[R] intersect of 2 data frames

[Peter Dalgaard]

Dimitris Rizopoulos wrote:
> look at ?merge(), e.g., try something like the following:
>
> merge(pretestm, posttest, by = "StuNum")
>
> check also the on-line help page for more info, especially for the  
> 'all' argument.
>
>   
Or, to follow the original line of reasoning:

subset(pretest, StuNum %in% common.id)

(This doesn't scale well to large sizes of common.id, but neither did 
the SQL statement...)
>   
>> Greetings!
>>
>> I am a new user of R.  The only one in my department, all others use SPSS.
>>
>> I have two data sets of students with test scores, pretest and
>> posttest.  The first column StuNum and then columns of scores.
>>
>> The problem is that the lists are slightly different not all students
>> are in both data sets.
>>
>> So I did:
>>
>> intersect(pretest$StuNum, posttest$StuNum) -> common.id
>>
>> At this point I need to select from pretest the students in common.id
>> and do the same with posttest.
>>
>> I have tried several things but, basically I do not know how to do so.
>>
>> In SQL it should be something like:
>>
>> select pretest$StuNum in common.id
>>
>>
>> Any pointers?
>>
>> Thanks,
>> Enrico
>>
>>
>> --
>> Enrico Indiogine
>>
>> Mathematics Education
>> Texas A&M University
>> hindiogine at gmail.com
>>
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>>
>>
>>     
>
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