Create vector satisfying two factor conditions in a data frame

Question

I'm new to R and having a heck of a time grappling with the syntax. Let's say I've got the following data frame data:

value   label    second
1       a        q
2       a        q
3       a        ASDF
4       b        q
6       b        QWERTY
6       b        QWERTY
7       c        q
8       c        q
9       c        q
10      d        q

Now, I want to get a vector of df$second which correspond to the maxima of df$value for a given value of df$label. So for instance, given df$label = 'a', I want to return 'ASDF'. For df$label = 'b', I want to return 'QWERTY', 'QWERTY'.

Here's what I'm trying:

max_value <- max(data$value[data$label == 'a'])
    result <- c()
    for (x in data$value){
        if (x == max_value){
            result <- c(result, data$second)
        }
    }

Now this does not generate the proper results vector. I'd like to figure out a way to do this with sapply, tapply, mapply etc. I'm just having trouble getting my head around these functions. Any help would be greatly appreciated.

Answer 1

Straight forward in data.table:

library(data.table)
DT <- data.table(df, key="label")
DT[.(lab)][value==max(value), second]

# where `lab` is whatever label value you are trying to find

Note that if you want to do this for all values of label, just use the by argument:

DT[, c(.SD, mx=max(value)), by=label][value==mx, second, by=label]

   label second
1:     a   ASDF
2:     b QWERTY
3:     b QWERTY
4:     c      q
5:     d      q