R help - Mar 2011 - How create vector that sums correct responses for multiple subjects?

I have a data file with indicates pretest scores for a linguistics
experiment.  The data are in long form so for each of 33 subjects there are
400 rows, one for each item on the test, and there is a column called
‘Correct’ that shows ‘C’ for a correct response and ‘E’ for an incorrect
response.  I am trying to write a formula that will create a vector that
indicates the number of correct answers for each subject.



The following:



nrow(pretestdata[(pretestdata$Subject=="1" &
pretestdata$Correct=="C"),])



gives the number of correct responses for subject 1, but I would like a
vector that indicates the number correct for each of 33 subjects.



 Thank you sincerely for your assistance.



Kevin Burnham

	[[alternative HTML version deleted]]

Try this:

tapply(mydata$Correct, mydata$Subject, function(x)sum(x == "C"))

Sarah

On Thu, Mar 24, 2011 at 3:24 PM, Kevin Burnham <kburnham at gmail.com>
wrote:
> I have a data file with indicates pretest scores for a linguistics
> experiment. ?The data are in long form so for each of 33 subjects there are
> 400 rows, one for each item on the test, and there is a column called
> ?Correct? that shows ?C? for a correct response and ?E? for an incorrect
> response. ?I am trying to write a formula that will create a vector that
> indicates the number of correct answers for each subject.
>
>
>
> The following:
>
>
>
> nrow(pretestdata[(pretestdata$Subject=="1" &
pretestdata$Correct=="C"),])
>
>
>
> gives the number of correct responses for subject 1, but I would like a
> vector that indicates the number correct for each of 33 subjects.
>
>
>
> ?Thank you sincerely for your assistance.
>
>
>
> Kevin Burnham
>
-- 
Sarah Goslee
http://www.functionaldiversity.org

Hi Kevin,

Try this (untested):

sapply(split(pretestdata, Subject), function(l) with(l$Correct ==
"C"))

HTH,
Jorge


On Thu, Mar 24, 2011 at 3:24 PM, Kevin Burnham <> wrote:
> I have a data file with indicates pretest scores for a linguistics
> experiment.  The data are in long form so for each of 33 subjects there are
> 400 rows, one for each item on the test, and there is a column called
> ‘Correct’ that shows ‘C’ for a correct response and ‘E’ for an incorrect
> response.  I am trying to write a formula that will create a vector that
> indicates the number of correct answers for each subject.
>
>
>
> The following:
>
>
>
> nrow(pretestdata[(pretestdata$Subject=="1" &
pretestdata$Correct=="C"),])
>
>
>
> gives the number of correct responses for subject 1, but I would like a
> vector that indicates the number correct for each of 33 subjects.
>
>
>
>  Thank you sincerely for your assistance.
>
>
>
> Kevin Burnham
>
>        [[alternative HTML version deleted]]
>
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
>
	[[alternative HTML version deleted]]

On Thu, Mar 24, 2011 at 2:24 PM, Kevin Burnham <kburnham at gmail.com>
wrote:
> I have a data file with indicates pretest scores for a linguistics
> experiment. ?The data are in long form so for each of 33 subjects there are
> 400 rows, one for each item on the test, and there is a column called
> ?Correct? that shows ?C? for a correct response and ?E? for an incorrect
> response. ?I am trying to write a formula that will create a vector that
> indicates the number of correct answers for each subject.
>
> nrow(pretestdata[(pretestdata$Subject=="1" &
pretestdata$Correct=="C"),])
>
>
>
> gives the number of correct responses for subject 1, but I would like a
> vector that indicates the number correct for each of 33 subjects.
How about

with(pretestdata, table(Subject, Correct))

?

Hadley

-- 
Assistant Professor / Dobelman Family Junior Chair
Department of Statistics / Rice University
http://had.co.nz/