[R] fast way to check an object is a member of a list?

[Tony Plate]

if you are talking about atomic objects, is.element() and %in% may do what 
you are looking for:

 > 5 %in% 1:4
[1] FALSE
 > 3 %in% 1:4
[1] TRUE
 > is.element(3, as.list(1:4))
[1] TRUE
 > is.element(5, as.list(1:4))
[1] FALSE
 > 3 %in% as.list(1:4)
[1] TRUE
 > 5 %in% as.list(1:4)
[1] FALSE

For general objects (both atomic and non-atomic), I know of no better way 
than the one already suggested by John Fox:

 > LIST <- as.list(1:4)
 > X <- as.integer(3)
 > any(sapply(LIST, function(y,x) identical(y,x), X))
[1] TRUE
 >

The main problem with using identical() is that it is sensitive to the 
storage modes, which can lead to unexpected non-matches:

 > LIST <- as.list(1:4) # elements are integer
 > X <- 3 # a double value
 > any(sapply(LIST, function(y,x) identical(y,x), X))
[1] FALSE
 > identical(X, LIST[[3]])
[1] FALSE
 > X==LIST[[3]]
[1] TRUE
 > storage.mode(X)
[1] "double"
 > storage.mode(LIST[[3]])
[1] "integer"
 >

hope this helps,

tony plate

At 01:38 PM 11/19/2002 -0800, Francisco J Molina wrote:

>Is there any fast way to check that an object is a member of a list?
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