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Allow nchar to be greater than string length in first/last #24556

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Allow nchar to be greater than string length in first/last #24556

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1272653
allow nchar to be geater than string length in first/last without thr…
bkamins Nov 9, 2017
1792389
correct behavior for empty strings and add more tests
bkamins Nov 21, 2017
93f56db
improve first/last docstring
bkamins Nov 22, 2017
6d6de2a
fix corner case of empty string and negative nchar
bkamins Nov 22, 2017
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19 changes: 14 additions & 5 deletions base/strings/basic.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -595,7 +595,7 @@ end
"""
first(str::AbstractString, nchar::Integer)

Get a string consisting of the first `nchar` characters of `str`.
Get a string consisting of at most the first `nchar` characters of `str`.
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This wording makes it sound like the function may just randomly decide to give you any number of characters up to the number you asked for. It would be clearer to leave this sentence as is but add:

unless str is shorter than nchar characters, in which case a string equal to str is returned.

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fixed


```jldoctest
julia> first("∀ϵ≠0: ϵ²>0", 0)
Expand All @@ -606,19 +606,24 @@ julia> first("∀ϵ≠0: ϵ²>0", 1)

julia> first("∀ϵ≠0: ϵ²>0", 3)
"∀ϵ≠"

julia> first("1234", 10)
"1234"
```
"""
function first(str::AbstractString, nchar::Integer)
s = start(str)
if 0 <= nchar <= 1
return str[1:nchar]
return str[s:(s-1+nchar)]
end
str[1:nextind(str, 1, nchar-1)]
idx = min(endof(str), nextind(str, s, nchar-1))
str[s:idx]
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I guess you can use @inbounds here now? That would (or could, if @boundscheck is implemented where needed) offset the cost of callind endof.

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I am not sure that @inbounds will help (but please correct me if I am wrong or I have misunderstood something what you have meant), because neither
getindex(s::String, r::UnitRange{Int}) nor getindex(s::AbstractString, r::UnitRange{Int})
call endof (they work differently, but perform the checks without calling endof and what they do does not seem to me to be influenced by @inbounds (but maybe I am wrong).

The optimization would be to write a specialized method for first for String only which would call unsafe_string instead of getindex. It would be a bit faster, but I do not know if we want to have such a messy code (the same could be done for last).

Any thoughts on this?

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OK. I guess leave it that way for now if the performance impact isn't too large.

end

"""
last(str::AbstractString, nchar::Integer)

Get a string consisting of the last `nchar` characters of `str`.
Get a string consisting of at most the last `nchar` characters of `str`.
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Similar comment as above.

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fixed


```jldoctest
julia> last("∀ϵ≠0: ϵ²>0", 0)
Expand All @@ -629,14 +634,18 @@ julia> last("∀ϵ≠0: ϵ²>0", 1)

julia> last("∀ϵ≠0: ϵ²>0", 3)
"²>0"

julia> last("1234", 10)
"1234"
```
"""
function last(str::AbstractString, nchar::Integer)
e = endof(str)
if 0 <= nchar <= 1
return str[(e-nchar+1):e]
end
str[prevind(str, e, nchar-1):e]
idx = max(start(str), prevind(str, e, nchar-1))
str[idx:e]
end

# reverse-order iteration for strings and indices thereof
Expand Down
35 changes: 18 additions & 17 deletions test/strings/basic.jl
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Expand Up @@ -637,23 +637,24 @@ end
end

@testset "first and last" begin
s = "∀ϵ≠0: ϵ²>0"
@test_throws ArgumentError first(s, -1)
@test first(s, 0) == ""
@test first(s, 1) == "∀"
@test first(s, 2) == "∀ϵ"
@test first(s, 3) == "∀ϵ≠"
@test first(s, 4) == "∀ϵ≠0"
@test first(s, length(s)) == s
@test_throws BoundsError first(s, length(s)+1)
@test_throws ArgumentError last(s, -1)
@test last(s, 0) == ""
@test last(s, 1) == "0"
@test last(s, 2) == ">0"
@test last(s, 3) == "²>0"
@test last(s, 4) == "ϵ²>0"
@test last(s, length(s)) == s
@test_throws BoundsError last(s, length(s)+1)
let s = "∀ϵ≠0: ϵ²>0"
@test_throws ArgumentError first(s, -1)
@test first(s, 0) == ""
@test first(s, 1) == "∀"
@test first(s, 2) == "∀ϵ"
@test first(s, 3) == "∀ϵ≠"
@test first(s, 4) == "∀ϵ≠0"
@test first(s, length(s)) == s
@test first(s, length(s)+1) == s
@test_throws ArgumentError last(s, -1)
@test last(s, 0) == ""
@test last(s, 1) == "0"
@test last(s, 2) == ">0"
@test last(s, 3) == "²>0"
@test last(s, 4) == "ϵ²>0"
@test last(s, length(s)) == s
@test last(s, length(s)+1) == s
end
end

@testset "invalid code point" begin
Expand Down