R help - Nov 2009 - when use which()

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

S?ren

> -----Original Message-----
> From: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of 
> soeren.vogel at eawag.ch
> Sent: Friday, November 13, 2009 12:24 PM
> To: r-help at r-project.org
> Subject: [R] when use which()
> 
> 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?

When using "[" there should be no difference between
using x[condition] and x[which(condition)].  which() is
just a waste of typing, cpu time, and memory there.
If you want all x that don't satisfy the condition
x[!condition] works but x[-which(condition)]
silently gives the wrong answer when all(condition==FALSE).

There is one weird case, in [[, not [, where there is a
difference, and this might be considered a bug:
   > list(a=10,b=20,c=30)[[c(FALSE,TRUE,FALSE)]]
   Error in list(a = 10, b = 20, c = 30)[[c(FALSE, TRUE, FALSE)]] :
     attempt to select less than one element
   > list(a=10,b=20,c=30)[[which(c(FALSE,TRUE,FALSE))]]
   [1] 20
[[ is treating the vector of logicals as a vector of integers
and it can only work if all of them are TRUE and the list has
at least 3 levels of recursion
   > list(list(list(10,20),list(30,40)))[[c(TRUE,TRUE,TRUE)]]
   [1] 10

Of course, someone can write a package that defines a method for
[ that doesn't work this way, but that would be unfriendly.

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com 
> 
> Thanks for help
> 
> Regards
> 
> S?ren
> 
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