是否有将字符串截断到一定长度的便利功能？
它相当于这样的东西
test_str = "test"
if length(test_str) > 8
out_str = test_str[1:8]
else
out_str = test_str
end
答案 0 :(得分：7)
在天真的ASCII世界中：
truncate_ascii(s,n) = s[1:min(sizeof(s),n)]
会这样做。如果最好与原始字符串共享内存并避免复制SubString可以使用：
truncate_ascii(s,n) = SubString(s,1,min(sizeof(s),n))
但是在Unicode世界中（它是一个Unicode世界），这更好：
truncate_utf8(s,n) = SubString(s,1, (eo=endof(s) ; neo=0 ;
for i=1:n
if neo<eo neo=nextind(s,neo) ; else break ; end ;
end ; neo) )
最后，@IsmaelVenegasCastelló提醒我们grapheme复杂性（arrrgh），然后这就是所需要的：
function truncate_grapheme(s,n)
eo = endof(s) ; tt = 0 ; neo=0
for i=1:n
if (neo<eo)
tt = nextind(s,neo)
while neo>0 && tt<eo && !Base.UTF8proc.isgraphemebreak(s[neo],s[tt])
(neo,tt) = (tt,nextind(s,tt))
end
neo = tt
else
break
end
end
return SubString(s,1,neo)
end
最后两个实现尝试避免计算length（可能很慢）或分配/复制，甚至只在n较短时循环length次。
这个答案借鉴了@MichaelOhlrogge，@ FengyangWang，@ Oxinabox和@IsmaelVenegasCastelló
的贡献答案 1 :(得分：4)
我会做strtruncate(str, n) = join(take(str, n))。
示例：
julia> strtruncate("αβγδ", 3)
"αβγ"
julia> strtruncate("αβγδ", 5)
"αβγδ"
请注意，您的代码对Unicode字符串不完全有效。
答案 2 :(得分：3)
如果字符串是ASCII，这非常有效：
String(resize!(str.data, n))
或就地：
resize!(str.data, n)
对于unicode，@ Fengyang Wangs的方法速度非常快，但如果你只截断字符串的最后一部，转换为Char数组可能会稍快一些：
trunc1(str::String, n) = String(collect(take(str, n)))
trunc2(str::String, n) = String(Vector{Char}(str)[1:n])
trunc3(str::String, n) = String(resize!(Vector{Char}(str), n))
trunc4(str::String, n::Int)::String = join(collect(graphemes(str))[1:n])
function trunc5(str::String, n)
if isascii(str)
return String(resize!(str.data, n))
else
trunc1(str, n)
end
end
定时：
julia> time_trunc(100, 100000, 25)
0.112851 seconds (700.00 k allocations: 42.725 MB, 7.75% gc time)
0.165806 seconds (700.00 k allocations: 91.553 MB, 11.84% gc time)
0.160116 seconds (600.00 k allocations: 73.242 MB, 11.58% gc time)
1.167706 seconds (31.60 M allocations: 1.049 GB, 11.12% gc time)
0.017833 seconds (100.00 k allocations: 1.526 MB)
true
julia> time_trunc(100, 100000, 98)
0.367191 seconds (700.00 k allocations: 83.923 MB, 5.23% gc time)
0.318507 seconds (700.00 k allocations: 132.751 MB, 9.08% gc time)
0.301685 seconds (600.00 k allocations: 80.872 MB, 6.19% gc time)
1.561337 seconds (31.80 M allocations: 1.122 GB, 9.86% gc time)
0.061827 seconds (100.00 k allocations: 1.526 MB)
true
编辑：哎呀......我刚才意识到我实际上正在摧毁trunc5中的原始字符串。这应该是正确的，但性能较差：
function trunc5(str::String, n)
if isascii(str)
return String(str.data[1:n])
else
trunc1(str, n)
end
end
新时间：
julia> time_trunc(100, 100000, 25)
0.123629 seconds (700.00 k allocations: 42.725 MB, 7.70% gc time)
0.162332 seconds (700.00 k allocations: 91.553 MB, 11.41% gc time)
0.152473 seconds (600.00 k allocations: 73.242 MB, 9.19% gc time)
1.152640 seconds (31.60 M allocations: 1.049 GB, 11.54% gc time)
