[R] Modifying a data frame based on a vector that contains column numbers

[arun]

HI,
Try this:
 mydf1<- mydf
 mydf1[]<-lapply(1:3,function(i) {mydf[which(i== myindex),i]<-1; mydf[,i]})
 mydf1
#  c1 c2 c3
#1  1 NA NA
#2 NA  1 NA
#3 NA NA  1
#4 NA  1 NA
#5  1 NA NA


 identical(mydf1,mygoal)
#[1] TRUE
A.K.



----- Original Message -----
From: Dimitri Liakhovitski <dimitri.liakhovitski at gmail.com>
To: r-help <r-help at r-project.org>
Cc: 
Sent: Wednesday, March 13, 2013 8:10 PM
Subject: [R] Modifying a data frame based on a vector that contains column numbers

Hello!

# I have a data frame:
mydf<-data.frame(c1=rep(NA,5),c2=rep(NA,5),c3=rep(NA,5))

# I have an index whose length is always the same as nrow(mydf):
myindex<-c(1,2,3,2,1)

# I need c1 to have 1s in rows 1 and 5 (based on the information in myindex)
# I need c2 to have 1s in rows 2 and 4 (also based on myindex)
# I need c3 to have 1 in row 3
# In other words, I am trying to achieve this result:
mygoal<-data.frame(c1=c(1,NA,NA,NA,1),c2=c(NA,1,NA,1,NA),c3=c(NA,NA,1,NA,NA))

I know how to do it with a loop that runs through rows of mydf.
However, in real life I have a huge data frame with tons of rows, dozens of
columns (instead of 3 in this example) - I am afraid it'll take forever.
Any hint on how to do it faster, maybe using subindexing somehow?

Thank you very much!

-- 
Dimitri Liakhovitski

    [[alternative HTML version deleted]]

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