R help - Oct 2011 - Calculating difference between values in data frame based on separate column

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

	[[alternative HTML version deleted]]

Hi,

It shouldn't be so complicated. What about simply:
> td
  vial measure value
2    1       A    12
1    1       B    26
4    2       A    30
3    2       B    45
6    3       A    27
5    3       B    32
8    4       A     6
7    4       B    34
> td <- td[order(td$vial, td$measure),] # make sure the samples are in
order
> # how to get the differences
> td[td$measure == "B", "value"] - td[td$measure ==
"A", "value"]
[1] 14 15  5 28
>
> td.diff <- data.frame(vial = td[td$measure == "A",
"vial"], diff = td[td$measure == "B", "value"] -
td[td$measure == "A", "value"])
> td.diff
  vial diff
1    1   14
2    2   15
3    3    5
4    4   28
>
Sarah

On Fri, Oct 21, 2011 at 6:31 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
>
-- 
Sarah Goslee
http://www.functionaldiversity.org

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
>
> ? ? ? ?[[alternative HTML version deleted]]
>
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