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Conditional operator in Python?

thp at cs.ucr.edu thp at cs.ucr.edu
Tue Sep 4 03:58:56 EDT 2001 
On Tue, Sep 04, 2001 at 09:48:13AM +0200, Mikael Olofsson wrote:
> 
> On 04-Sep-2001 thp at cs.ucr.edu wrote:
>  >  Okay, factorial is a somewhat simplistic example.  Consider the
>  >  following function, which counts the k-way partitions of n:
>  >  
>  >    p = lambda k, n : (
>  >        ( k < 1  or k > n  ) and [0] or 
>  >        ( n == 1 or n == k ) and [1] or 
>  >        [p(k-1,n-1)+p(k,n-k)] 
>  >      )[0]
>  >  
>  >  Using, say, ?: notation this would be written:
>  >  
>  >    p = lambda k, n : 
>  >      k < 1  or k > n  ? 0 :
>  >      n == 1 or n == k ? 1 :
>  >      p( k-1, n-1 ) + p (k, n-k )
>  >  
>  >  which seems much more concise and readable.
> 
> Well, both are just as hard to read to me, but then I have never used 
> C. I (almost) always use if-then-else, since that fits my small brain 
> best. Sure, it needs more rows, but I prefer that to both variations of
> line noise above.

So, replace "?" by "then" and ":" by "else", obtaining:

    p = lambda k, n : 
      k < 1  or k > n  then 
        0 
      else n == 1 or n == k then 
        1 
      else 
        p( k-1, n-1 ) + p (k, n-k )

which reads better with a couple of if's added:

    p = lambda k, n : 
      if k < 1  or k > n  then 
        0 
      else if n == 1 or n == k then 
        1 
      else 
        p( k-1, n-1 ) + p (k, n-k )

I'm not lobbying for ?: notation in particular.  I simply want
something better than (a and [b] or [c])[0].  In fact, the
if/then/else notation is my personal favorite.

Regards,
Tom Payne
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