Re: Spelling corrector

[Dirk Laurie <dirk.laurie@...>]

2018-07-13 22:43 GMT+02:00 Hisham <h@hisham.hm>:
> On 7 July 2014 at 10:12, Paco Zamora Martínez <pakozm@gmail.com> wrote:
>>>
>>> > yield( splits[i-1][1]..word[i+1]..word[i]..splits[i+2][2] )
>>>
>> I tried table.concat and string.format, but both perform worst. This was
>> counter-intuitive to me, because Lua string concat generates copies of
>> intermediate strings. However, seems that for short strings and small number
>> of concatenated strings, string __concat performs better than string.format
>> or table.concat. Does anyone know if my observation is true?
>
> The "folk wisdom" about copies of intermediate strings in Lua is often
> mis-stated, I think.
>
> ("aa"):upper() .. ("bb"):upper() .. ("cc"):upper() .. ("dd"):upper()
>
> It translates to a single concatenation bytecode in both Lua and
> LuaJIT, so it produces the following strings:
>
> "aa"
> "bb"
> "cc"
> "AA"
> "BB"
> "CC"
> "DD"
> "AABBCCDD"
>
> This does generate intermediate strings:
>
> local s = ""
> for _, w in ipairs({"aa", "bb", "cc", "dd"})
>    s = s .. w:upper()
> end
>
> It produces
>
> ""
> "aa"
> "bb"
> "cc"
> "AA"
> "BB"
> "CC"
> "AABB"
> "AABBCC"
> "AABBCCDD"
>
> And this second pattern is the one that people tell to avoid when they
> talk about "intermediate strings". For that, one should do instead:
>
>
> local t = {}
> for _, w in ipairs({"aa", "bb", "cc", "dd"})
>    table.insert(s, w:upper())
> end
> local s = table.concat(t)
>
> That will produce:
>
> "aa"
> "bb"
> "cc"
> "AA"
> "BB"
> "CC"
> "AABBCCDD"
>
> plus an extra table. Of course this is an oversimplified example for
> illustration purposes, but often the loop is long and the naive
> approach above can produce a huge pyramid of intermediate strings.
>
> Over the years, the sensible advice was somehow distorted into some
> "all string concatenation is evil" cargo-cult, but that is not true,
> especially for short sequences of concatenations in an expression.
> Using a..b..c will usually be cheaper and produce less garbage than
> either string.format("%s%s%s", a, b, c) or table.concat({a, b, c}).
>
> I guess I should turn this email into a blog post titled "Lua string
> concatenation considered not harmful" :)

Now that you have resurrected an old issue, let me do likewise :-)

There is a difference between table.concat on the one hand, and
the concatenation operator and the API function lua_concat on
the other. table.concat fails if anything except strings and numbers
is among the things to be concatenated. The other two respect the
__concat metamethod.

Would it not be  agood thing to expose lua_concat at the Lua level,
e.g. by a function concat(...)?