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

Answer 1

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.)