0.066662 seconds (200.00 k allocations: 12.207 MB)
true
julia> time_trunc(100, 100000, 98)
0.369576 seconds (700.00 k allocations: 83.923 MB, 5.10% gc time)
0.312237 seconds (700.00 k allocations: 132.751 MB, 9.42% gc time)
0.297736 seconds (600.00 k allocations: 80.872 MB, 5.95% gc time)
1.545329 seconds (31.80 M allocations: 1.122 GB, 10.02% gc time)
0.080399 seconds (200.00 k allocations: 19.836 MB, 5.07% gc time)
true
Aaand new edit： Aargh，忘记了计时功能。我输入了一个ascii字符串：
function time_trunc(m, n, m_)
str = randstring(m)
@time for _ in 1:n trunc1(str, m_) end
@time for _ in 1:n trunc2(str, m_) end
@time for _ in 1:n trunc3(str, m_) end
@time for _ in 1:n trunc4(str, m_) end
@time for _ in 1:n trunc5(str, m_) end
trunc1(str, m_) == trunc2(str, m_) == trunc3(str, m_) == trunc4(str, m_) == trunc5(str, m_)
end
最终编辑（我希望）：
试用@Dan Getz＆＃39; truncate_grapheme并使用unicode字符串：
function time_trunc(m, n, m_)
# str = randstring(m)
str = join(["αβγπϕ1t_Ω₃!" for i in 1:100])
@time for _ in 1:n trunc1(str, m_) end
@time for _ in 1:n trunc2(str, m_) end
@time for _ in 1:n trunc3(str, m_) end
# @time for _ in 1:n trunc4(str, m_) end # too slow
@time for _ in 1:n trunc5(str, m_) end
@time for _ in 1:n truncate_grapheme(str, m_) end
trunc1(str, m_) == trunc2(str, m_) == trunc3(str, m_) == trunc5(str, m_) == truncate_grapheme(str, m_)
end
定时：
julia> time_trunc(100, 100000, 98)
0.690399 seconds (800.00 k allocations: 103.760 MB, 3.69% gc time)
1.828437 seconds (800.00 k allocations: 534.058 MB, 3.66% gc time)
1.795005 seconds (700.00 k allocations: 482.178 MB, 3.19% gc time)
0.667831 seconds (800.00 k allocations: 103.760 MB, 3.17% gc time)
0.347953 seconds (100.00 k allocations: 3.052 MB)
true
julia> time_trunc(100, 100000, 25)
0.282922 seconds (800.00 k allocations: 48.828 MB, 4.01% gc time)
1.576374 seconds (800.00 k allocations: 479.126 MB, 3.98% gc time)
1.643700 seconds (700.00 k allocations: 460.815 MB, 3.70% gc time)
0.276586 seconds (800.00 k allocations: 48.828 MB, 4.59% gc time)
0.091773 seconds (100.00 k allocations: 3.052 MB)
true
所以最后一个看起来显然是最好的（这篇文章现在太长了。）
答案 3 :(得分：2)
您可以使用：
"test"[1:min(end,8)]
另外
SubString("test", 1, 8)
答案 4 :(得分：2)
您可以使用graphemes功能：
C:\Users\Ismael
λ julia5
_
_ _ _(_)_ | By greedy hackers for greedy hackers.
(_) | (_) (_) | Documentation: http://docs.julialang.org
_ _ _| |_ __ _ | Type "?help" for help.
| | | | | | |/ _' | |
| | |_| | | | (_| | | Version 0.5.0-rc3+0 (2016-08-22 23:43 UTC)
_/ |\__'_|_|_|\__'_| | Official http://julialang.org/ release
|__/ | x86_64-w64-mingw32
help?> graphemes
search: graphemes
graphemes(s) -> iterator over substrings of s
Returns an iterator over substrings of s that correspond to the extended
graphemes in the string, as defined by Unicode UAX #29.
