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

[Petr PIKAL]

Hi


Michael Bach <phaebz at gmail.com> napsal dne 19.04.2011 14:21:13:

> Petr PIKAL <petr.pikal at 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 ...
 - attr(*, "dimnames")=List of 2
  ..$ x: chr [1:3] "2" "3" "5"
  ..$ y: chr [1:8] "5" "6" "7" "8" ...
 - attr(*, "class")= chr [1:2] "xtabs" "table"
 - attr(*, "call")= language xtabs(formula = zdata ~ x + y, data = data)

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!