[R] how to find a row index in a matrix or a data frame ?

[Gabor Grothendieck]

On Thu, Nov 25, 2010 at 4:59 AM, Luedde, Mirko <mirko.luedde at sap.com> wrote:
> Dear all,
>
> this looks pretty much a standard problem, but I couldn't find a
> satisfying and understandable solution.
>
> (A) Given a data frame (or matrix), e.g.
>
>      x <- data.frame(A=c(1, 2, 2), B=c(4, 5, 5))
>
>    and a row of this data frame, e.g.
>
>      r <- c(2, 5)
>
>    I need to find one row index i (or all such indices) such that r
>    is at the i-th row in x, that is, the expression
>
>      all(x[i,]==as.list(r))
>
>    evaluates to TRUE.  I can not evaluate an expression like
>
>      x[x[,1]==2 & x[,2]==5,]
>
>    because I do not know in advance how many columns x will have.
>
>    Basically, thus, I'm looking for an equivalent of "vectorfind" in
>    Scilab.
>
> (B) Which would be the most appropriate data type for x, matrix or
>    data frame or another type?
>
> (C) What will be better, searching for rows or searching for columns?
>

Try this:

   which(apply(t(x) == r, 2, all))

or this:

   which(colSums((t(x) != r) > 0) == 0)

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