[R] indexing into a data.frame using another data.frame that also contains values for replacement

[Dennis Murphy]

Hi:

Here are two possibilities:

 df1 <- data.frame(rows=c("A","B","C", "B", "C", "A"),
 columns=c("21_2", "22_2", "23_2", "21_2", "22_2", "23_2"),
 values=c(3.3, 2.5, 67.2, 44.3, 53, 66))

with(df1, xtabs(values ~ rows + columns))

    columns
rows 21_2 22_2 23_2
   A  3.3  0.0 66.0
   B 44.3  2.5  0.0
   C  0.0 53.0 67.2

library(reshape2)
dcast(df1, rows ~ columns)

Using values as value column: use value_var to override.
  rows 21_2 22_2 23_2
1    A  3.3   NA 66.0
2    B 44.3  2.5   NA
3    C   NA 53.0 67.2

HTH,
Dennis

On Sat, Apr 30, 2011 at 4:18 PM, Alice Wines <aabroadh at ncsu.edu> wrote:
> Hello all,
>
>     I have a quandry I have been scratching my head about for a
> while. I've searched the manual and the web and have not been able to
> find an acceptable result, so I am hoping for some help.
>
>     I have two data frames and I want to index into the first using
> the second, and replace the specific values I have indexed with more
> values from the second data.frame. I can do this using a loop, but I
> wanted a quicker solution with no loops involved.
>
> Although my data set is much larger than this, a small example of what
> I am trying to do is as follows:
>
> df1 <- data.frame(rows=c("A","B","C", "B", "C", "A"),
> columns=c("21_2", "22_2", "23_2", "21_2", "22_2", "23_2"),
> values=c(3.3, 2.5, 67.2, 44.3, 53, 66))
> df2 <- data.frame(matrix(rep(NA, length(df1$values)),nrow=3, ncol=3))
> names(df2) <- c("21_2", "22_2", "23_2")
> row.names(df2) <- c("A", "B", "C")
>
>> df1
>  rows columns values
> 1    A    21_2    3.3
> 2    B    22_2    2.5
> 3    C    23_2   67.2
> 4    B    21_2   44.3
> 5    C    22_2   53.0
> 6    A    23_2   66.0
>
>
>
>> df2
>  21_2 22_2 23_2
> A   NA   NA   NA
> B   NA   NA   NA
> C   NA   NA   NA
>
>
>     Note that none of the same locations in df2 are specified twice
> in df2, so I'm not worried about over-writing it.
>
>    I have tried 'mapply' and 'replace', but apparently either they do
> not work well for this or I don't understand how to use them properly
> for this purpose. My understanding is that 'replace' needs a vector
> input and that one cannot create a vector of vectors, so I couldn't
> pass my indices to 'replace'.
>
>     When I tried mapply, the code I used was something like what follows:
>
> df3 <- mapply('[<-' , df2, paste(as.character(df1$rows),
> as.character(df1$columns), sep=', '), df1$values)
>
> but it yields the following strange result
>
>> df3
>        21_2 22_2 23_2 <NA> <NA> <NA>
>          NA   NA   NA   NA   NA   NA
>          NA   NA   NA   NA   NA   NA
>          NA   NA   NA   NA   NA   NA
> A, 21_2  3.3  2.5 67.2 44.3   53   66
>
>
> What I want to see is the following:
>
>>df3
>    21_2   22_2  23_2
> A   3.3    NA    66.0
> B   44.3  2.5    NA
> C   NA   53.0   67.2
>
>
>     I will greatly appreciate any help that can be given as I am
> completely bamboozled by this problem and although I found many useful
> things in my search for an answer, I did not find out how to do this.
>
> Thanks,
>
> Alice
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>