• Subject: Re: string like arrays
• From: Luis Carvalho <carvalho@...>
• Date: Tue, 15 Jan 2008 19:39:13 -0500

```> 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

```