unexpected output with pandas merge_asof (missing rows)-CodePudding

I want to merge the two data frames based on the nearest timestamp, the case is as following

Time stamp  Value1  Time Stamp 2    Value 2
13:30   A   13:32   40
13:30   B       
13:30   C       
13:30   D       
13:30   E       
13:30   F

the above is the dataframe where value 1 is (A,B,C,D,E) and value 2 is 40 with a different time stamp, i want to merge the 40 with (A,B,C,D,E) so i get the following result:

13:30   A   40
13:30   B   40
13:30   C   40
13:30   D   40
13:30   E   40
13:30   F   40

I have already used merge_asof method and i got this result

13:30   F   40

I used the following code:

df = pd.merge_asof(df_downtime,df_centerline,on='Time',direction='nearest',by='Desc Variable',allow_exact_matches=False)

CodePudding user response：

Although your example is ambiguous and not reproducible, if you obtained the result you provided it is likely that you have used an incorrect order of the parameters.

merges_asof is a left merge, so you should probably use:

df = pd.merge_asof(df_centerline, df_downtime,
                   on='Time', direction='nearest',
                   by='Desc Variable', allow_exact_matches=False)