finding the max value for values the row before through the row after for each row in R

Question

I have looked for an answer high and low. It seems so simple but I am struggling with getting anything to work.

Using R 3.0 in Win 7.

I am looking for a way to find the max value (row by row) for: the row of interest, the row before, and row after.

an example would look something like this:

   x  max
1  1   NA
2  7    7
3  3    7
4  4    5
5  5    5

I could do this with a loop but I would like to avoid that if possible. I have explored things similar to rowSums and rollmean but they do not quite fit the bill since I want a max for a row after it as well.

Any thoughts are greatly appreciated!!

Answer 1

You could use embed and pmax in base R for this.

d <- data.frame(x=c(1,7,3,4,5))
transform(d, max=c(NA, do.call(pmax, as.data.frame(embed(d$x, 3))), NA))
#   x max
# 1 1  NA
# 2 7   7
# 3 3   7
# 4 4   5
# 5 5  NA

Answer 2

Here's an approach using dplyr's lead() and lag() functions:

library(dplyr)

d <- data.frame(x = c(1,7,3,4,5))
mutate(d, max = pmax(lead(x), x, lag(x)))
#>   x max
#> 1 1  NA
#> 2 7   7
#> 3 3   7
#> 4 4   5
#> 5 5  NA

Answer 3

Assuming that you want to do this for a matrix, not for a vector(use rollapply for vectors), this is straightforward solution, probably not the best in terms of the speed:

library(Hmisc)
x <- matrix(runif(10), ncol=2)
rowMaxs  <- apply(x, 1, max)

row3Maxs <- apply(cbind(rowMaxs, 
                        Lag(rowMaxs, 1), 
                        Lag(rowMaxs, -1)), 1, max)

cbind(x, row3Maxs)

however from the performance standpoint the following might be better:

row3Maxsc <- c(NA, 
          sapply(2:(length(rowMaxs)-1), 
                 function(i)
                   max(rowMaxs[i], rowMaxs[i-1], rowMaxs[i+1])
                 ),
          NA)
cbind(x, row3Maxs)