[R] Problem with ddply in the plyr-package: surprising output of a date-column

From: Christoph Jäckel <christoph.jaeckel_at_wi.tum.de>
Date: Mon, 25 Apr 2011 19:19:27 +0200

Hi Together,

I have a problem with the plyr package - more precisely with the ddply function - and would be very grateful for any help. I hope the example here is precise enough for someone to identify the problem. Basically, in this step I want to identify observations that are identical in terms of certain identifiers (ID1, ID2, ID3) and just want to save those observations (in this step, without deleting any rows or manipulating any data) in a separate data.frame. However, I get the warning message below and the column with dates is messed up. Interestingly, the value column (the type is factor here, but if you change that with as.integer it doesn't make any difference) is handled correctly. Any idea what I do wrong?

df <- data.frame(cbind(ID1=c(1,2,2,3,3,4,4),ID2=c('a','b','b','c','d','e','e'),ID3=c("v1","v1","v1","v1","v2","v1","v1"),


df[,1] <- as.character(df[,1])
df[,2] <- as.character(df[,2])
df$Date <- strptime(df$Date,"%Y-%m-%d")

#Apparently there are two observation that have the same IDs: ID1=2 and ID1=4
#I want to save those IDs in a separate data.frame, so the desired output is:

#My idea: Write a custom function that only returns observations with
multiple rows.
#Seems to work except that the Date column doesn't make any sense anymore

#Warning message: In output[[var]][rng] <- df[[var]]: number of items
to replace is not a multiple of replacement length ddply(df,.(ID1,ID2,ID3),function(df) if(nrow(df)<=1){NULL}else{df})

#Notice that it works perfectly if I only have one observation with
multiple rows
ddply(df[1:6,],.(ID1,ID2,ID3),function(df) if(nrow(df)<=1){NULL}else{df})

Thanks in advance,


Christoph Jäckel (Dipl.-Kfm.)

Research Assistant

Chair for Financial Management and Capital Markets | Lehrstuhls für Finanzmanagement und Kapitalmärkte

TUM School of Management | Technische Universität München

Arcisstr. 21 | D-80333 München | Germany

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