Question

I am trying to make a function that will determine if value in a column of a data frame is a new high. So for example if I had the following data:

x <- rnorm(10,100,sd=5)
x <- data.frame(x)

How can I return, TRUE or FALSE in a new column that only takes into account all the previous values. The resulting table would look something like:

           x   new.max
1  102.42810   NA
2  109.22762   TRUE
3  101.97970   FALSE
4  101.49303   FALSE
5   93.30595   FALSE
6   96.77199   FALSE
7  110.96441   TRUE
8   96.27485   FALSE
9  101.77163   FALSE
10 100.78992   FALSE

If I try

x$new.max <- ifelse ( x$x == max(x$x) , TRUE, FALSE )

The resulting table is below, as it calculates the maximum value of the entire column instead of a subset of all the previous values.

           x new.max
1  102.42810   FALSE
2  109.22762   FALSE
3  101.97970   FALSE
4  101.49303   FALSE
5   93.30595   FALSE
6   96.77199   FALSE
7  110.96441   TRUE
8   96.27485   FALSE
9  101.77163   FALSE
10 100.78992   FALSE

Answer 1

There is a built-in function that computes the running maximum, called cummax().

diff(cummax(x)) will be non-zero at positions where a new maximum is achieved (there’s no entry for the first element of x, which is always a new maximum).

Putting the pieces together:

new.max <- c(TRUE, diff(cummax(x)) > 0)

I’ve set the first element to TRUE, but it could just as well be NA.