How to iterate individual characters in Lua string?

Question

I have a string in Lua and want to iterate individual characters in it. But no code I've tried works and the official manual only shows how to find and replace substrings :(

str = "abcd" for char in str do -- error   print( char ) end  for i = 1, str:len() do   print( str[ i ] ) -- nil end 

Answer 1

In lua 5.1, you can iterate of the characters of a string this in a couple of ways.

The basic loop would be:

for i = 1, #str do     local c = str:sub(i,i)     -- do something with c end 

But it may be more efficient to use a pattern with string.gmatch() to get an iterator over the characters:

for c in str:gmatch"." do     -- do something with c end 

Or even to use string.gsub() to call a function for each char:

str:gsub(".", function(c)     -- do something with c end) 

In all of the above, I've taken advantage of the fact that the string module is set as a metatable for all string values, so its functions can be called as members using the : notation. I've also used the (new to 5.1, IIRC) # to get the string length.

The best answer for your application depends on a lot of factors, and benchmarks are your friend if performance is going to matter.

You might want to evaluate why you need to iterate over the characters, and to look at one of the regular expression modules that have been bound to Lua, or for a modern approach look into Roberto's lpeg module which implements Parsing Expression Grammers for Lua.

Answer 2

Depending on the task at hand it might be easier to use string.byte. It is also the fastest ways because it avoids creating new substring that happends to be pretty expensive in Lua thanks to hashing of each new string and checking if it is already known. You can pre-calculate code of symbols you look for with same string.byte to maintain readability and portability.

local str = "ab/cd/ef" local target = string.byte("/") for idx = 1, #str do    if str:byte(idx) == target then       print("Target found at:", idx)    end end