From: Mariano Kamp <mariano.kamp@...>
Date: 2006-10-10T15:41:49+09:00
Subject: Re: Learn to Program, by Chris Pine

Hi Mike,

   what your guard should do is prevent the recursion go across the  
boundaries of your world, right?

   I would put the guard at the beginning of the method to make sure  
that the rest of the method can be sure of handling valid data.

   if x < 0 || x > 10 || y < 0 || y > 10
     return 0 # respect the world's boundaries
   end


> def continent_size world, x, y
>   if world[y][x] != 'land'
>     # either it's water or we've already counted it; we don't want to
> count it
>     # again
>     return 0
>   end
That's a bit of a problem as your code might already call with out of  
bounds coordinates.
Put the boundary guard (see above) before that.

>   if ((y <= 10 && y > 0) && (x <= 10 && x > 0))
An array starts to count at zero ... so it needs to be (y <= 10 && y  
 >= 0), right?
You have eleven columns and eleven rows. So the array indices are 0..10.

>   # So, first we count this tile...
>   size = 1
>   world[y][x] = 'counted land'
>
>
>     # ...then we count all of the
>     # neighboring eigth tiles (and,
>     # of course, their neighbors via recursion)
>     size = size + continent_size(world, x-1, y-1)
>     size = size + continent_size(world, x  , y-1)
>     size = size + continent_size(world, x+1, y-1)
>     size = size + continent_size(world, x-1, y  )
>     size = size + continent_size(world, x+1, y  )
>     size = size + continent_size(world, x-1, y+1)
>     size = size + continent_size(world, x  , y+1)
>     size = size + continent_size(world, x+1, y+1)
>     size
Ok, you're returning the size here, but

>   end
what are you returning here, if you haven't been in this if-block?

nil. That is a major part of your problem...

> end
Use the code above and put it after the definition of the method.  
Remove your own "if" and your method should work.


> puts continent_size(world, 5, 5) #this should be fine; but what about
> (11,11)?
That's up to you. In my code (the guard) it returns 0. So that 11,11  
will return 0.
You could also raise an exception that the coordinates are out-of- 
bounds, but I guess that will come later in your book and is not that  
straight forward applicable here.

Cheers,
Mariano