[R] when use which()

[soeren.vogel at eawag.ch]

Hello:

# some code to assign with and without "which"
q <- 1:20; q[c(9, 12, 14)] <- NA
r <- 1:20; r[c(8:9, 12:15)] <- NA
s <- 1:20; s[c(8:9, 12:15)] <- NA
r[q < 16] <- 0
s[which(q < 16)] <- 0
r;s # both: 0  0  0  0  0  0  0  0 NA  0  0 NA  0 NA  0 16 17 18 19 20
r <- 1:20; r[c(8:9, 12:15)] <- NA
s <- 1:20; s[c(8:9, 12:15)] <- NA
r[is.na(q)] <- 30
s[which(is.na(q))] <- 30
r;s # both: 1  2  3  4  5  6  7 NA 30 10 11 30 NA 30 NA 16 17 18 19 20

So it appears to me that "a[b] <- c" delivers the same results as  
a[which(b)] <- c". Is there any situation where the assignment with/ 
out which indeed makes a difference?

Thanks for help

Regards

Sören