Hello R User

Hi,

You could also use library(data.table) to do this faster.
dat1<-read.table(text="
ID? ? Time
1? ? 3
1? ? 6
1? ? 7
1? ? 10
1? ? 16
2? ? 12
2? ? 18
2? ? 19
2? ? 25
2? ? 28
2? ? 30
",sep="",header=TRUE)
library(data.table)
dat2<-data.table(dat1)
res<-dat2[,Time1:=c(0,diff(Time)),by=ID]
? head(res,3)
? #? ID Time Time1
#1:? 1? ? 3? ?  0
#2:? 1? ? 6? ?  3
#3:? 1? ? 7? ?  1

#Comparing different approaches:
set.seed(55)
dat3<- data.frame(ID=rep(1:1000,each=500),Value=sample(1:800,5e5,replace=TRUE))
dat4<-data.table(dat3)
system.time(dat3$Value1<-ave(dat3$Value,dat3$ID,FUN=function(x) c(0,diff(x))))
#?  user? system elapsed
? # 0.312?  0.000?  0.313

ids<-unique(dat3$ID)
? system.time({
? ? for(id in ids){
? ? dat3$Value[dat3$ID==id]<-c(0,diff(dat3$Value[dat3$ID==id]))
? ? } })
#?  user? system elapsed
# 36.938?  0.868? 37.873

system.time(dat5<-dat4[,Value1:=c(0,diff(Value)),by=ID])
#?  user? system elapsed
? # 0.036?  0.000?  0.037
head(dat5)
#?  ID Value Value1
#1:? 1?  439? ? ? 0
#2:? 1?  175?  -264
#3:? 1? ? 28?  -147
#4:? 1?  634? ? 606
#5:? 1?  449?  -185
#6:? 1? ? 60?  -389
? head(dat3)
#? ID Value Value1
#1? 1? ?  0? ? ? 0
#2? 1? -264?  -264
#3? 1? -147?  -147
#4? 1?  606? ? 606
#5? 1? -185?  -185
#6? 1? -389?  -389

A.K.












----- Original Message -----
From: bibek sharma <mbhpathak at gmail.com>
To: R help <r-help at r-project.org>
Cc:
Sent: Friday, December 14, 2012 10:51 AM
Subject: [R] Hello R User

Hello R User,
In the sample data given below, time is recorded for each id
subsequently. For the analysis, for each id, I would like to set 1st
recorded time to zero and thereafter find the difference from previous
time. I.e. for ID==1, I would like to see Time=0,3,1,3,6. This needs
to be implemented to big data set.
Any suggestions are much appreciated!
Thanks,
Bibek

ID? ? Time
1? ? 3
1? ? 6
1? ? 7
1? ? 10
1? ? 16
2? ? 12
2? ? 18
2? ? 19
2? ? 25
2? ? 28
2? ? 30