[R] two questions

Mark Myatt mark at myatt.demon.co.uk
Tue Apr 16 11:27:29 CEST 2002

Ambrosini Alessandro <klavan at tiscalinet.it> writes:
>The first: if I have a vector as (1,1,3,2,1,1), which is the command that
>gives all the positions of the min value? From the vector of the example a
>would like to obtain a new vector as (1,2,5,6) that give me all the
>positions of the minimum value 1.

Something like:

        (1:length(x))[x == min(x)]

Does it. There is also a which() function:

        which(x == min(x))

This does it too.

>The second: if I have a matrix "A" and I want to obtain a new matrix
>deleting a column or a row of A, what have I to do?

Use negated row and column indices:

        m <- matrix(1:12, nrow = 3)
        # drop the second row
        m[-2, ]
        # drop the second column
        m[ ,-2]

>Hello! If I have a matrix as 1 2
>                             2 3
>and I want to change the value 2 in 0, what can I do?

        m <- matrix(c(1, 2, 2, 3), nrow = 2)
        m[m == 2] <- 0

I Hope that helps. These sorts of questions are dealt with very fully in
most of the introductory texts available in the help system and from the
CRAN website.


Mark Myatt

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