[R] find most repeated item from column in dataframe

[Bill.Venables at csiro.au]

Do you expect this to be easy?  It may be, but I can't see a particularly graceful way to do it.  Here is one possible solution.

> dat
   StandID PlotNum HerbNum  Woody
1      001       1       1    low
2      001       2       2 medium
3      001       3       1    low
4      001       4       3    low
5      001       5       1   high
6      001       6       2 medium
7      002       1       1   high
8      002       2       2   high
9      002       3       2    low
10     002       4       3   high
11     002       5       1   high
12     002       6       2 medium
> getMostCommon <- function(x) {
	tx <- table(x)
	m <- which(tx == max(tx))[1]
	as(names(tx)[m], class(x))
}
> val <- unclass(by(dat[,-1], dat$StandID, function(x) lapply(x, getMostCommon)))
> (newDat <- cbind(StandID = names(val), as.data.frame(do.call(rbind, val))))
    StandID PlotNum HerbNum Woody
001     001       1       1   low
002     002       1       2  high

This sort of gets you the answer, but it is not quite what it seems.  One way to make it more manageable is

> for(j in 2:ncol(newDat)) newDat[[j]] <- unlist(newDat[[j]])
> newDat
    StandID PlotNum HerbNum Woody
001     001       1       1   low
002     002       1       2  high

This is now a data frame with columns (more or less) what they appear to be.

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of Randy Cass
Sent: Wednesday, 25 August 2010 11:33 AM
To: r-help at r-project.org
Subject: [R] find most repeated item from column in dataframe

R users,

I am trying to find some way to find the value of a column that is repeated
the most for each StandID of a dataframe.  I have research methods online
and the help page, but have had no success in finding a solution.  I have
tried using the table function but it returns items for the whole dataset
and not by the StandID.  Any help will be appreciated.  Thanks in advance.

R version 2.11.1
Windows 7
Dataframe is imported from text file

StandID     PlotNum    HerbNum      Woody
001            1               1                    low
001            2               2                    medium
001            3               1                    low
001            4               3                    low
001            5               1                    high
001            6               2                    medium
002            1               1                    high
002            2               2                    high
002            3               2                    low
002            4               3                    high
002            5               1                    high
002            6               2                    medium

I would like to get the following from the dataframe

StandID    HerbNum      Woody
001          1                    low
002          2                    high

Thanks,

Randy

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