[R] Calculating difference between values in data frame based on separate column

[Dennis Murphy]

Here's another way, using the reshape2 package:

library(reshape2)
d <- dcast(df, vial ~ measure, value_var = 'value')
d$diff <- with(d, B - A)
> d
  vial  A  B diff
1    1 12 26   14
2    2 30 45   15
3    3 27 32    5
4    4  6 34   28

HTH,
Dennis

On Fri, Oct 21, 2011 at 3:31 PM, Nathan Miller <natemiller77 at gmail.com> wrote:
> Hi all,
>
> Say I have a data frame something like the one below with different sample
> vials, measured before(B) and after(A) some process, with a value recorded
> at each measurement point
>
> vial    measure    value
> 1       B                26
> 1       A                12
> 2       B                 45
> 2       A                 30
> 3       B                 32
> 3       A                 27
> 4       B                 34
> 4       A                 6
>
> Is there an easy means by which I can subtract the after (A) measurements
> from the before (B) measurement for each vial in this data frame so I am
> left with a data frame that contains the vial number and difference between
> A and B? I've played around with writing a function and applying it with
> ddply, but haven't stumbled on a technique that works, though I feel there
> should be something simple means of doing this that I'm just not seeing.
>
> Thanks for you help,
> Nate
>
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>
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