difference in behavior within if statements for python

Question

i had two statements and my friend is claiming to me they are not the same. i believe they are. I was wondering if they are and if they aren't is there an example where the behavior is different.

if (n != p and c/n > 5.15):

if (c/n > 5.15 and n !=p):

Run them with n = p = 0 and see.

Martijn Pieters
– Martijn Pieters

2014-07-28 20:03:47 +00:00
Commented Jul 28, 2014 at 20:03 — Martijn Pieters
– Martijn Pieters, Commented Jul 28, 2014 at 20:03

BrenBarn · Accepted Answer · 2014-07-28 20:01:32Z

6

They could be different due to the short-circuiting behavior of and. If the first operand of and is false, the second is not evaluated. So if c/n > 5.15 raises an exception (for instance if n is zero), the first if may work (that is, not raise any error) while the second causes an error. Here is an example:

c = 0
n = 0
p = 0

# No error, no output because the condition was not true
>>> if (n != p and c/n > 5.15):
...     print "Okay!"

# Raises an error
>>> if (c/n > 5.15 and n !=p):
...     print "Okay!"
Traceback (most recent call last):
  File "<pyshell#23>", line 1, in <module>
    if (c/n > 5.15 and n !=p):
ZeroDivisionError: division by zero

answered Jul 28, 2014 at 20:01

BrenBarn

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2 Comments

bruno desthuilliers Over a year ago

Just to make things clear: from a pure boolean logic POV, both expressions are indeed equals. From a Python code POV they are not.

BrenBarn Over a year ago

Sure, but this is a question tagged Python on StackOverflow, not a logic question on Math SE.

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