What's wrong with this code? - Printable Version

What's wrong with this code? - Printable Version

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+---- Thread: What's wrong with this code? (/showthread.php?tid=337)

What's wrong with this code? - LTee - 2011-03-24

Okay, this is driving me mad. :-D

I wrote a little routine to 'shuffle' the numbers 1 to 10 into an array, randomly. It's not exactly the best way of doing this but it was supposed to be a quick test. Regardless, it doesn't work. Or rather, it does work - the first time you run it. The second loop of the same code never completes and I can't decide if I found a bug or if I've made a mistake.

Here's the code:

Code:
dim levelMap(10) as UBYTE

randomize
for x = 1 to 10
	cls
	generateLevelMap()
	for n = 1 to 10: print levelMap(n): next n
	pause 0
next x

SUB generateLevelMap()
	dim lm as UBYTE
	dim lmPos as UBYTE = 0
	
	'clear the map first
	for lm = 1 to 10
		levelMap(n) = 0
	next lm
	
	'debug code to print the map, to make sure it's empty
	print
	for lm = 1 to 10
		print levelMap(n);",";
	next lm
	print
	pause 0
	
	'now fill it randomly
	print
	for lm = 1 to 10
		do
			'get a random position in the array
			lmPos = int(rnd * 10) + 1
			
			'debug code to print the number we randomly picked
			print lmPos;",";
			
			'debug code to print the array element if it ISN'T empty
			if levelMap(lmPos) <> 0 then
				print ink 2;levelMap(lmPos);",";
			end if
			
		loop until levelMap(lmPos) = 0
		
		'write the current number into the empty random element
		levelMap(lmPos) = lm
		
		'debug code to show that we handled this number okay
		print inverse 1;lm
		
	next lm
	
	pause 0
	cls
END SUB

I've stuck a load of debug info in there so that you can see what it's doing.

Basically it does this:
1. Clear the array to all zeroes
2. For each number, 1 to 10....
3. Look in a random array position to see if it's zero.
3a. if it isn't, loop around to 3 and try a different one
3b. if it is, write the number into that array position and move on to the next number
4. The result should be the numbers 1 to 10 in a random order in the array, after a variable number of iterations.

First run, no problem.

Second run, although the debug code shows that the array was cleared, the do...loop until loop never completes - by the time it's checking the array within that loop it contains the OLD values again, despite me having zeroed them out just a few seconds earlier.

Am I missing an obvious goof? :-D

Re: What's wrong with this code? - LCD - 2011-03-24

Code:
for lm = 1 to 10
      levelMap(n) = 0
   next lm

Maybe if you change it to:

Quote:for lm = 1 to 10
levelMap(lm) = 0
next lm

Then it will really put a zero in each position of the array.

Look also here:

Code:
for lm = 1 to 10
      print levelMap(n);",";
   next lm

But this is only a suspection...

Re: What's wrong with this code? - LTee - 2011-03-24

Ha ha, you're right! What a clown I am. :-D

That'll teach me to copy/paste stuff from one file to another! Thanks, LCD!

Re: What's wrong with this code? - boriel - 2011-03-24

Another pending feature: a command-line flag to enforce the user to *declare* (DIM) any used variable. This way, the levelMap(n) should complain about undeclared "n" var (or 2 Variable not Found) ;-) Free Basic also has this feature. I will put it, but need to re-re-check everything in the 1.2.8 version which now seems to be relatively stable at the moment...

Re: What's wrong with this code? - LTee - 2011-03-24

I'd like that! That's not the first time I've tripped up because of a bad copy/paste, and occasionally I've forgotten to declare something entirely and ended up with a float when a ubyte would've done.

And I'd agree on 1.2.8, by the way - it seems to be working great for me!