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Thanks everyone,

I already tried something like this, but it introduces more code in the middle reducing performance. About to write, I thought the answer will be this one (immutable).

Anyway, thank you a lot.

Marco Antonio Abreu
falecomigo@marcoabreu.eti.br
http://www.marcoabreu.eti.br



Luis Carvalho wrote:
there is some how, in Lua, to access string bytes like an array; something like what we can use in C or Pascal?

examples:

s = 'Lua race'
print( a[5] ) --> prints "r"

It would be very desirable to be able to write too, like:

s[5] = 'f'
print( s ) --> prints "Lua face"

Not with Lua strings since they're immutable. But you can roll something like:

function mutstring (s)
  assert(type(s) == "string", "string expected")
  local ms = s or ""
  local u = newproxy(true)
  local mt = getmetatable(u)
  local relatpos = function(p)
    local l = #ms
    if p < 0 then p = l + p + 1 end
    if p < 1 then p = 1 end
    return p, l
  end
  mt.__index = function(_, k)
    assert(type(k) == "number", "number expected as key")
    local k, l = relatpos(k)
    if k <= l then
      return ms:sub(k, k)
    end
  end
  mt.__newindex = function(_, k, v)
    assert(type(k) == "number", "number expected as key")
    assert(type(v) == "string" and #v == 1, "character expected as value")
    local k, l = relatpos(k)
    if k <= l + 1 then
      ms = ms:sub(1, k - 1) .. v .. ms:sub(k + 1, l)
    end
  end
  mt.__len = function(_) return #ms end
  mt.__tostring = function(_) return ms end
  return u
end

Test:

s = mutstring("Lua race")
print(s[5])
r
s[5] = "f"
print(s)
Lua face

Cheers,
Luis.