[R] Selecting rows/columns of a matrix

[William Dunlap]

Use logical vectors and the "[" operator:  x[condition] can be read as
'x such that condition [is true]'.  E.g.,
   > a<-matrix(1:16, 4,4,byrow=T)
   > j <- c(TRUE, FALSE, TRUE, FALSE)
   > a[j,,drop=FALSE]
        [,1] [,2] [,3] [,4]
   [1,]    1    2    3    4
   [2,]    9   10   11   12
   > a[,j,drop=FALSE]
        [,1] [,2]
   [1,]    1    3
   [2,]    5    7
   [3,]    9   11
   [4,]   13   15
   > a[j,j,drop=FALSE] # apply t() to the result if you want the transpose
        [,1] [,2]
   [1,]    1    3
   [2,]    9   11
The drop=FALSE is not needed in these examples, but prevents single-row
or -column results from be converted to vectors.

See "An Introduction to R" (under Help>Manuals in the Windows GUI,
or look on the R website) for details on these and many other issues.

Bill Dunlap
TIBCO Software
wdunlap tibco.com


On Sun, Oct 26, 2014 at 11:35 AM, Steven Yen <syen04 at gmail.com> wrote:
> Dear
>
> I am interested in selecting rows and columns of a matrix with a criterion
> defined by a binary indicator vector. Let  matrix a be
>
>> a<-matrix(1:16, 4,4,byrow=T)
>> a
>      [,1] [,2] [,3] [,4]
> [1,]    1    2    3    4
> [2,]    5    6    7    8
> [3,]    9   10   11   12
> [4,]   13   14   15   16
>
> Elsewhere in Gauss, I select the first and third rows and columns of a by
> defining a column vector j = [1,0,1,0]. Then, select the rows of a using j,
> and then selecting the rows of the transpose of the resulting matrix using j
> again. I get the 2 x 2 matrix as desired. Is there a way to do this in R?
> below are my Gauss commands. Thank you.
>
> ---
>
> j
>
> 1
> 0
> 1
> 0
>
> a=selif(a,j); a
>
> 1  2  3  4
> 9 10 11 12
>
> a=selif(a',j); a
>
> 1  9
> 3 11
>
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