logical condition in vector operation

Dear Frederico

From your example it is not clear to me what you like to obtain: 
Please have a look on the slightly changed example here (I
changed two values to show a potentially undesired side effect of
your coding.


test <- data.frame(rbind(c(3,3,10,21,0), c(2,3,11,12,0), c(3,4,12,23,0),
                         c(3,5,13,24,0)))
names(test) <- c("x","y","p","d","su")
test

  x y  p  d su
1 3 3 10 21  0
2 2 3 11 12  0
3 3 4 12 23  0
4 3 5 13 24  0

j <- 3
test[test[,1] == j, 5] <- test[test[,1] == j,2] + test[test[,2] == j,1]

Warning message:

longer object length
	is not a multiple of shorter object length in: 
        test[test[, 1] == j, 2] + test[test[, 2] == j, 1] 

Your code example produces now a warning for the adapted
data frame "test", since one tries to add two vectors of length 2
and 3, respectively. The result is based on recycling of the
smaller vector. In your example there was no warning since the
second column had only one entry.
The result with the adapted data frame is:

test

  x y  p  d su
1 3 3 10 21  6
2 2 3 11 12  0
3 3 4 12 23  6
4 3 5 13 24  8

Is that kind of recycling desired in your application. Otherwise
you should be careful with the coding example above.