> From: Knut erik Teigen
> Sent: Thursday, August 03, 2006 12:21 PM
> 
> Hello
> I've just started learning Ruby, and for a first project, I 

Welcome!

> #!/usr/bin/ruby -w
> 
> def solve(i,j,grid)
> 
>   #termination condition
>   if j==9 #completed one row, start next
>     j=0
>     i+=1
>     if i==9 #then we're done!
>       return true
>     end
>   end
> 
>   #we don't want to test for given values, so
>   #jump to next cell
>   return solve(i,j+1,grid) if grid[i*9+j] != 0
> 
>   #recursive part.
>   #check each value for this cell,
>   #if value gets set, continue with next cell
>   for value in 1..9
>     if possible(i,j,value,grid)
>       grid[i*9+j]=value
>       return true if solve(i,j+1,grid)
>     end
>   end

You want to return nil or false here.
Plus something like grid[i*9+j]=0 is missing.
 
> end
> 
> #checks if value can be placed in this cell without
> #conflicts in rows, columns or boxes
> def possible(i,j,value,grid)
> 
>   #checks row
>   row = i*9
>   return false if grid[row...row+9].index(value)!=nil
> 
>   #checks column
>   col = j
>   for i in 0..8

this destroys your parameter 'i'

>     if grid[i*9+col]==value
>       return false
>     end
>   end
> 
>   #checks box
>   #integer math to find start of box
>   i0=(i/3).to_i*3
>   j0=(j/3).to_i*3
>   for i in i0...i0+3
>     for j in j0...j0+3
>       if grid[i*9+j]==value
>         return false
>       end
>     end
>   end
> 
>   #value not found, so it's a possible choice for this cell
>   return true
> 
> end
> ... snip ...

cheers

Simon