Hi,
May be this helps:
dat1<- read.table(text="
a b c d e
1 12 15 65 6
1 65 85 36 5
2 69 84 35 8
2 45 78 65 8
",sep="",header=TRUE)
library(data.table)
dat2<- data.table(dat1)
dat2[,head(sapply(.SD,sum)/sapply(.SD,sum)[4],-1),by="a"]
# a V1
#1: 1 7.000000
#2: 1 9.090909
#3: 1 9.181818
#4: 2 7.125000
#5: 2 10.125000
#6: 2 6.250000
outputdat<-list()
ColNames<-colnames(dat2)
x<- ncol(dat2)-1
ColNames<-colnames(dat2)
x<- ncol(dat2)-1
for(z in 2:x)
{
outputdat[[z]]<-dat2[,sum(get(ColNames[z]))/sum(e),by="a"]
}
do.call(rbind,outputdat)
# a V1
#1: 1 7.000000
#2: 2 7.125000
#3: 1 9.090909
#4: 2 10.125000
#5: 1 9.181818
#6: 2 6.250000
A.K.
----- Original Message -----
From: Camilo Mora <cm...@dal.ca>
To: r-help@r-project.org
Cc:
Sent: Wednesday, March 13, 2013 11:27 PM
Subject: [R] loop in a data.table
I would like to clarify my previous email about using data.table.
imagine the following data.frame called "data":
a b c d e
1 12 15 65 6
1 65 85 36 5
2 69 84 35 8
2 45 78 65 8
I want to aggregate the rows of columns b:d by the rows of column a. the
aggregation is sum(col[b:d]/sum(col[e]).
For this I am using a data.table with a loop of the form:
##########################################
ColNames<-colnames(data) #gets the names of the columns
x=ncol(data)-1 #number of columns to process minus the last column.
data<-data.table(data) #converts to data.table
for (z in 2:x) #I start the loop in the second column and finish in column d
{
outputdata<-data[, sum(get(ColNames[z]))/sum(e), by="a"]
}
############################################
this works fine but the function "get" slowdown the aggregation of the rows by
about 20 times. I wonder if there is an alternative fucntion to "get" or an
alternative way to aggregate all columns at once. I am reading into the
function .SD but have not yet figure out how to put more than one operation in
the function.
right now I have:
###############
outputdata=data[, lapply(.SD, sum), by="a", .SDcols=2:x]
##############
this later code aggregates all columns at once but only by summing. eventually
I need to divide the sum of each column by the sum of column e as well.
ANy help will be greatly appreciate.
Thanks,
Camilo
Camilo Mora, Ph.D.
Department of Geography, University of Hawaii
Currently available in Colombia
Phone: Country code: 57
Provider code: 313
Phone 776 2282
From the USA or Canada you have to dial 011 57 313 776 2282
http://www.soc.hawaii.edu/mora/
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______________________________________________
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
