finding the index of a max value in R

Question

I have the following data frame called surge:

MeshID    StormID Rate Surge Wind
1         1412 1.0000E-01   0.01 0.0
2         1412 1.0000E-01   0.03 0.0
3         1412 1.0000E-01   0.09 0.0
4         1412 1.0000E-01   0.12 0.0
5         1412 1.0000E-01   0.02 0.0
6         1412 1.0000E-01   0.02 0.0
7         1412 1.0000E-01   0.07 0.0
1         1413 1.0000E-01   0.06 0.0
2         1413 1.0000E-01   0.02 0.0
3         1413 1.0000E-01   0.05 0.0

I used the following code to find the max value of surge per storm:

MaxSurge <- data.frame(tapply(surge[,4], surge[,2], max))

It returns:

1412 0.12
1413 0.06

This is great, except I'd also like it to include the MeshID value at the point where the surge is the maximum. I know I can probably use which.max, but I can't quite figure out how to put this in action. I'm VERY new to R programming.

Answer 1

And a data.table solution for coding elegance

library(data.table)
surge <- as.data.table(surge)
surge[, .SD[which.max(surge)], by = StormID]

Answer 2

here is another data.table solution, but not relying on .SD (thus 10x faster)

surge[,grp.ranks:=rank(-1*surge,ties.method='min'),by=StormID]
surge[grp.ranks==1,]

Answer 3

If you have 2 data.points at the maximum, which.max will only refer to the first one. A more complete solution would involve rank:

# data with a tie for max  
surge <- data.frame(MeshID=c(1:7,1:4),StormID=c(rep(1412,7),
rep(1413,4)),Surge=c(0.01,0.03,0.09,0.12,0.02,0.02,0.07,0.06,0.02,0.05,0.06))

# compute ranks  
surge$rank <- ave(-surge$Surge,surge$StormID,FUN=function(x) rank(x,ties.method="min"))
# subset on the rank  
subset(surge,rank==1)
   MeshID StormID Surge rank
4       4    1412  0.12    1
8       1    1413  0.06    1
11      4    1413  0.06    1