CODEWITHSUNDEEP
lua
lbaby
lbaby

Reputation: 2588

Does the 'for in ' loop call function in Lua?

There is a piece of code confuses me in Programming in Lua 

local iterator   -- to be defined later
function allwords ()
  local state = {line = io.read(), pos = 1}
  return iterator, state
end

function iterator (state)
  while state.line do        -- repeat while there are lines
    -- search for next word
    local s, e = string.find(state.line, "%w+", state.pos)
    if s then                -- found a word?
      -- update next position (after this word)
      state.pos = e + 1
      return string.sub(state.line, s, e)
    else    -- word not found
      state.line = io.read() -- try next line...
      state.pos = 1          -- ... from first position
    end
  end
  return nil                 -- no more lines: end loop
end
--here is the way I use this iterator:
for i ,s in allwords() do
     print (i)
end

It seems that the 'for in ' loop call the function iterator implicitly with argument state: i(s)

Anyone can tell me ,what happened?

Upvotes: 3

Views: 2092

Answers (1)

Yu Hao
Yu Hao

Reputation: 122383

Yes. Quoting Lua Manual

The generic for statement works over functions, called iterators. On each iteration, the iterator function is called to produce a new value, stopping when this new value is nil.

The generic for statement is just a syntax sugar:

A for statement like

for var_1, ···, var_n in explist do block end

is equivalent to the code:

do
   local f, s, var = explist
   while true do
     local var_1, ···, var_n = f(s, var)
     if var_1 == nil then break end
     var = var_1
     block
   end
 end

Upvotes: 3

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