[R] Fastest Way to Divide Elements of Row With Its RowSum

[Thomas Lumley]

On Thu, 17 Sep 2009, William Revelle wrote:

> At 2:40 PM +0900 9/17/09, Gundala Viswanath wrote:
>> I have a data frame (dat). What I want to do is for each row,
>> divide each row  with the sum of its row.
>> 
>> The number of row can be large > 1million.
>> Is there a faster way than doing it this way?
>> 
>> datnorm;
>> for (rw in 1:length(dat)) {
>>     tmp <- dat[rw,]/sum(dat[rw,])
>>     datnorm <- rbind(datnorm, tmp);
>> }
>> 
>> 
>> - G.V.
>
>
> datnorm <- dat/rowSums(dat)
>
> this will be faster if dat is a matrix rather than a data.frame.
>

Even if it's a data frame and he needs a data frame answer it might be faster to do
   mat<-as.matrix(dat)
   matnorm<-mat/rowSums(mat)
   datnorm<-as.data.frame(dat)

The other advantage, apart from speed, of doing it with dat/rowSums(dat) rather than the loop is he gets the right answer. The loop goes from 1 to the number of columns if dat is a data frame and 1 to the number of entries if dat is a matrix, not from 1 to the number of rows.

      -thomas

Thomas Lumley			Assoc. Professor, Biostatistics
tlumley at u.washington.edu	University of Washington, Seattle