Subsetting a vector using a boolean vector with different length

[lobshi]

Hi,
How can expect the output when I subset a vector using a vector of boolean values with different length. For example:

a <- c(1,2,3)
b <- c(TRUE,FALSE)
d <- c(FALSE,TRUE)
e <- c(TRUE)
f <- c(FALSE)
a[b]; a[c]; a[d]; a[e]; a[f]

Is there any rule for this situation?

Thanks!

[martin.R]

The values get recycled from the start to match the length of a.

This can be useful, but can also lead to unexpected output.

[Yarnabrina]

Just to illustrate upon Martin's point:

a <- 1:5 # modified from c(1, 2, 3) to make the illustration easier

b <- c(TRUE, FALSE)
a[b] # behaves like a[c(TRUE, FALSE, TRUE, FALSE, TRUE)]
#> [1] 1 3 5

d <- c(FALSE, TRUE)
a[d] # behaves like a[c(FALSE, TRUE, FALSE, TRUE, FALSE)]
#> [1] 2 4

e <- c(TRUE)
a[e] # behaves like a[c(TRUE, TRUE, TRUE, TRUE, TRUE)]
#> [1] 1 2 3 4 5

f <- c(FALSE)
a[f] # behaves like a[c(FALSE, FALSE, FALSE, FALSE, FALSE)]
#> integer(0)

^{Created on 2019-02-27 by the reprex package (v0.2.1)}

[lobshi]

Thank you so much for the explanation.

[cderv]

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