CODEWITHSUNDEEP
lua
Victor
Victor

Reputation: 47

Trying to understand Custom Iterators

im trying to understand the iterators, in many examples I founf something like this:

function square(iteratorMaxCount,currentNumber)

   if currentNumber<iteratorMaxCount
   then
      currentNumber = currentNumber+1
      return currentNumber, currentNumber*currentNumber
   end

end

function squares(iteratorMaxCount)
   return square,iteratorMaxCount,0 // why not return square(iteratorMAxCount,0)????
end  

for i,n in squares(3)
do 
   print(i,n)
end

First I dont understand the line I comment, and I dont find an easy example of how to do a Stateful Iterator and a Stateless iterator. Can anybody help me? thanks

Upvotes: 0

Views: 238

Answers (1)

Piglet
Piglet

Reputation: 28968

From Lua Reference Manual 3.3.5:

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 Note the following:

explist is evaluated only once. Its results are an iterator function, a state, and an initial value for the first iterator variable. f, s, and var are invisible variables. The names are here for explanatory purposes only. You can use break to exit a for loop. The loop variables var_i are local to the loop; you cannot use their values after the for ends. If you need these values, then assign them to other variables before breaking or exiting the loop.

So squares() has to return a function (square) a state (iteratorMaxCount) and an initial value (0) in order to work with a generic for loop.

Read the reference manual, Programming in Lua.

Upvotes: 2

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