[R] Syntax Check: rshape2 melt()

[Nordlund, Dan (DSHS/RDA)]

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Rich Shepard
> Sent: Thursday, October 27, 2011 8:26 AM
> To: r-help at r-project.org
> Subject: [R] Syntax Check: rshape2 melt()
> 
> 
>    This is my first excursion into using reshape2 and I want to ensure
> that
> the melt() function call is syntactically correct.
> 
>    The unmodifed data frame is organized this way:
> 
> head(tds.anal)
>        site   sampdate param quant
> 1    UDS-O 2006-12-06   TDS 10800
> 4   STC-FS 1996-06-14  Cond   280
> 7    UDS-O 2007-10-04    Mg  1620
> 9    UDS-O 2007-10-04   SO4  7580
> 19 JCM-10B 2007-06-21    Ca    79
> 20 JCM-10B 2007-06-21    Cl   114
> 
>    What I want looks like this:
> 
>      site    sampdate  TDS   Cond   Mg  Ca   Cl  Na  SO4
>     UDS-O  2006-12-06  10800  NA   1620 NA   NA  NA  7580
> 
> with the actual data for each param, of course.
> 
>    I've read the reshape.pdf, reshape2.pdf, the ?melt help page, and
> the
> ?melt.data.frame help page. I'm still unclear on the differences among
> measure.vars, variable.name, and value.name. After several attempts I
> have
> what may be what the melted tds.anal should look like:
> 
> m.tds.anal <- melt(tds.anal, id.vars = c('site', 'sampdate', 'param'),
> \
> measure.vars = 'quant', value.name = 'quant', na.rm = F)
> > head(m.tds.anal)
>       site   sampdate param variable quant
> 1   UDS-O 2006-12-06   TDS    quant 10800
> 2  STC-FS 1996-06-14  Cond    quant   280
> 3   UDS-O 2007-10-04    Mg    quant  1620
> 4   UDS-O 2007-10-04   SO4    quant  7580
> 5 JCM-10B 2007-06-21    Ca    quant    79
> 6 JCM-10B 2007-06-21    Cl    quant   114
> 
>    Is the melt() function call correct? Should the melted result look
> like
> the unmelted ("long" form in Paul Dalgaard's book) data with the
> additional
> 'variable' column containing 'quant' for each row?
> 
> Rich
> 

Rich,

What I think you want is as simple as 


test.melted <- melt(test)
wanted <- cast(test.melted, site + sampdate ~ param)


Hope this is helpful,

Dan

Daniel J. Nordlund
Washington State Department of Social and Health Services
Planning, Performance, and Accountability
Research and Data Analysis Division
Olympia, WA 98504-5204