My backwards logic
davea at davea.name
Fri Sep 5 19:15:23 CEST 2014
Seymore4Head <Seymore4Head at Hotmail.invalid> Wrote in message:
> I'm still doing practice problems. I haven't heard from the library
> on any of the books I have requested.
> This is not a hard problem, but it got me to thinking a little. A
> prime number will divide by one and itself. When setting up this
> loop, if I start at 2 instead of 1, that automatically excludes one of
> the factors. Then, by default, Python goes "to" the chosen count and
> not "through" the count, so just the syntax causes Python to rule out
> the other factor (the number itself).
> So this works:
> while True:
> print (a)
> for x in range(2,a):
> if a%x==0:
> print ("Number is not prime")
> wait = input (" "*40 + "Wait")
> But, what this instructions want printed is "This is a prime number"
> So how to I use this code logic NOT print (not prime) and have the
> logic print "This number is prime"
The traditional way of telling whether something happened in a
loop is to set a flag to False outside the loop, and
conditionally set it to True in the if test. Then after the loop,
check your flag.
Python however has a better way. You can put an else clause on the
for x in range(2,a):
print ("Number is not prime")
print ("Number is prime")
The else clause fires if no break executed.
There are also ways to do it using not, any, and a list
comprehension, no explicit loop at all.
More information about the Python-list