(Roughly, these are what users would perceive as single characters, even
though they may contain more than one codepoint; for example a letter
combined with an accent mark is a single grapheme.)
julia> s = "αβγπϕ1t_Ω₃!"; n = 8;
julia> length(s)
11
julia> graphemes(s)
length-11 GraphemeIterator{String} for "αβγπϕ1t_Ω₃!"
julia> collect(ans)[1:n]
8-element Array{SubString{String},1}:
"α"
"β"
"γ"
"π"
"ϕ"
"1"
"t"
"_"
julia> join(ans)
"αβγπϕ1t_"
查看truncate功能：
julia> methods(truncate)
# 2 methods for generic function "truncate":
truncate(s::IOStream, n::Integer) at iostream.jl:43
truncate(io::Base.AbstractIOBuffer, n::Integer) at iobuffer.jl:140
help?> truncate
search: truncate
truncate(file,n)
Resize the file or buffer given by the first argument to exactly n bytes,
filling previously unallocated space with '\0' if the file or buffer is
grown.
所以解决方案看起来像这样：
julia> @doc """
truncate(s::String, n::Int)::String
truncate a `String`; `s` up to `n` graphemes.
# Example
```julia
julia> truncate("αβγπϕ1t_Ω₃!", 8)
"αβγπϕ1t_"
julia> truncate("test", 8)
"test"
```
""" ->
function Base.truncate(s::String, n::Int)::String
if length(s) > n
join(collect(graphemes(s))[1:n])
else
s
end
end
Base.truncate
测试它：
julia> methods(truncate)
# 3 methods for generic function "truncate":
truncate(s::String, n::Int64)
truncate(s::IOStream, n::Integer) at iostream.jl:43
truncate(io::Base.AbstractIOBuffer, n::Integer) at iobuffer.jl:140
help?> truncate
truncate(file,n)
Resize the file or buffer given by the first argument to exactly n bytes,
filling previously unallocated space with '\0' if the file or buffer is
grown.
truncate(s::String, n::Int)::String
truncate a String; s up to n graphemes.
Example
≡≡≡≡≡≡≡≡≡
julia> truncate("αβγπϕ1t_Ω₃!", 8)
"αβγπϕ1t_"
julia> truncate("test", 8)
"test"
julia> truncate("αβγπϕ1t_Ω₃!", n)
"αβγπϕ1t_"
julia> truncate("test", n)
"test"
简介：
julia> Pkg.add("BenchmarkTools")
INFO: Nothing to be done
INFO: METADATA is out-of-date — you may not have the latest version of BenchmarkTools
INFO: Use `Pkg.update()` to get the latest versions of your packages
julia> using BenchmarkTools
julia> @benchmark truncate("αβγπϕ1t_Ω₃!", 8)
BenchmarkTools.Trial:
samples: 10000
evals/sample: 9
time tolerance: 5.00%
memory tolerance: 1.00%
memory estimate: 1.72 kb
allocs estimate: 48
minimum time: 1.96 μs (0.00% GC)
median time: 2.10 μs (0.00% GC)
mean time: 2.45 μs (7.80% GC)
maximum time: 353.75 μs (98.40% GC)
julia> Sys.cpu_info()[]
Intel(R) Core(TM) i7-4710HQ CPU @ 2.50GHz:
speed user nice sys idle irq ticks
2494 MHz 937640 0 762890 11104468 144671 ticks
答案 5 :(得分：0)
这里可以处理任何UTF-8字符串：
function trim_str(str, max_length)
edge = nextind(str, 0, max_length)
if edge >= ncodeunits(str)
str
else
str[1:edge]
end
end