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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