basic python question about for loop

Reedick, Andrew jr9445 at ATT.COM
Wed Apr 9 23:23:30 CEST 2008



> -----Original Message-----
> From: python-list-bounces+jr9445=att.com at python.org [mailto:python-
> list-bounces+jr9445=att.com at python.org] On Behalf Of jmDesktop
> Sent: Wednesday, April 09, 2008 5:04 PM
> To: python-list at python.org
> Subject: Re: basic python question about for loop
> 
> >
> > > >>> for n in range(2, 10):
> > > ...     for x in range(2, n):
> > > ...         if n % x == 0:
> > > ...             print n, 'equals', x, '*', n/x
> > > ...             break
> > > ...     else:
> > > ...         # loop fell through without finding a factor
> > > ...         print n, 'is a prime number'
> > > ...
> >
> > > first time 2 mod 2, 2/2, no remainder == 0, what am I doing wrong?
> > > Why did it fall through?
> >

> 
> So what is n and x in the first iteration?  Sorry.  I'm trying.


You're never getting to n and x in the first iteration, because the 'for x in range(2, n)' loop isn't looping.

This:  
	for x in range(2, n)
is equivalent in C/Perl/etc. to:
	for(x=2; x<n; x++)
which for the first iteration is:
	for(x=2; x<2; x++)

Since (2 < 2) is false, you never call 'if n %x == 0:' in the first iteration.

Or to put it another way:
	Range(2, n) starts at 2, and stops _before_ n.
	Range(2, n) starts at 2, and stops _before_ 2.




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