[R] code rows depending on the value of other rows in multilevel dataframe

[Junqian Gordon Xu]

I have a multilevel dataframe (df):

   ID             Date      Segment Slice Tract     Lesion
1 CSPP005 12/4/2007       1     1      LCST         0
2 CSPP005 12/4/2007       1     1      LPC           2
3 CSPP005 12/4/2007       1     1      RPC          3
4 CSPP005 12/4/2007       1     1      RCST        1
5 CSPP005 12/4/2007       1     1      LGM          0
6 CSPP005 12/4/2007       1     1      RGM          0
7 CSPP005 12/4/2007       1     1      Whole        NA

The five levels are:

ID->Date->Segment->Slice->Tract

The observation variable is: Lesion

What I want to do is (in pseudo code)

if  (any of the Tract (for a given ID, Date, Segment, and Slice),
which( Lesion == 2) )
    df [ for that particular (ID, Date,Segment, and slice) ,
which(Tract == Whole) ] $Lesion <- 2
else
    df [ for that particular (ID, Date,Segment, and slice) ,
which(Tract == Whole) ] $Lesion <- 0



I started with (don't know if this is the right path),

Lesion2<-df[which(df$Lesion == 2),]
Where.Lesion2<-unique(Lesion2[,1:4])
Whole<-subset(df, Tract == "Whole")

But stuck at how to match the ID/Date/Segment/Slice from Where.Lesion2 to Whole.

Regards
Gordon