Dcast Warning: 'Aggregation Function Missing: Defaulting to Length'

R reshape2 'Aggregation function missing: defaulting to length'

Thanks to @akrun who pointed it out.

Well, there's a high chance that your data has duplicate row that look either like this:

student    test    score
Adam Exam1 80
Adam Exam1 85
Adam Exam2 90
John Exam1 70
John Exam2 60

Or like this:

student   class     test    score
Adam Biology Exam1 80
Adam Theology Exam1 85
Adam Theology Exam2 90
John Biology Exam1 70
John Theology Exam2 60

When you cast it like this: dcast(data, student + class ~ test, value.var='score')

Can dcast be used without an aggregate function?

I don't think there is a way to do it directly but we can add in an additional column which will help us out

df2 <- structure(list(id = c("A", "B", "C", "A", "B", "C", "C"), cat = c("SS", 
"SS", "SS", "SV", "SV", "SV", "SV"), val = c(220L, 222L, 223L,
224L, 225L, 220L, 1L)), .Names = c("id", "cat", "val"), class = "data.frame", row.names = c(NA,

# Add a variable for how many times the id*cat combination has occured
tmp <- ddply(df2, .(id, cat), transform, newid = paste(id, seq_along(cat)))
# Aggregate using this newid and toss in the id so we don't lose it
out <- dcast(tmp, id + newid ~ cat, value.var = "val")
# Remove newid if we want
out <- out[,-which(colnames(out) == "newid")]
> out
# id SS SV
#1 A 220 224
#2 B 222 225
#3 C 223 220
#4 C NA 1

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