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How to get values in an array using another array with the indexes of the values?

April 23, 2022

I have an array of floats (size 9300), and another array of ints (size 2600) with indexes of the first array. I am trying to get the values in the first array based on the index in the second array. Example:

index   1st_array
0       12.00
1       3.89 
2       24.20
3       34.60

index  2nd_array
0       0
1       2

The output:

index      3nd_array
0          12.00
1          24.20

>Solution :

If you have two Series s1 and s2, use:

s1.loc[s2]

Example:

s1 = pd.Series(['a', 'b', 'c', 'd'])
s2 = pd.Series([0, 2])

s1.loc[s2]
0    a
2    c
dtype: object

For numpy arrays

a1[a2]

Example:

a1 = np.array(['a', 'b', 'c', 'd'])
a2 = np.array([0, 2])

a1[a2]
array(['a', 'c'], dtype='<U1')

For DataFrames:

df1.set_index('index').loc[df2['2nd_array']].reset_index()

NB. the set_index/reset_index are only required if "index" are columns

Example:

df1 = pd.DataFrame({'index': [0, 1, 2, 3], 
                    '1st_array': [12.0, 3.89, 24.2, 34.6]})

df2 = pd.DataFrame({'index': [0, 1],
                    '2nd_array': [0, 2]})

(df1
 .set_index('index')
 .loc[df2['2nd_array']]
 .reset_index()
 )

   index  1st_array
0      0       12.0
1      2       24.2