python - Sum rows where value equal in column

Question

How can I sum across rows that have equal values in the first column of a numpy array? For example:

In: np.array([[1,2,3],
             [1,4,6], 
             [2,3,5],
             [2,6,2],
             [3,4,8]])

Out: [[1,6,9], [2,9,7], [3,4,8]]

Any help would be greatly appreciated.

Answer 1

Pandas has a very very powerful groupby function which makes this very simple.

import pandas as pd

n = np.array([[1,2,3],
             [1,4,6], 
             [2,3,5],
             [2,6,2],
             [3,4,8]])

df = pd.DataFrame(n, columns = ["First Col", "Second Col", "Third Col"])

df.groupby("First Col").sum()