first(x) # [not always the] same as x[1]

Because of this...

if (!length(x)) 
                return(x)

...in the code of first, the result of first(x) is not the same as x[1] when x is a zero-length vector: x vs. NA. Which is perfectly fine, but you may consider clarifying this difference in the help text.

A small example to illustrate the setting where this difference bit me. Find the first index of x == 1, within each group, where group "b" have no 1s

d <- data.table(grp = c("a", "a", "a", "b", "b", "c", "c"),
                x = c(0, 1, 1, 0, 0, 1, 1))

#    grp x
# 1:   a 0
# 2:   a 1
# 3:   a 1
# 4:   b 0
# 5:   b 0
# 6:   c 1
# 7:   c 1

# select first element using [1]
d[ , .(first.true = which(x == 1)[1]), by = grp]
#    grp first.true
# 1:   a          2
# 2:   b         NA <~~ zero-length result of grp b appears as NA
# 3:   c          1

# select first element using `first`
d[ , .(first.true = first(which(x == 1))), by = grp]
#    grp first.true
# 1:   a          2 <~~ zero-length result of grp b is "gone"
# 2:   c          1

related #4065 (comment)

@jangorecki

Hi @jangorecki @tdhock
I'd like to propose a documentation update that addresses both this issue and #2487 as they share same principle: first() is not a synonym for x[1], but a separate, optimized function.
My plan is to add a section - "Differences from base R subsetting" to the help page. This would clarify both the zero-length vector behavior and the stripping of attributes like names.
Is this seems right to you, and if so can I open a PR for this?

related to a previous issue #5390 (comment)

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first(x) # [not always the] same as x[1] #2002

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first(x) # [not always the] same as x[1] #2002

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jangorecki commented on Nov 26, 2019

Mukulyadav2004 commented on Jun 7, 2025

tdhock commented on Jun 8, 2025

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