Reshape2 Aggregation function missing: defaulting to length. Need help understanding what is going on

Question

I am trying to take a very large and messy dataset, melt the data, then dcast it. I keep running into this "Aggregation function missing: defaulting to length" problem and I don't fully understand what the issue is. I have found a possible solution, but before I put my solution into production, I was hoping someone could explain what is going on.

code to build the DF:

 z <- data.frame(ID = c("A","A","A","A","A","A","A","A"), 
 Disease =      "Flu",variable = "event_date", value = "11/30/15")

what I thought would work

dcast(z, ID + Disease ~ variable)

But this code produces the Aggregation function missing: defaulting to length and outputs:

ID     Disease     event_date
A       Flu          8

when I use

dcast(z, ID + Disease ~ variable, fun.aggregate = levels)

I get the correct output of

ID     Disease     event_date
A       Flu          11/30/15

but I'm not confident that using fun.aggregate = levels is appropriate. Any advice or education would be much appreciated. Thanks!

Your LHS of the formula is not unique as an ID, so it defaults to `length`. — A5C1D2H2I1M1N2O1R2T1, Feb 05 '16 at 15:28
What part of it isn't unique though? All 8 rows are duplicates — Eric Bakota, Feb 05 '16 at 15:31
Um.... exactly. If it's a duplicate, how can it be unique? Anyway, I think it's addressed in the *duplicate* question/answer. — A5C1D2H2I1M1N2O1R2T1, Feb 05 '16 at 15:33
oh man, I feel really silly. I think I understand the error in my thinking. I was, for reasons unknown, under the impression that dcast would automatically deduplicate. I really appreciate your feedback, thanks! — Eric Bakota, Feb 05 '16 at 15:37
Anyway, if it's just this transformation you want, then just try something like: `dcast(unique(z), ID + Disease ~ variable)`. — A5C1D2H2I1M1N2O1R2T1, Feb 05 '16 at 15:37
You also need to think about what you want to have happen if you have multiple values for each ID + Disease combination. In that case, you can create a secondary ID to help resolve the issue. — A5C1D2H2I1M1N2O1R2T1, Feb 05 '16 at 15:39

Reshape2 Aggregation function missing: defaulting to length. Need help understanding what is going on

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