How to iterate individual characters in Lua string?

Eye_of_Hell_69882 0
Viewed 111406

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
lua
6 Answers

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.

RBerteig_682040

If you're using Lua 5, try:

for i = 1, string.len(str) do
    print( string.sub(str, i, i) )
end
Aaron_Saarela_489400

Iterating to construct a string and returning this string as a table with load()...

itab=function(char)
local result
for i=1,#char do
 if i==1 then
  result=string.format('%s','{')
 end
result=result..string.format('\'%s\'',char:sub(i,i))
 if i~=#char then
  result=result..string.format('%s',',')
 end
 if i==#char then
  result=result..string.format('%s','}')
 end
end
 return load('return '..result)()
end

dump=function(dump)
for key,value in pairs(dump) do
 io.write(string.format("%s=%s=%s\n",key,type(value),value))
end
end

res=itab('KOYAANISQATSI')

dump(res)

Puts out...

1=string=K
2=string=O
3=string=Y
4=string=A
5=string=A
6=string=N
7=string=I
8=string=S
9=string=Q
10=string=A
11=string=T
12=string=S
13=string=I
user_117407580
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