list subsetting question

Hi, all.

I'm curious about the following behavior in list subsetting:

t1 <- as.list(1:10)
t2 <- 1:10
t2 == 5

[1] FALSE FALSE FALSE FALSE  TRUE FALSE FALSE FALSE FALSE FALSE

t1[t2==5]

[[1]]
[1] 5

t1[[t2==5]]

Error:  attempting to select more than one element

t1[[which(t2==5)]]

[1] 5


I don't understand why the second-to-last command gives an error, rather
than the same result as the third-to-last command.  After all, only one
element of the indexing vector is TRUE, so as far as I can tell I'm really
only trying to select one element.

The final command does what I want (as does t1[t2==5][[1]]), so this isn't
an urgent question, but I've looked through the documentation and can't
figure out why t1[[t2==5]] doesn't work.

Thanks for any help.

Matt Wiener



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