Re: string like arrays

[Luis Carvalho <carvalho@...>]

> 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.

-- 
A mathematician is a device for turning coffee into theorems.
        -- P. Erdos 

-- 
Luis Carvalho
Applied Math PhD Student
Brown University