Nested for loop problem

[Don Low]

Hi,

I'm studying the python tutorial at python.org. In introducing the
concept of *break* and *continue* Statements, and *else* Clauses on
Loops, there's an example that goes as follows:

#!/usr/bin/python

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'

I have a problem with this nested for loop, because the answer it yields
doesn't make sense to me. Here is the answer:

2 is a prime number
3 is a prime number
4 equals 2 * 2
5 is a prime number
6 equals 2 * 3
7 is a prime number
8 equals 2 * 4
9 equals 3 * 3

Where I have the problem is as follows:

n = 2 in the first iteration of range(2, 10); therefore x = [] in the
first iteration of range(2, n). More specifically:
[color=blue][color=green][color=darkred]
>>> range(2, 10)[/color][/color][/color]
[2, 3, 4, 5, 6, 7, 8, 9][color=blue][color=green][color=darkred]
>>> range(2, 2)[/color][/color][/color]
[]
[color=blue][color=green][color=darkred]
>>> for x in range(2, 2):[/color][/color][/color]
.... print x
....

Since x prints nothing, I guess x = NULL. If I try:

if 2 % NULL or blank, I get an error message.

So the question is when I run the script, why doesn't it produce an
error message?

OK, I'm sure I'm misunderstandin g something, but could someone explain?


--
Thanks,

Don

[Irmen de Jong]

Re: Nested for loop problem

Don Low wrote:[color=blue][color=green][color=darkred]
>>>>for x in range(2, 2):[/color][/color]
>
> ... print x
> ...
>
> Since x prints nothing, I guess x = NULL. If I try:[/color]

That's your misunderstandin g. That loop does *nothing*,
it doesn't enter the loop body. When you're looping over
an empty sequence, the loop body is never entered.

(also, NULL is meaningless in Python. I think you meant
None, right?)

--Irmen

[Don Low]

Re: Nested for loop problem

In article <3fad715f$0$587 15$e4fe514c@new s.xs4all.nl>, Irmen de Jong wrote:[color=blue]
> Don Low wrote:[color=green][color=darkred]
>>>>>for x in range(2, 2):[/color]
>>
>> ... print x
>> ...
>>
>> Since x prints nothing, I guess x = NULL. If I try:[/color]
>
> That's your misunderstandin g. That loop does *nothing*,
> it doesn't enter the loop body. When you're looping over
> an empty sequence, the loop body is never entered.[/color]

OK, so python knows not to even bother going through the loop if the
range is [].[color=blue]
>
> (also, NULL is meaningless in Python. I think you meant
> None, right?)[/color]

Yes, as in range(2, 2) yields nothing.

I have another question which I hope you can answer. If I do

2 % 2 in the python interpreter, the answer is 0, and yet 2 is a prime
number.

Thanks for your time,

Don[color=blue]
>
> --Irmen
>[/color]


--
Thanks,

Don

[Michael Geary]

Re: Nested for loop problem

> #!/usr/bin/python[color=blue]
>
> 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'[/color]
[color=blue]
> I have another question which I hope you can answer. If I do
>
> 2 % 2 in the python interpreter, the answer is 0, and yet 2 is
> a prime number.[/color]

2 modulo 2 is zero. Any number modulo itself is zero.

If the question is why the code above reports 2 as a prime number, that's
because it never gets to the "if n %x == 0" when n is 2. "for x in
range(2,2)" does not execute the loop at all but goes directly to the else.

I think you would find it helpful to step through this code line by line in
any Python debugger such as PythonWin or IDLE--that way you can see exactly
what it does as it executes.

-Mike

[Duncan Smith]

Re: Nested for loop problem


"Don Low" <m_tessier@symp atico.ca> wrote in message
news:boju9f$1fg li4$1@ID-145503.news.uni-berlin.de...[color=blue]
> In article <3fad715f$0$587 15$e4fe514c@new s.xs4all.nl>, Irmen de Jong[/color]
wrote:[color=blue][color=green]
> > Don Low wrote:[color=darkred]
> >>>>>for x in range(2, 2):
> >>
> >> ... print x
> >> ...
> >>
> >> Since x prints nothing, I guess x = NULL. If I try:[/color]
> >
> > That's your misunderstandin g. That loop does *nothing*,
> > it doesn't enter the loop body. When you're looping over
> > an empty sequence, the loop body is never entered.[/color]
>
> OK, so python knows not to even bother going through the loop if the
> range is [].[color=green]
> >
> > (also, NULL is meaningless in Python. I think you meant
> > None, right?)[/color]
>
> Yes, as in range(2, 2) yields nothing.
>[/color]

It doesn't return nothing; it returns an empty list. You *can* write a
function that returns None.
[color=blue][color=green][color=darkred]
>>> def func():[/color][/color][/color]
return
[color=blue][color=green][color=darkred]
>>> a = func()
>>> a is None[/color][/color][/color]
True[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

Duncan