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Sep 12th, 2009

While and raw_input

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Can someone please tell me what I am doing wrong. I know this is easy, but I haven't managed to make it work properly.

python Syntax (Toggle Plain Text)
overwrite = None
while overwrite != 'Y'.lower() or overwrite != 'N'.lower():
    overwrite = raw_input("Do you want to overwrite (Y/N)? ")
overwrite = None
while overwrite != 'Y'.lower() or overwrite != 'N'.lower():
    overwrite = raw_input("Do you want to overwrite (Y/N)? ")

It doesn't exit the while loop.

Thanks.

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nunos

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Sep 12th, 2009

Re: While and raw_input

You are creating a situation where the while loop exit condition is always True. Test this ...

python Syntax (Toggle Plain Text)
overwrite = None
while overwrite != 'y' or overwrite != 'n':
    overwrite = raw_input("Do you want to overwrite (Y/N)? ").lower()
    print overwrite != 'y'
    print overwrite != 'n'
    # overall condition
    print False or True
overwrite = None
while overwrite != 'y' or overwrite != 'n':
    overwrite = raw_input("Do you want to overwrite (Y/N)? ").lower()
    print overwrite != 'y'
    print overwrite != 'n'
    # overall condition
    print False or True

Easier to understand is this approach ...

python Syntax (Toggle Plain Text)
while True:
    overwrite = raw_input("Do you want to overwrite (Y/N)? ").lower()
    if overwrite == 'y' or overwrite == 'n':
        break
while True:
    overwrite = raw_input("Do you want to overwrite (Y/N)? ").lower()
    if overwrite == 'y' or overwrite == 'n':
        break

... or even simpler ...

python Syntax (Toggle Plain Text)
while True:
    overwrite = raw_input("Do you want to overwrite (Y/N)? ").lower()
    if overwrite in 'yn':
        break
while True:
    overwrite = raw_input("Do you want to overwrite (Y/N)? ").lower()
    if overwrite in 'yn':
        break

Last edited by vegaseat; Sep 12th, 2009 at 3:49 pm.

vegaseat

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DaniWeb's Hypocrite

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Sep 12th, 2009

Re: While and raw_input

For future coding conundrums, an "or" can be viewed as an if / elif, and "and" can be viewed as two nested if statements, which you would adapt to while() statements.

Python Syntax (Toggle Plain Text)
while overwrite != 'Y'.lower() or overwrite != 'N'.lower():  becomes
if overwrite != 'Y'.lower():
    do something
elif overwrite != 'N'.lower():
    do the same thing
 
For completeness, the 'and' would be
if overwrite != 'Y'.lower() and overwrite != 'N'.lower():  becomes
if overwrite != 'Y'.lower():
    if overwrite != 'N'.lower():
       do something
while overwrite != 'Y'.lower() or overwrite != 'N'.lower():  becomes
if overwrite != 'Y'.lower():
    do something
elif overwrite != 'N'.lower():
    do the same thing

For completeness, the 'and' would be
if overwrite != 'Y'.lower() and overwrite != 'N'.lower():  becomes
if overwrite != 'Y'.lower():
    if overwrite != 'N'.lower():
       do something

Hopefully this will help sort out the and/or's in the future.

woooee

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Nearly a Posting Maven

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Sep 13th, 2009

Re: While and raw_input

What woooee said was correct:

python Syntax (Toggle Plain Text)
overwrite=""
while(overwrite!="Y".lower() and overwrite!="N".lower()):
    overwrite=raw_input()
overwrite=""
while(overwrite!="Y".lower() and overwrite!="N".lower()):
    overwrite=raw_input()

sravan953

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Sep 13th, 2009

Re: While and raw_input

Thanks vegaseat, woooee and sravan953. You were all very helpful.

nunos

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Sep 13th, 2009

Re: While and raw_input

Click to Expand / Collapse Quote originally posted by sravan953 ...

What woooee said was correct:
python Syntax (Toggle Plain Text)
overwrite=""
while(overwrite!="Y".lower() and overwrite!="N".lower()):
    overwrite=raw_input()
overwrite="" while(overwrite!="Y".lower() and overwrite!="N".lower()): overwrite=raw_input()

Here "Y".lower() does not make any sense since it is simply 'y'

sneekula

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Nearly a Posting Maven

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Sep 13th, 2009

Re: While and raw_input

@sneekula, that is what nunos wanted.

sravan953

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