[R] How to sum only a few elements in a line

[William Dunlap]

> -----Original Message-----
> From: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of Marcio Resende
> Sent: Wednesday, November 25, 2009 12:27 PM
> To: r-help at r-project.org
> Subject: [R] How to sum only a few elements in a line
> 
> 
> Hello,
> 
> I have a matrix with the numbers 0,1 and 9
> I would like to write a function that could sum each line 
> skiping everytime
> a number 9 appears
> for example
> [0 1 0 1 1 9 1]
> the sum would be 4.
> However I cannot replace 9 by 0 otherwise after the sum is 
> done I wouldn´t
> be able to distiguish which ones were real zeros and which 
> ones were nines
> replaced by zero just to sum.

One of the nice things about the S language is
that arguments functions are not altered by the
function.  When the function appears to alter
an argument it is really altering a copy of it.
Thus you can write a function like
    f <- function(matrix) {
        matrix[matrix==9] <- 0
        rowSums(matrix)
    }
and use it as
    > myMatrix <- rbind(c(0,1,0,1,1,9,1), c(9,9,9,9,9,17,9))
    > f(myMatrix)
    [1]  4 17
    > myMatrix # not altered by running f over it
         [,1] [,2] [,3] [,4] [,5] [,6] [,7]
    [1,]    0    1    0    1    1    9    1
    [2,]    9    9    9    9    9   17    9

By the way, it would help if you wrote your example data
as an S expression, e.g., rbind(c(...),c(...)), and not
as an expression in some other language, "[ 1 1 9 ]".

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com 

> Thank you very much
> 
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