Converting python loop output into an array

Question

I am trying to convert a python from loop output into an array. For example in the following code, I wanted the output as b = [6.0, 14.0, 0.0, 0.0, 0.0] but it gives the output as a column.

import numpy as np
j  = np.arange(1.5, 10.0, 2)
for m in j: 
    a = 2*m    
    if a <= 8:
        b = 2*a
        print(b)        
    else:
         b = 0.0
         print(b)

I have tried to define the output b as numpy.array but it does not work. Any idea?

You print one value after the other. You have multiple options - if it's just about how it's displayed, you could tell print to not print newlines and add a space. Otherwise, you'd probably want to save the values of b in a list and in the end print that - or do the whole loop as a list comprehension instead. Do you want to continue processing it later, or do you just care about printing? And will the loop remain this simple? — lucidbrot
– lucidbrot, Commented Apr 16, 2021 at 6:46
Thank you. I want to continue processing it later. I put the print just to see how it looks. — hm1
– hm1, Commented Apr 16, 2021 at 6:55

joao · Accepted Answer · 2021-04-16 06:58:14Z

1

It's good practice to separate the creation of a data structure from whatever use you make of it, such as printing it, so you could create the b first, then print it:

import numpy as np
j  = np.arange(1.5, 10.0, 2)
b = []
for m in j: 
    a = 2*m    
    if a <= 8:
        b.append(2*a)
    else:
         b.append(0.0)
print(b)

As it happens, python's default is to print the array horizontally:

[6.0, 14.0, 0.0, 0.0, 0.0]

answered Apr 16, 2021 at 6:58

joao

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4 Comments

tobias_k Over a year ago

Or as a list comprehension: [4*n*(n<=4) for n in j]

joao Over a year ago

Multiplying by the boolean, that's a neat trick, first time I've seen it :-)

hm1 Over a year ago

tobias_k, must admit your solution is very unique!

tobias_k Over a year ago

Of course you can also use a more conventional list comprehension with a ternary expression like [4*n if n <= 4 else 0.0 for n in j] instead of that "multiply by boolean" trick.

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