Re: string immutability

[Tom N Harris <telliamed@...>]

On Thursday, January 02, 2014 10:28:43 AM Enrico Colombini wrote:
> ".." is a dangerous operator when doing many concatenations or when
> operating on large strings.
> table.concat and/or the C-level luaL_Buffer can be useful to reduce
> memory usage in these cases.

The VM concat instruction is very efficient so a chained .. is no worse than 
table.concat. Where you get in trouble is building a string piecemeal in a 
loop.

Using a variation of your example...

    function GCAndShow(prompt)
      local before = collectgarbage "count"
      collectgarbage"collect"
      local after = collectgarbage "count"
      io.write(string.format('%s: %.1fKiB (%.1fKiB garbage)\n', 
                             prompt, after, before-after))
    end
    GCAndShow "initial state"

    local s1 = string.rep('x', 99999)
    GCAndShow "allocated a large string"

    local s2 = s1..s1..s1..s1..s1..s1..s1..s1..s1..s1
    GCAndShow "concatenated string with `..'"

    local s3 = table.concat{s1,s1,s1,s1,s1,s1,s1,s1,s1,s1}
    assert(s2==s3)
    GCAndShow "concatenated string with table.concat"

    local s4 = ""
    for i=1,10 do s4 = s4..s1 end
    assert(s2==s4)
    GCAndShow "concatenated string with loop"

    local s5 = string.format("%s%s%s%s%s%s%s%s%s%s",
                             s1,s1,s1,s1,s1,s1,s1,s1,s1,s1)
    assert(s2==s5)
    GCAndShow "concatenated string with string.format"

    s1,s2,s3,s4,s5 = nil
    GCAndShow "deleted all strings"


With Lua 5.1.5 (and nearly the same with LuaJIT 2.0.2)
initial state: 29.1KiB (3.6KiB garbage)
allocated a large string: 151.2KiB (163.0KiB garbage)
concatenated string with `..': 1347.5KiB (732.5KiB garbage)
concatenated string with table.concat: 1347.5KiB (732.7KiB garbage)
concatenated string with loop: 1347.5KiB (5029.5KiB garbage)
concatenated string with string.format: 1348.3KiB (732.5KiB garbage)
deleted all strings: 90.9KiB (1257.5KiB garbage)

With Lua 5.2.2
initial state: 24.9KiB (3.3KiB garbage)
allocated a large string: 122.6KiB (97.8KiB garbage)
concatenated string with `..': 1099.2KiB (0.0KiB garbage)
concatenated string with table.concat: 2075.8KiB (2344.0KiB garbage)
concatenated string with loop: 3052.3KiB (2441.4KiB garbage)
concatenated string with string.format: 4029.1KiB (3028.1KiB garbage)
deleted all strings: 25.1KiB (4004.1KiB garbage)

-- 
tom <telliamed@whoopdedo.org>