Rounding down numbers to the nearest even number

Question

I have a dataframe with a column of numbers that look like this:

data = [291.79, 499.31, 810.93, 1164.25]
df = pd.DataFrame(data, columns=['Onset'])

Is there an elegant way to round down the odd numbers to the nearest even number? The even numbers should be rounded to the nearest even integer. This should be the output:

data = [290, 498, 810, 1164]
df = pd.DataFrame(data, columns=['Onset'])

Thank you!

Answer 1

I'd use modulo operator:

df['Onset_new'] = (i:=df['Onset'].astype(int)) - i % 2
print(df)

Prints:

     Onset  Onset_new
0   291.79        290
1   499.31        498
2   810.93        810
3  1164.25       1164

Answer 2

df.apply(lambda x:np.floor(x)-np.floor(x)%2)