Getting index of first occurrence of a value in every column of a matrix

Question

If I have a single vector, I can get the 1st occurrence which is below a value:

test <- c(0.5,0.8,0.1,0.08,0.06,0.04)
which(test<0.1)[1]    

Answer 1

Try this:

test2 <- matrix(c(5,8,3,4,7,5,6,2),ncol=2)
> test2
     [,1] [,2]
[1,]    5    7
[2,]    8    5
[3,]    3    6
[4,]    4    2
> foo <- function(x){which(x < 5)[1]}
> apply(test2,2,foo)

The key here being that you take the piece that you know works on a single vector, and simply wrap it in a function. apply will, well, apply that function to each column.