[R] Creating a new by variable in a dataframe

[arun]

Hi,

May be this helps you: 

dat1<-read.table(text="
tdate  event_tim  transaction
1/10/2012   2   14
1/10/2012   4   28
1/10/2012   6   42
1/10/2012   8   14
2/10/2012   6   46
2/10/2012   9   64
2/10/2012   8   71
3/10/2012  3   85
3/10/2012   1   14
3/10/2012   4   28
9/10/2012   5   51
9/10/2012   9   66
9/20/2012  12   84
",sep="",header=TRUE,stringsAsFactors=FALSE)
dat2<-dat1[with(dat1,order(tdate,event_tim)),]
dat2$tdate<-as.Date(dat2$tdate,format="%m/%d/%Y")
dat3<-dat2
 dat3$last_trans<-NA
library(plyr)
dat4<-merge(dat3,ddply(dat2,.(tdate),tail,1))
dat4$last_trans<-dat4$transaction
 res<-merge(dat4,dat2,all=TRUE)
 res
#        tdate event_tim transaction last_trans
#1  2012-01-10         2          14         NA
#2  2012-01-10         4          28         NA
#3  2012-01-10         6          42         NA
#4  2012-01-10         8          14         14
#5  2012-02-10         6          46         NA
#6  2012-02-10         8          71         NA
#7  2012-02-10         9          64         64
#8  2012-03-10         1          14         NA
#9  2012-03-10         3          85         NA
#10 2012-03-10         4          28         28
#11 2012-09-10         5          51         NA
#12 2012-09-10         9          66         66
#13 2012-09-20        12          84         84





----- Original Message -----
From: ramoss <ramine.mossadegh at finra.org>
To: r-help at r-project.org
Cc: 
Sent: Friday, October 19, 2012 1:51 PM
Subject: [R] Creating a new by variable in a dataframe

Hello,

I have a dataframe w/ 3 variables of interest: transaction,date(tdate) &
time(event_tim).
How could I create a 4th variable (last_trans) that would flag the last
transaction of the day for each day?
In SAS I use:
proc sort data=all6;
by tdate event_tim;
run;
         /*Create last transaction flag per day*/
data all6;
  set all6;
  by tdate event_tim;
  last_trans=last.tdate;

Thanks ahead for any suggestions.



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