Python returns wrong truth table for logical implication

Question

I have implemented the above implication in Python but it does not return the expected results:

  True       True None
  True      False None
 False       True True
 False      False None

My python code is:

def implies(a,b):
    if a:
        return b
    else:True
    return
for p in (True, False):
    for q in (True, False):
        print("%10s %10s %s" %(p,q,implies((p or q) and (not p), q)))

I don't understand the contradiction here. None implies False doesn't it? And why not print True like it should?

`else: True` should be `else: return True`. As is it doesn't do anything. — RemcoGerlich, Sep 21 '16 at 11:32
Looks like you can replace the implementation of `implies` with the value `True`, if every result is expected to be `T`. — deceze, Sep 21 '16 at 11:37
Thank you guys, this function was actually provided in an exercise so I didnt look twice into it! — Sarah cartenz, Sep 21 '16 at 11:39

Juri Robl · Accepted Answer · 2016-09-21T11:41:30.653

2

def implies(a,b):
    if a:
        return b
    else:True
    return

Your error is in the last two lines, if !a, you aren't returning a specific value, so the result is None. You want:

def implies(a,b):
    if a:
        return b
    else:
        return True

edited Sep 21 '16 at 11:41

answered Sep 21 '16 at 11:33

Juri Robl

5,614
2
29
46

Python returns wrong truth table for logical implication

1 Answers1