Pandas Dataframe Fails To Assign Value To Slice Subset

I'm trying to change all values in the slice except the first one but it does not work... what am i doing wrong ? print(test) test.loc[(test.col_1==-5)&(test.index>'2018-07-

Solution 1:

You can use numpy.where and use indexing [1:] to exclude the first time the criterion is True. Here's a minimal example:

df = pd.DataFrame([[1, -5], [2, -5], [3, -1], [4, -5], [5, -5], [6, -1]],
                  columns=['col1', 'col2'])

df.iloc[np.where(df['col1'].between(2, 5))[0][1:], 1] = -1print(df)

   col1  col2
01-512-523-134-145-156-1

Solution 2:

There is problem join boolean indexing (filtering) with selecting, one possible solution is add new condiction:

test.index=pd.to_datetime(test.index)mask=(test.col_1==-5)&(test.index>'2018-07-1713:00:00')&(test.index<'2018-07-1714:00:00')m1=np.arange(len(test))>1test.loc[mask&m1,'col_1']=-1print(test)col_12018-07-17 13:51:00     -52018-07-17 13:52:00     -12018-07-17 13:53:00     -12018-07-17 13:54:00     -12018-07-17 13:55:00     -12018-07-17 13:56:00     -12018-07-17 13:57:00     -12018-07-17 13:58:00     -12018-07-17 13:59:00     -1