R help - Jun 2005 - RES: another aov results interpretation question

Dear All,

I created a script to calculate averages - two groups: "parcel" and
"date" - and, based on these averages, make a graph. The problem is
that
'R' does not recognize the first column even if I try to insert one. 

A brief example

Raw data: 

Data <- sample(1:100, 30, replace = FALSE, prob = NULL)
Date <- rep(c("02/30/2004","03/15/2004",
"04/16/2004", "06/14/2004",
"07/08/2004"), 6)
Parcel <- rep(c("DRY", "CONTROL"),15)

Df <- data.frame(Parcel, Date, Data)

Res <- tapply(Df$Data, list(Date = Df$Date, Parcel = Df$Parcel), mean) 

> Res
            Parcel
Date          CONTROL      DRY
  02/30/2004 53.00000 52.33333
  03/15/2004 54.00000 67.66667
  04/16/2004 54.66667 30.00000
  06/14/2004 27.66667 20.00000
  07/08/2004 59.00000 38.00000
> colnames(Res)
[1] "CONTROL" "DRY"
> matplot(Res[,1], Res[,-1], type = "l") 
### It does not recognize the colunm "Date". Why?

Thanks in advance!

Paulo

________________________________________
Paulo M. Brando
Instituto de Pesquisa Ambiental da Amazonia (IPAM)
Santarem, PA, Brasil.
Av. Rui Barbosa, 136.
Fone: + 55 93 522 55 38
www.ipam.org.br
E-mail: pmbrando at ipam.org.br

On 6/20/05, Paulo Brando <pmbrando at ipam.org.br>
wrote:
> 
> Dear All,
> 
> I created a script to calculate averages - two groups: "parcel"
and
> "date" - and, based on these averages, make a graph. The problem
is that
> 'R' does not recognize the first column even if I try to insert
one.
> 
> A brief example
> 
> Raw data:
> 
> Data <- sample(1:100, 30, replace = FALSE, prob = NULL)
> Date <- rep(c("02/30/2004","03/15/2004",
"04/16/2004", "06/14/2004",
> "07/08/2004"), 6)
> Parcel <- rep(c("DRY", "CONTROL"),15)
> 
> Df <- data.frame(Parcel, Date, Data)
> 
> Res <- tapply(Df$Data, list(Date = Df$Date, Parcel = Df$Parcel), mean)
> 
> 
> > Res
>            Parcel
> Date          CONTROL      DRY
>  02/30/2004 53.00000 52.33333
>  03/15/2004 54.00000 67.66667
>  04/16/2004 54.66667 30.00000
>  06/14/2004 27.66667 20.00000
>  07/08/2004 59.00000 38.00000
> 
> > colnames(Res)
> [1] "CONTROL" "DRY"
> 
> > matplot(Res[,1], Res[,-1], type = "l")
> 
> ### It does not recognize the colunm "Date". Why?
Date is a not a column in the matrix -- it is the name
of the row dimension (and Parcel is the name of the
column dimension).   Res is a 5 by 2 matrix whose
colnames are CONTROL and DRY.  Try this:

   dimnames(Res)
   dim(Res)  # 5 by 2

Also note that the first date (Feb 30th) is illegal and matplot
does not work with dates in any case.  Fix up the erroneous
date and convert the row names to an object, xx, of chron or Date 
class, using plot for the axes and matlines for the lines:

   xx <- format(rownames(Res), "%m/%d/%Y")  # Date class
   plot(range(xx), range(Res), type = "n")  # axes
   matlines(xx, Res)

More on dates is in R News 4/1.