Question

I have a matrix (1051*1051) that has 0zeros along its diagonal and values greater than 0zero everywhere else. The goal is to conditionally reassign some values in the matrix. For example, the criteria I would like to implement is this: If any element is greater than, say, 400, then that row/column element will be assigned a 0zero value.

This is how my code is setup as of now:

dl <- 400       # condition

for( i in 1:dim(DIST)[1] ) {
    for( j in 1:dim(DIST)[1] ) {
        if( DIST[i,j] > dl ) {
             DIS[i,j] <- 0
        }
    }
}

DIST is the original matrix (1051*1051).

DIS is the copy of DIST and to be edited.

My question:
Is there any other way to do this? A faster way?

I have read that loops in R should be avoided. If anyone has a more efficient way please share.

Thank you.

Answer 1

Just use [] assignment:

DIST[DIST>400] <- 0

See ?'[' for how this works. The key is that DIST>400 produces a logical vector of length length(DIST) (the number of elements in DIST), consisting of TRUE if the element is >400 and FALSE otherwise. That vector is then used to subset the matrix, and only the selected elements get assigned to.