[Tutor] exceptions problem

[Roelof Wobben]

Date: Fri, 10 Sep 2010 20:23:09 +0200
From: fal at libero.it
To: tutor at python.org
Subject: Re: [Tutor] exceptions problem

On 10/09/2010 18.12, Roelof Wobben wrote:
> ...
> def readposint():
>         x = raw_input("Please enter a positive integer :")
>         try:
>               if not (x == int(x) and x<  0): raise(ValueError)
>         except:
>                 print x , "is not a positive integer.    Try again."
>                 return False
>         return True
>
> y = readposint()
> print y
> while y == False:
>         readposint()
> print "You have entered : ", y
>
> But -9 and 2 are both true.
My fault, I didn't notice that after raw_input, whatever you enter is a 
STRING, not an integer! So, without any exception thrown, the comparison
x == int(x) is always False. Let's make it better:
    if (int(x)<0 or (float(x) - int(x) > 0)): raise(ValueError)
 
 
Then, if the input value x is indeed a positive integer, you should 
return x, not True or False. Try returning -1 if the exception is 
thrown, in line 7, and returning x in line 8. Then, you should change 
also line 12... ok, here's to you:
 
def readposint():
     x = raw_input("Please enter a positive integer :")
     try:
         if (int(x)<0 or (float(x) - int(x) > 0)): raise(ValueError)
     except:
         print x , "is not a positive integer.    Try again."
         return -1
     return x
 
y = readposint()
print y
while y == -1:
     readposint()
print "You have entered : ", y
 
>
> Roelof
FrancescoThank you.I never thought that you can use a float and a integer to look if the number is a integer.Roelof
 
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