Chapter 3, Exercise 5 confusion

Calion · #1 (**permalink**) November 17th, 2010, 12:14 AM

I'm completely new to Lua and relatively new to programming, not having done more than BASIC, HyperCard, AppleScript, etc. I'm working my way through the book, but I'm stuck on Exercise 5 in Chapter3, page 114. Here it is:

Code:

-- Returns a function that appends N dots to its argument:
function MakeDotter(N)
  Dots = ""
  for I = 1, N do
    Dots = Dots .. "."
  end
  return function(Str)
    return Str .. Dots
  end
end

The question, of course, is What is wrong with this program? The answer the book gives is that Dots is not local. What I don't see is why. True, when you follow the book and do

Code:

OneDotter=MakeDotter(1)
print(OneDotter("A"))

you get

Code:

A.

and when you do

Code:

ThreeDotter=MakeDotter(3)
print(ThreeDotter("B"))

you get

Code:

B...

and then if you do

Code:

print(OneDotter("C"))

you get

Code:

C...

.

But I don't see what that has to do with Dots. Because if you then do

Code:

print(MakeDotter(1)("D"))

you get

Code:

D.

just like you should. It seems to me that the problem is with N, not Dots. After all, the value of Dots gets reset to "" --blank--every time the function is called.

Now I'm aware--after trying it with the Lua interpreter at <http://www.lua.org/cgi-bin/demo> (since there is no functional Lua interpeter for Windows Mobile), that making Dots local fixes the problem. What I don't know is WHY.

Help please?

Need to download code?

Navigation