[R] reshape a matrix

[Uwe Ligges]

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? 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.)