IT, Programming, & Web Development › Forums › CS50’s Introduction to Computer Science by Harvard University on Edx › Week 6: Python › MITx 6.00.1x Introduction to Computer Science and Programming Using Python › Week 1: Python Basics › Why logical or fails in conditional checks and how to fix it in Python
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September 12, 2024 at 12:05 pm #3509
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Let’s break it down again, focusing specifically on how Python processes the expression:
The Problematic Expression:
if x == 'a' or 'i' or 'o' or 'e' or 'u':
Step-by-Step Explanation:
- How Python Evaluates the Condition:
Python evaluates conditions involvingor
from left to right. Here’s what happens when it encounters the expression:
- The first part is
x == 'a'
. - If
x
is equal to'a'
, then this part evaluates toTrue
. - If
x
is not'a'
, this part evaluates toFalse
. -
Next, Python moves to the second part,
'i'
. - In Python, any non-empty string is considered
True
. So'i'
is alwaysTrue
, regardless of the value ofx
. - Because of this, Python doesn’t even check the remaining parts (
'o'
,'e'
,'u'
).
- Why Non-Empty Strings Are
True
:
In Python, non-empty values such as strings, lists, or numbers are treated asTrue
when evaluated in conditions. So, the string'i'
is treated asTrue
, which causes the entireor
expression to be consideredTrue
as soon as it encounters'i'
.
Therefore, the expression simplifies like this:
if (x == 'a') or True or True or True or True:
Since
or
returnsTrue
as soon as it encounters aTrue
value, the entire condition becomesTrue
for every character in the string, not just vowels.Why This is Wrong:
- What you intended: You want to check if
x
is equal to'a'
,'e'
,'i'
,'o'
, or'u'
. - What actually happens: Python only checks if
x == 'a'
, and after that, it treats the other strings ('i'
,'o'
, etc.) asTrue
, regardless of the value ofx
.
Correcting the Condition:
To properly check if
x
is any of the vowels, you should use thein
operator, which checks membership in a collection (like a string or list):if x in 'aeiou':
This way, Python checks if
x
is in the string'aeiou'
, meaning it comparesx
against all the vowels.Summary:
- Your original expression:
x == 'a' or 'i' or 'o' or 'e' or 'u'
doesn’t work because'i'
,'o'
,'e'
, and'u'
are treated as alwaysTrue
, so the condition always evaluates toTrue
. - Correct way: Use
x in 'aeiou'
to check ifx
is one of the vowels, ensuring the condition behaves as intended.
This is why your code increments
y
for every character in the string, not just for vowels. - How Python Evaluates the Condition:
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