While loop condition

Question

This is a really basic question, and I apologize for its simplicity, but I have been searching for the answer and trying different syntax for hours without luck.

I am using python to create a text menu for a cipher program. I am using a while loop for an error message when an invalid key is pressed, but it loops even when the condition is false.

purpose = input("Type 'C' for coding and 'D' for decoding: ")

while purpose.upper() != "D" or "C":
    purpose = input("Error, please type a 'C' or a 'D': ")

if (purpose.upper() == "C"):
    do_something()

if (purpose.upper() == "D"):
    do_something()

For some reason the error message is displayed regardless of key press. Thank you so much for the help!

score 3 · Answer 1 · answered Sep 11 '14 at 04:01

3

You need to think of conditions on either side of or and and as logically independent.

When the computer sees:

while purpose.upper() != "D" or "C":

It reads it as

(purpose.upper() != "D")

OR

"C"

The second part, "C" alone, is always true.

You probably wanted:

while purpose.upper() != "D" or purpose.upper() != "C":

or better yet:

while purpose.upper() not in ("C", "D"):

answered Sep 11 '14 at 04:01

sapi

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The last option will return `True` when the user enters `CD` :) – Nir Alfasi Sep 11 '14 at 04:03
@alfasin - Good point, I assumed it was a char. I'll fix it :P – sapi Sep 11 '14 at 04:04

Nir Alfasi · Accepted Answer · 2014-09-11T15:32:55.797

1

Change:

while purpose.upper() != "D" or "C":

to:

while purpose.upper() != "D" and purpose.upper() != "C":

As Saish suggested in the comments below, a more pythonic way of doing that would be:

while purpose.upper() not in ("C", "D"):

edited Sep 11 '14 at 15:32

answered Sep 11 '14 at 04:01

Nir Alfasi

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1

The pythonic way of doing this is really: `while purpose.upper() not in ("C", "D"):` – Saish Sep 11 '14 at 05:18
@Saish +1 very true! (answer updated) – Nir Alfasi Sep 11 '14 at 15:33

score 0 · Answer 3 · answered Sep 11 '14 at 04:42

Here, try this one. it seems to work

 reason = ['D', 'C']
 while True:
     purpose = input("Type 'C' for coding and 'D' for decoding: ").upper()
     if reason.__contains__(purpose):
         print("You've got it this time")
         break

Let me know how it works out

score 0 · Answer 4 · answered Sep 11 '14 at 04:48

while purpose.upper() != "D" or "C":

The above line will be evaluated to :

while (purpose.upper() != "D") or "C":

Expressions are evaluated left to right. Here "C" is always true, hence the loop is always executed.

You need to have something like this:

while purpose.upper() != "D" or purpose.upper() != "C":

or

#You can put it all into a list ["C", "D"] and then check 
while purpose.upper() not in ["C", "D"]:

or

#You can put it all into a tuple ("C", "D") and then check 
while purpose.upper() not in ("C", "D"):

While loop condition

4 Answers4