[R] loop in a data.table

[arun]

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 <cmora at DAL.CA>
To: r-help at 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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