Question

I have an excel csv with a date/time column and a value associated with that date/time. I’m trying to write a script that will go through this format (see below), and find 1) the maximum value per day, and 2) the time on that day that the maximum occurs. Preferably R would return both values to me in a new dataframe.

The data looks something like this:

         V1    V2 V3
1  5/1/2012  3:00  1
2  5/1/2012  6:00  2
3  5/1/2012  9:00  5
4  5/1/2012 12:00  3
5  5/1/2012 15:00  6
6  5/1/2012 18:00  2
7  5/1/2012 21:00  1
8  5/2/2012  0:00  2
9  5/2/2012  3:00  3
10 5/2/2012  6:00  6
11 5/2/2012  9:00  4
12 5/2/2012 12:00  6
13 5/2/2012 15:00  7
14 5/2/2012 18:00  9
15 5/2/2012 21:00  1

So the function I’m envisioning would return:

1 5/1/2012 15:00 6
2 5/2/2012 18:00 9

Any ideas?

Answer 1

here you go:

dat.str <- '         V1    V2 V3
1  5/1/2012  3:00  1
2  5/1/2012  6:00  2
3  5/1/2012  9:00  5
4  5/1/2012 12:00  3
5  5/1/2012 15:00  6
6  5/1/2012 18:00  2
7  5/1/2012 21:00  1
8  5/2/2012  0:00  2
9  5/2/2012  3:00  3
10 5/2/2012  6:00  6
11 5/2/2012  9:00  4
12 5/2/2012 12:00  6
13 5/2/2012 15:00  7
14 5/2/2012 18:00  9
15 5/2/2012 21:00  1'

dat <- read.table(textConnection(dat.str), row.names=1, header=TRUE)

do.call(rbind, 
        by(dat, INDICES=dat$V1, FUN=function(x) tail(x[order(x$V3), ], 1)))