nrow(matrix) function

Question

I have assignment using R and have a little problem. In the assignment several matrices have to be generated with random number of rows and later used for various calculations. Everything works perfect, unless number of rows is 1.

In the calculations I use nrow(matrix) in different ways, for example if (i <= nrow(matrix) ) {action} and also statements like matrix[,4] and so on.

So in case number of rows is 1 (I know it is actually vector) R give errors, definitely because nrow(1-dimensional matrix)=NULL. Is there simple way to deal with this? Otherwise probably whole code have to be rewritten, but I'm very short in time :(

Answer 1

It is not that single-row/col matrices in R have ncol/nrow set to NULL -- in R everything is a 1D vector which can behave like matrix (i.e. show as a matrix, accept matrix indexing, etc.) when it has a dim attribute set. It seems otherwise because simple indexing a matrix to a single row or column drops dim and leaves the data in its default (1D vector) state.

Thus you can accomplish your goal either by directly recreating dim attribute of a vector (say it is called x):

dim(x)<-c(length(x),1)
x #Now a single column matrix

dim(x)<-c(1,length(x))
x #Now a single row matrix

OR by preventing [] operator from dropping dim by adding drop=FALSE argument:

x<-matrix(1:12,3,4)
x                #OK, matrix
x[,3]            #Boo, vector
x[,3,drop=FALSE] #Matrixicity saved!