[R] using a value in a column to "lookup" data in a certian column of a dataset?

[Gabor Grothendieck]

>From ?"]" note that:

          "When indexing arrays by '[' a single argument 'i' can be a
          matrix with as many columns as there are dimensions of 'x';
          the result is then a vector with elements corresponding to
          the sets of indices in each row of 'i'. "

so:

> mm <- matrix(1:25,5)
> mm
     [,1] [,2] [,3] [,4] [,5]
[1,]    1    6   11   16   21
[2,]    2    7   12   17   22
[3,]    3    8   13   18   23
[4,]    4    9   14   19   24
[5,]    5   10   15   20   25
> mm[cbind(1:5, mm[,1])]
[1]  1  7 13 19 25


On 3/14/06, r user <ruser2006 at yahoo.com> wrote:
> I have a dataset with 20 columns and ~600,000 rows.
>
> Column 1 has a number from 2-19.  This number tells
> me, for each row, which column has the "applicable"
> data.  (i.e. the data that I wish to use for each
> individual row)
>
> I want to create a vector that contains the data from
> the value in column 1.
>
> e.g.
> If column 1, row 1, has a value of "6", I want to
> obtain the value in column 6, row1.
>
> If column1, row 2, has value of "2", I want to obtain
> the value in column 2, row2. etc
>
>
> I have created a for next loop to do this, but am
> looking for a more efficient manner.
>
> Thanks.
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
>