I have a vector:

df <- c(5,9,-8,-7,-1)

How can I identify the position prior to a change in sign? ie df[2]

• r

This is pretty simple, if you know about the sign function...

which(diff(sign(df))!=0)
# [1] 2

I prefer Joshua's answer, but here's an alternative, more complicated one just for fun:

head(cumsum(rle(sign(df))$lengths),-1)

If you want to be a terrible person, you could always use a for loop:

signchange <- function(x) {
    index = 0
    for(i in 1:length(x))
    {
        if(x[i] < 0)
        {
            return (index)
        }
        else
        {
            index = index + 1
        }
    }
    return (index)
}