[R] reshape a matrix

[David Winsemius]

On Aug 20, 2011, at 1:21 PM, Uwe Ligges wrote:

>
>
> On 20.08.2011 19:04, David Winsemius wrote:
>>
>> On Aug 20, 2011, at 12:32 PM, Uwe Ligges wrote:
>>
>>> On 20.08.2011 17:04, Wendy wrote:
>>>> Hi all,
>>>>
>>>> I have a data.frame like following
>>>>
>>>> A<-c('d0','d0','d1','d1','d2','d2')
>>>> B<-rep(c('control','sample'),3)
>>>> C<-c(rep(100000,2),200,300,400,500)
>>>> dataframe<-data.frame(A,B,C)
>>>>
>>>> I want to reshape the matrix, so the matrix with 'd0', 'd1' and  
>>>> 'd2'
>>>> in rows
>>>> and 'control' and 'sample' in columns. Is there a function for  
>>>> doing
>>>> this
>>>> easily?
>>>
>>> See ?reshape
>>>
>>> reshape(data=dataframe, idvar="A", timevar="B", direction="wide")
>>>
>>> Uwe Ligges
>>
>> Many people have experienced problems understanding the mechanics  
>> of the
>> base function 'reshape'. If you do not and if do continue to use  
>> it, you
>> would be doing the world a great service by writing a tutorial manual
>> with a bunch of worked examples. I have never found a tutorial that
>> clarified how I should use it in the variety of situations where I  
>> have
>> needed it.
>
> David,
>
> I think there are some good examples on the help page. What is  
> missing? What is not clearly explained?

The stumbling blocks I have encountered are in trying to figure out  
which of the multiple arguments are needed a) in going from wide to  
long and b) in going from long to wide, c) and what are the reasons  
for the various error messages I provoke . I am almost never able to  
do it correctly on the first try and rarely able to do it even on the  
fourth try. I bought Spector's book in hopes of understanding it  
better, but his efforts did not take root in my brain.

In the instance above, how would I have know how to apply the help  
page description of timevar (below) to this problem?

timevar
the variable in long format that differentiates multiple records from  
the same group or individual.

To my reading that does not distinguish the purpose of 'timevar' from  
the purpose of 'idvar' and then reading the 'idvar' definition just  
below it does not help at all.

-- 
David.


> If a longer tutorial is needed, that may be an article for the R  
> Help Desk in The R Journal. Anybody volunteering?
>
> Best,
> Uwe
>
>
>>
>> So Hadley wrote an alternate facility ... the reshape package that  
>> does
>> not have a reshape function in it but rather two functions 'melt' and
>> 'cast'.
> >
> > Your data is all ready "molten", i.e. it is in the long format
>> (in the terminology of the base reshape function) with identifier  
>> values
>> in each row and a single column of values.
>>
>> > library(reshape)
>> > cast(dataframe,A~B)
>> Using C as value column. Use the value argument to cast to override  
>> this
>> choice
>> A control sample
>> 1 d0 1e+05 1e+05
>> 2 d1 2e+02 3e+02
>> 3 d2 4e+02 5e+02
>>
>> Basically the cast formula keeps the LHS variables in the rows and  
>> hte
>> RHD variables get arranges in columns. (For reasons that are  
>> unclear to
>> me the dataframe argument was placed first when using positional
>> argument passing, unlike most other formula methods in R.)

David Winsemius, MD
West Hartford, CT