R help - Feb 2010 - Count variables by ID

Hello,

I have an education dataset and I'd like to know how to count the number of
courses each student has taken in a particular subject. For instance, say
the course codes for "History" are 20-26 & 31 and I have the
following data

Student ID  	Course Code
1	                16
1	                17
1	                21
1	                31

2	                10
2	                22
2	                23
2	                24


I would like a separate variable that reads like this
Student ID    Hist Courses
1                  2
2                  3

I've read other "count" tutorials on the forum, but couldn't
still find an
answer to my problem. Thanks for helping out!

Greg
-- 
View this message in context:
http://n4.nabble.com/Count-variables-by-ID-tp1476618p1476618.html
Sent from the R help mailing list archive at Nabble.com.

On Feb 10, 2010, at 5:58 PM, travelmail wrote:
>
> Hello,
>
> I have an education dataset and I'd like to know how to count the  
> number of
> courses each student has taken in a particular subject. For  
> instance, say
> the course codes for "History" are 20-26 & 31 and I have the
> following data
>
> Student ID  	Course Code
> 1	                16
> 1	                17
> 1	                21
> 1	                31
>
> 2	                10
> 2	                22
> 2	                23
> 2	                24
>
>
> I would like a separate variable that reads like this
> Student ID    Hist Courses
> 1                  2
> 2                  3
 > table( courses[courses$CourseCode %in% c(20:26, 31), ]$StudentID)

1 2
2 3

 > data.frame( ctab)
   Var1 Freq
1    1    2
2    2    3

You can rename as you desire but please try to stop using spaces. Very  
unReliable.

>
> I've read other "count" tutorials on the forum, but
couldn't still
> find an
> answer to my problem. Thanks for helping out!
>
> Greg
> -- 
> View this message in context:
http://n4.nabble.com/Count-variables-by-ID-tp1476618p1476618.html
> Sent from the R help mailing list archive at Nabble.com.
>
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David Winsemius, MD
Heritage Laboratories
West Hartford, CT