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

[Peter Ehlers]

Marcio Resende wrote:
> 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.
> Thank you very much
> 
In order to distinguish 'real' 0s from those resulting
from elimination of 9s (without going back to the original
matrix), you can do this:

f <- function(x){
   Sum <- sum(x[x != 9])
   has_9 <- 9 %in% x
   c(Sum = Sum, has_9 = has_9)
}

# make a sample matrix
M <- matrix(sample(c(0,1,9), 30, TRUE), 6, 5)

# get sums and whether the row contained one or more 9s
t(apply(M, 1, f))

  -Peter Ehlers