[R] Syntax Check: rshape2 melt()

[Dennis Murphy]

Try this, based on your small example:

> tds.a <- read.table(textConnection("
+      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"), header = TRUE, stringsAsFactors = FALSE)
> closeAllConnections()

# Define param so that all of its levels are represented:
tds.a <- within(tds.a, {
           param = factor(param, levels = c('TDS', 'Cond', 'Mg', 'Ca',
'Cl', 'Na', 'SO4'))
           sampdate = as.Date(sampdate)  } )
library('reshape2')
dcast(tds.a, site + sampdate ~ param, value_var = 'quant')

# Result:
     site   sampdate   TDS Cond   Mg Ca  Cl  SO4
1 JCM-10B 2007-06-21    NA   NA   NA 79 114   NA
2  STC-FS 1996-06-14    NA  280   NA NA  NA   NA
3   UDS-O 2006-12-06 10800   NA   NA NA  NA   NA
4   UDS-O 2007-10-04    NA   NA 1620 NA  NA 7580

HTH,
Dennis

On Thu, Oct 27, 2011 at 8:26 AM, Rich Shepard <rshepard at appl-ecosys.com> wrote:
>  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
>
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