# [R] Speeding up a loop

wwreith reith_william at bah.com
Fri Jul 20 20:34:36 CEST 2012

```That is faster than what I was doing and reducing 15% of my iterations it

Next question.

I need to multiply each row x[i,] of the matrix x by another matrix A.
Specifically

for(i in 1:n)
{
If (x[i,]%*%A[,1]<.5 || x[i,]%*%A[,2]<42 || x[i,]%*%A[,3]>150)
{
x<-x[-i,]
n<-n-1
}. #In other words remove row i from x if it does not meet criteria (>=.5,
>=42, <=150). When multiplied to A
}
Is there a better way than using a for loop for this or x<-x[-i,] for that
matter? I assume building a new matrix would be worse.

Ideally I want to also exclude some x[,i] as well example if x[1,] is better
than x[2,] in all three categories i.e. bigger, bigger, and smaller than
x[2,] when multiplied to A then I want to exclude x[2,] as well. Any
suggestions on whether it is better to do this all at once or in stages?

Thanks for helping!

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