[Tutor] Else Clause In A Loop

[kinuthia muchane]

On Mon, 2008-05-12 at 14:08 -0400, "Simón A. Ruiz" wrote:
> I'll try my hand at this:
> 
> The outside for loop is looking through every number up to the variable 
> "number".
No quarrel here.
> 
> For each of those numbers, it checks to see if any number between 2 and 
> i is divisible into i. If it finds anything, we know it's not a prime, 
> and so it breaks out of that second loop without completing it, which 
> means the else block isn't executed.

This is where I am getting lost. When the variable 'number' is 3, it
means that in that instance the inner 'for' statement  will be 'for j in
range(2,3)', hmmm which means that we will be dividing each element by 2
in the first 'for' statement and checking whether it is true , no? But
2%2 is zero, so, in my warped logic, the inner 'for' loop should break
and the else clause will not execute!
> 
> If it can't find anything that i is divisible by, then that inside for 
> loop finishes without breaking, we know that i is a prime number, and 
> the "else" clause is executed.

This is where it gets even more interesting for me. Wont 'i' in one
instance be 8, so at that particular moment there will be a 7 in the
inner 'for' loop which will be divisible by one of our 'prime' numbers
ie 7?! I know I am wrong but for some reason I cannot see the light! :-)
> 
> range(2,2) is is an empty list, so the loops ends uneventfully without 
> doing anything and the else clause is executed.
> 
> I'm not an expert, and this seems to me to be what's happening there.
> 
> Simón

All the same, thanks...

> 
> kinuthia muchane wrote:
> > Hi,
> > 
> > I learnt that a loop can have an else clause. And that this clause
> > executes when the loop TERMINATES. In a while loop when the condition
> > becomes false, and in a for loop when a sequence is exhausted. When I
> > write the following code it seems to work:
> > 
> > for n in [1,2,3,4,5]:
> > 	print 'we are in the loop',n
> > else:
> > 	print 'we are now EXITING the loop',n
> > 
> > which results in:
> > 
> > we are in the loop 1
> > we are in the loop 2
> > we are in the loop 3
> > we are in the loop 4
> > we are in the loop 5
> > we are now EXITING the loop 5
> > 
> > Or:
> > 
> > n = 1
> > while n <= 5:
> > 	print 'we are in the loop',n
> > 	n += 1
> > else:
> > 	print 'we are now EXITING the loop',n
> > 
> > ...which gives:
> > 
> > we are in the loop 1
> > we are in the loop 2
> > we are in the loop 3
> > we are in the loop 4
> > we are in the loop 5
> > we are now EXITING the loop 6 (it spills over here!)
> > 
> > This has served to confuse me more. Would someone please kindly explain
> > how all this fits into the code below which searches (and finds!) for
> > prime numbers...
> > 
> > 
> > def prime():
> >    number = int(raw_input("Enter a number :"))
> >    for i in range(2,number):
> > 	for j in range(2,i):
> > 		if i%j == 0:
> > 			break
> > 	else:
> > 		print  i, "is a prime number"
> > prime()
> > 
> > ...especially in the instance when number is 2 in the first for
> > statement, for then we will have for j in range(2,2)! Or what is going
> > on?? 
> > 
> > Thanks,
> > Kinuthia...
> > 
> > 
> > _______________________________________________
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