Rounding down values in Pandas dataframe column with NaNs

You could just apply numpy.floor;

import numpy as np

tempDF['int_measure'] = tempDF['measure'].apply(np.floor)

    id  measure  int_measure
0   12      3.2            3
1   12      4.2            4
2   12      6.8            6
...
9   51      2.1            2
10  51      NaN          NaN
11  51      3.5            3
...
19  91      7.3            7

You could also try:

df.apply(lambda s: s // 1)

Using np.floor is faster, however.