is:", np.vstack((a,b)))
if a.shape[0] != b.shape[0] and a.shape[1] != b.shape[1]:
    print("These arrays cannot be concatenated.")

edited Feb 14, 2019 at 22:08

answered Feb 14, 2019 at 21:58

mkiever

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