How to parse nested statements????

[twitchyliquid64]

Hi all,

This is hard to explain so bear with me.

Consider the following code.

for $x = 0 to 20 step 1
    $f += 1
Next

If I was writing an interpreter to parse that, to process FOR loops I would just look for the FOR keyword and the NEXT keyword, and process normally everything in the middle.

But if I had this...

for $x = 0 to 20 step 1
    for $e = 0 to 10 step 1
        ConsoleWrite( "Rebecca Black dies on Friday")
    Next
Next

It has a nested for loop, so now when I go to interpret it, the interpreter will find the first FOR, and the first NEXT, but then process the wrong things and eventually get to the point where it was trying to process a FOR or NEXT statment that wasnt matched by its partner FOR or NEXT statement.

How can I write an interpreter that can deal with a unlimited number of nested statements??? I just cant get my head around how to do it.

[bwochinski]

I've never dealt with anything like that really, but couldn't you just call a parsing function recursively when you hit the "FOR" statement, and then parse forward until you come across the "NEXT" and then unwind the recursion (then re-enter the recursive call to parse the next value for $x).

Hopefully that makes sense.

[twitchyliquid64]

I've never dealt with anything like that really, but couldn't you just call a parsing function recursively when you hit the "FOR" statement, and then parse forward until you come across the "NEXT" and then unwind the recursion (then re-enter the recursive call to parse the next value for $x).

Hopefully that makes sense.

Do you mean...

Func Parse($linenum)
  For $linenum in $linecount step 1
    if $firstwordofline = "Next" then return $linenum
    if $firstwordofline = "For" then 
      ;repeat for loop
        $linenum = Parse($linenum)
      ;end repeat
      continueloop
    Endif
  Next
Endfunc

Edited April 1, 2011 by hyperzap

[bwochinski]

Yeah that looks about like what I was thinking

Basically it doesn't worry about finding the matching "NEXT" as it encounters each "FOR", it just chugs along until it can determine that one is missing. Like at the end of the script, or at the end of a function.

[jchd]

Time to hunt for a copy of one of the dragon books. Even the first edition '77 will be good!

[twitchyliquid64]

Ok thanks for the help guys; my interpreters are working!!!