How to concatenate two columns containing list (series) in Pandas

Question

I'd like to concatenate two columns in pandas. Each column consists of a list of floating points of 1x4 elements. I'd like to merge two columns such that the output is a vector of 1x8. The below shows a snippet of the dataframe

ue,bs
"[1.27932459e-01 7.83234197e-02 3.24789420e-02 4.34971932e-01]","[2.97806183e-01 2.32453145e-01 3.10236304e-01 1.69975788e-02]"
"[0.05627587 0.4113416  0.02160842 0.20420576]","[1.64862491e-01 1.35556330e-01 2.59050065e-02 1.42498115e-02]"

To concatenate two columns, I do the following:

df['ue_bs'] = zip(df_join['ue'], df_join['bs'])

With this, I get a new column 'ue_bs' which contains the following for the first row of df['ue_bs']:

(array([1.27932459e-01, 7.83234197e-02, 3.24789420e-02, 4.34971932e-01]),
 array([2.97806183e-01, 2.32453145e-01, 3.10236304e-01, 1.69975788e-02]))

However, they are still two arrays. In order to merge them, I did it as follows:

a = df['ue_bs'][0]
np.concatenate((a[0], a[1]), axis=0)

Then, I got

array([1.27932459e-01, 7.83234197e-02, 3.24789420e-02, 4.34971932e-01,
   2.97806183e-01, 2.32453145e-01, 3.10236304e-01, 1.69975788e-02])

I am wondering is there a neat way of doing this in single line of code, instead of having to loop through df['ue_bs'] and perform np.concatenate()?

Answer 1

To concatinate two lists in python, the easiest way is to use +. The same is true when concating columns in pandas. You can simply do:

df['ue_bs'] = df['ue'] + df['bs']

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If the column type is numpy arrays you can first convert them into normal python lists before the concatination:

df['ue_bs'] = df['ue'].apply(lambda x: x.tolist()) + df['bs'].apply(lambda x: x.tolist())