In R how do I sample one non-zero element from each column of a matrix and then -1 from it

Question

I have a matrix called MyMatrix:

a<-c(1,2,0)
b<-c(0,5,0)
c<-c(8,9,2)
d<-c(5,2,0)

MyMatrix<-cbind(a,b,c,d)

     a b c d
[1,] 1 0 8 5
[2,] 2 5 9 2
[3,] 0 0 2 0

From MyMatrix I would like to -1 from an element in every column, but only if the element is >0.

For example, the resulting matrix might be:

     a b c d
[1,] 0 0 8 5
[2,] 2 4 8 1
[3,] 0 0 2 0

How might I achieve this?

Answer 1

Try this:

safeSample<-function(x) {if (length(x)==1) x else sample(x, 1)}
apply(MyMatrix, 2, 
      function(x) {
          ind<-safeSample(which(x>0)) 
          x[ind]<-x[ind]-1 
          x
      }
)