Sudoku

Eric Parry joan4eric at gmail.com
Fri Mar 29 06:07:27 CET 2013


On Friday, March 29, 2013 9:58:27 AM UTC+10:30, Dave Angel wrote:
> On 03/28/2013 06:11 PM, Eric Parry wrote:
> 
> > On Thursday, March 28, 2013 3:06:02 PM UTC+10:30, Dave Angel wrote:
> 
> 
> 
> 
> 
>     <SNIP>
> 
> >>
> 
> >>
> 
> >> Are you familiar with recursion?  Notice the last line in the function
> 
> >> r() calls the function r() inside a for loop.
> 
> >>
> 
> >> So when r() returns, you're back inside the next level up of the
> 
> >> function, and doing a "backtrack."
> 
> >>
> 
> >> When the function succeeds, it will be many levels of recursion deep.
> 
> >> For example, if the original pattern had 30 nonzero items in it, or 51
> 
> >> zeroes, you'll be 51 levels of recursion when you discover a solution.
> 
> >>
> 
> >> If you don't already understand recursion at all, then say so, and one
> 
> >> or more of us will try to explain it in more depth.
> 
> >>
> 
>        <SNIP>
> 
> >
> 
> > Thank you for that explanation.
> 
> > No, I do not understand recursion. It is missing from my Python manual. I would be pleased to receive further explanation from anyone.
> 
> > Eric.
> 
> >
> 
> 
> 
> Recursion is not limited to Python.  It's a general programming term, 
> 
> and indeed applies in other situations as well.  Suppose you wanted to 
> 
> explain what the -r switch meant in the cp command.  "r" stands for 
> 
> recursion.
> 
> 
> 
> (example is from Linux.  But if you're more familiar with Windows, it's 
> 
> the /s switch in the DIR command.)
> 
> 
> 
> The cp command copies all the matching files in one directory to another 
> 
> one.  If the -r switch is specified, it then does the same thing to each 
> 
> subdirectory.
> 
> 
> 
> Notice we did NOT have to specify sub-subdirectories, since they're 
> 
> recursively implied by the first description.
> 
> 
> 
> 
> 
> Closer to the current problem, suppose you defined factorial in the 
> 
> following way:  factorial(0) is 1, by definition.  And for all n>0, 
> 
> factorial(n) is n*factorial(n-1).
> 
> 
> 
> So to directly translate this definition to code, you write a function 
> 
> factorial() which takes an integer and returns an integer.  If the 
> 
> parameter is zero, return one.  If the parameter is bigger than zero, 
> 
> then the function calls itself with a smaller integer, building up the 
> 
> answer as needed  (untested).
> 
> 
> 
> def  factorial(n):
> 
>      if n==0:
> 
>          return 1
> 
>      val = n *factorial(n-1)
> 
>      return val
> 
> 
> 
> 
> 
> 
> 
> 
> 
> -- 
> 
> DaveA

Thank you for that Dave, I've started writing the VBA code.
Eric.



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