conditional expression sought

Elaine Jackson elainejackson7355 at
Sat Jan 31 06:57:39 CET 2004

Thanks. I've got this straightened around in my head now. I just sort-of
'panicked' when I saw a boolean literal where I didn't expect one to be (ie:
returned by the expression with the A's and B's when all the A's are False). I
realized later that that's what I should have expected, and that you could
always just add "...or undefined()" to such an expression, where

def undefined():
    raise "undefined conditional expression"

This is all just part of my newbie efforts to assimilate the language. Anyway,
thanks again for your help.


"Dave K" <dk123456789 at> wrote in message
news:ulnl10teov6ceq1q52laduq13d7u9edddd at
| On Fri, 30 Jan 2004 01:06:55 GMT in comp.lang.python, "Elaine Jackson"
| <elainejackson7355 at> wrote:
| >Sorry to take so long but I wasn't sure what a "unit test" was (the other
| >post clarified it). Tell me if this isn't what you're looking for:
| >
| >falsies=[0,0.0,[],(),{},'',None]
| >truies=[49,3.14,[1,2,3],(4,5,6),{7:8,9:10},'nonempty']
| >
| >def demo(A,B):
| >    print "If A is ",A
| >    print "and B is ",B
| >    print "then (A[0] and B[0]) or (A[1] and B[1]) or (A[2] and B[2]) = ",
| >    print (A[0] and B[0]) or (A[1] and B[1]) or (A[2] and B[2])
| >
| >A=[]
| >from random import randint
| >for i in range(3):
| >    A.append(bool(randint(0,1)))
| >B=truies[0:3]
| >demo(A,B)
| >
| >A=[False,False,False]
| >B=falsies[0:3]
| >demo(A,B)
| >print "I would have liked this to be B[2] = ",B[2]
| >
| (snip)
| Do you mean that the expression should return the last element in B if
| all elements in A are false? If all subexpressions before the last are
| false, the whole conditional reduces to:
|     A[-1] and B[-1]
| So simply force (a copy of) A[-1] to always be true. Instead of
| rewriting demo, I'll cheat by modifying the call:
| >>> A=[False, False, False]
| >>> B=[0, 0.0, []]
| >>> demo(A[:-1]+[True], B)
| If A is  [False, False, True]
| and B is  [0, 0.0, []]
| then (A[0] and B[0]) or (A[1] and B[1]) or (A[2] and B[2]) =  []
| >>> print A
| [False, False, False]
| For complete generality, you should also consider the case where
| len(A) != len(B). Truncate the longer list, or extend the shorter?
| Does it matter which list is shorter? Or forget the whole mess and
| raise an exception? There are lots of reasonable possibilities, but
| they won't all lead to the same result for certain input values.
| You're in charge of this project, you decide :)
| Dave

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