Re: [R] Odp: Data frame with 3 columns to matrix

From: Petr PIKAL <petr.pikal_at_precheza.cz>
Date: Tue, 19 Apr 2011 15:51:52 +0200

Hi

Michael Bach <phaebz_at_gmail.com> napsal dne 19.04.2011 14:21:13:

> Petr PIKAL <petr.pikal@precheza.cz> writes:
>
> > Hi
> >
> > r-help-bounces_at_r-project.org napsal dne 19.04.2011 09:46:47:
> >
> >> Dear R Users,
> >>
> >> Lets assume I have this data frame:
> >>
> >> x y z
> >> 1 1.00 5 0.5
> >> 2 1.02 5 0.7
> >> 3 1.04 7 0.1
> >> 4 1.06 9 0.4
> >>
> >> x and y columns are sorted and the values not necessarily integers. z
> >> values are not sorted. Now I would like to create a matrix out of
this
> >> with x as first column values and y as first row values. Matrix
element
> >> a_11 shall be left NA. The a_ij should have the z value for the
> >> corresponding x and y pair. The result shall be some sort of a grid
and
> >> then e.g. look like:
> >>
> >> [,1] [,2] [,3] [,4] [,5]
> >> [1,] NA 5 6 7 9 (y)
> >> [2,] 1.00 0.5 NA NA NA
> >> [3,] 1.02 0.7 NA NA NA
> >> [4,] 1.04 NA NA 0.1 NA
> >> [5,] 1.06 NA NA NA 0.4
> >> (x)
> >>
> >> This example is just for illustration. The resulting matrix should
have
> >> more numeric values than NA's.
> >
> > I am not sure if this is the solution you want
> >
> > tab<-xtabs(z~x+y, data=df)
> > tab[tab==0]<-NA
> >
>
> This looks right, but the resulting table does not have rows == columns
> and there are also no missing values (0 or NA) which should be there. I

They are there

try str(tab)

With little effort you can get exactly the structure you want.

> data

  x y zdata

1 2  5     1
2 2  6     1
3 3  7     1
4 3  8     1
5 3  9     1
6 5 10     2
7 5 11     2
8 5 12     2

tab<-xtabs(zdata~x+y, data)
tab[tab==0]<-NA
tab

   y
x 5 6 7 8 9 10 11 12
  2 1 1

  3        1  1  1 
  5                 2  2  2

str(as.matrix(tab))
 xtabs [1:3, 1:8] 1 NA NA 1 NA NA NA 1 NA NA ...

tab1<-cbind(as.numeric(rownames(tab)), tab) rbind(c(NA,as.numeric(colnames(tab))), tab1)

      5 6 7 8 9 10 11 12
  NA 5 6 7 8 9 10 11 12
2 2 1 1 NA NA NA NA NA NA
3 3 NA NA 1 1 1 NA NA NA
5 5 NA NA NA NA NA 2 2 2
rbind(c(NA,as.numeric(colnames(tab))), tab1)

If you want to get rid of column and row names set them to NULL.

Regards
Petr

> think I did not make myself clear, please see also my reply to the
> previous answer to my question by David Winsemius.
>
> Thanks for your reply though!



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