Indexing strings in Lua?

[lesslucid]

Hallo,

I'm just trying to work my way through some tutorials and there's something that I find quite confusing. I thought I'd make my own simpler version by using the indices of a string to create a table and write a function so that, eg ->

tablifyString("word") -- would produce the output { "w", "o", "r", "d" }

...but unlike in, say, python, where "word"[1] = "o", there doesn't seem to be a way in lua to grab a character out of a string according to its index.
Uh, unless there is, and I just couldn't find it?

Thanks v. much for any help or advice!

[crow]

I think it should be like so

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MyStringTable = tablifyString("word");

then to call your letter

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-- w
MyStringTable[1];
-- o
MyStringTable[2];
-- r
MyStringTable[3];
-- d
MyStringTable[4];

OldString = "";
for x =1, 4 do
OldString = OldString..MyStringTable[x];
end
love.graphics.print(OldString, 400, 300);

[lesslucid]

Thanks for your advice!

...I'm still a little lost, though.

If I had some way to index characters in a string, I could write a tablifyString function, like this:

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function tablifyString(str)
  local tempTable = {}
  for i, char in ipairs(str)
    temptable[i] = str[i]
  end
  return tempTable
end

Or something like that. But I can't, because for lua then str doesn't mean anything.

[vrld]

You can either use string.sub for indexing or string.gmatch to convert a string to a table of strings:

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str = "Hello, world"
print(str:sub(1,1), str:sub(2,2)) --> H    e
print(string.sub(str, 3,3)) --> l

function tablify(str)
    local tbl = {}
    for char in str:gmatch(".") do -- matches every character
        tbl[#tbl+1] = char
    end
    return tbl
end

[crow]

Maybe something like this?

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function tablifyString(str)
  count = string.len(str);
  local tempTable = {};
  for i = 1, string.len(s) do
    temptable[i] = string.sub(str, i, i);
  end
  return tempTable;
end

Edit:

• Seems vrld got there before me lol

[lesslucid]

Ah, wonderful, thankyou both!

[Taehl]

...but unlike in, say, python, where "word"[1] = "o", there doesn't seem to be a way in lua to grab a character out of a string according to its index.
Uh, unless there is, and I just couldn't find it?

string.sub (s, i [, j])
Returns the substring of s that starts at i and continues until j; i and j can be negative. If j is absent, then it is assumed to be equal to -1 (which is the same as the string length). In particular, the call string.sub(s,1,j) returns a prefix of s with length j, and string.sub(s, -i) returns a suffix of s with length i.

[BlackBulletIV]

You could do something complex like this:

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local smt = getmetatable("")
function smt.__index(str, key)
  if type(key) == 'number' then
    return string.sub(str, key, key)
  end
end

What's happening is I'm getting the metatable for strings (yes, there is one), and creating an __index function to handle the indexing of a non-existent key (such as a number). We can then turn this into a string.sub call, getting only one character. Example:

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('goo')[1] -- 'g'

Note that you can't do this:

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'goo'[1]

Lua doesn't support that syntax.

[lesslucid]

Wow, I'm impressed that that's possible... thanks!

[BlackBulletIV]

Yeah, I was pretty excited when I first discovered it. Here's a cool thing you can do:

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local smt = getmetatable("")
local stringMethods = {}

function stringMethods:print()
  print(self)
end

function stringMethods:sub(i, j)
  return string.sub(self, i, j)
end

function smt.__index(str, key)
  if type(key) == 'number' then
    return string.sub(str, key, key)
  elseif stringMethods[key] then
    return stringMethods[key]
  end
end

This would allow you to define methods callable on strings via this syntax:

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s = 'hello'
s:print() -- prints 'hello'

You might want to just make the __index function look at the string library, so can call all string functions from a string itself.