Re: Numeric for loop: Changing control variable REALLY dangerous?

[Sean Conner <sean@...>]

It was thus said that the Great bil til once stated:
> In chapter 3.3.5, there is a bit a "strange sounding" warning:
> "You should not change the value of the control variable during the loop."
> 
> ... is this not a bit "over-cautious"?
> 
> e. g. in a typical loop "for i=1,count do ... end", it is quite often
> really useful to repeat a loop sequence by "i=i-1", or to repeat
> complete loop by "i=1" assignment.
> 
> This REALLY can be dangerous? (or can give anybody possibly give some
> illegal / "bad-running" example for such change?)

  I tried it, in Lua 5.1, 5.2, 5.3, 5,4 and LuaJIT 2.0:
	
	local count = 10
	for i = 1 , count do
	  print(i,count)
	  if i == 3 then
	    i = nil
	    count = nil
	    print(">",i)
	  end
	end
	
	print("<",count)

They all behaved the same way, and it appears to work like this:

	local count = 10

	do
	  local _Unnamed_value = 1
	  local _Unnamed_limit = count
	  local _Unnamed_step  = 1

	  while _Unnamed_value <= _Unnamed_limit do
	    i = _Unmamed_value
	    --
	    -- body of loop
	    --
	    _Unnamed_value = _Unnamed_value + _Unnamed_step
	  end
	end

  I suspect the warning is there because changing the control variable
doesn't work as one might expect, and not to rely upon changing either the
control variable, or the limit variable.

  -spc