Aggregation requires fun.aggregate: length used as default

Question

I have a file that I would like to reshape it to use R: These are the commands that I am running.

x <- data.frame(read.table(\"total.txt\", sep=\",\", header         


        

Answer 1

Clarification: In the discussion below, I refer to dcast() rather than cast(). As Maiasaura notes in the comments, the function cast() from the reshape package has been replaced in the reshape2 package by two functions: dcast() (for data.frame output) and acast() (for array or matrix output). In any case, my comments about the need for a fun.aggregate argument hold equally for cast(), dcast(), and acast().

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The error is being thrown because for at least one combination of the categorical variables in the call to cast(), your data.frame y must contain at least two rows of data. As documented in ?cast (or ?dcast):

If the combination of variables you supply does not uniquely identify one row in the original data set, you will need to supply an aggregating function, ‘fun.aggregate’.

Run the code below to see how this works, and how it can be remedied. In the last line of code, I use the fun.aggregate argument to tell dcast() to use mean() to combine values for any repeated combination of variables. In its place, you can put whatever aggregation function best fits your own situation.

library(reshape2)

## A toy dataset, with one row for each combination of variables
d <- expand.grid(Hostname = letters[1:2],
                 Date = Sys.Date() + 0:1,
                 MetricType = LETTERS[3:4])
d$Value <- rnorm(seq_len(nrow(d)))

## A second dataset, in which one combination of variables is repeated
d2 <- rbind(d, d[1,])

## Runs without complaint
dcast(d, Hostname + Date ~ MetricType)

## Throws error asking for an aggregation function
dcast(d2, Hostname + Date ~ MetricType)

## Happy again, with a supplied aggregation function
dcast(d2, Hostname + Date ~ MetricType, fun.aggregate=mean)