How to use "cast" in reshape without aggregation

Question

In many uses of cast I've seen, an aggregation function such as mean is used.

How about if you simply want to reshape without information loss. For example, if I want to take this long format:

ID     condition    Value
John   a            2
John   a            3
John   b            4
John   b            5
John   a            6
John   a            2
John   b            1
John   b            4

To this wide-format without any aggregation:

ID    a  b
John  2  4
John  3  5
Alex  6  1
Alex  2  4

I suppose that this is assuming that observations are paired and you were missing value would mess this up but any insight is appreciated

Two things: 1. your wide-format does not match the long format data since Alex does not appear in the long format. 2. How do you know that a==2 and b==4 go together instead of a==2 and b==5 or some other combination of the values? I'm guessing you are not showing a column that indicates an experiment or some other identifier... — Chase
@Chase my guess is that they just have two rows for each ID*condition combination. But more info from aleph4 wouldn't hurt... — Dason

G. Grothendieck G. Grothendieck · Accepted Answer · 2013-05-12T00:03:42

In such cases you can add a sequence number:

library(reshape2)

DF$seq <- with(DF, ave(Value, ID, condition, FUN = seq_along))
dcast(ID + seq ~ condition, data = DF, value.var = "Value")

The last line gives:

    ID seq a b
1 John   1 2 4
2 John   2 3 5
3 John   3 6 1
4 John   4 2 4

(Note that we used the sample input from the question but the sample output in the question does not correspond to the sample input.)

How to use "cast" in reshape without aggregation

1 Answers