On Nov 25, 2009, at 3:45 PM, William Dunlap wrote:
-----Original Message-----
From: r-help-boun...@r-project.org
[mailto:r-help-boun...@r-project.org] On Behalf Of Marcio Resende
Sent: Wednesday, November 25, 2009 12:27 PM
To: r-help@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
How about using the element-wise behavior of "*" when working with
matrices?
> rowSums( myMatrix*(myMatrix != 9) )
[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
--
David Winsemius, MD
Heritage Laboratories
West Hartford, CT
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