Re: Newbie question about variables, arrays and where they all go
On Mon Jan 19 2009 @ 4:21, dolphin_sonar wrote:
> Again, you answered my question. You should be teaching Perl as your
> responses are very clear and that's not easy for people to do...give
> clear, concise responses.
Actually, I had a big goof in my response. The program and the print
statement are fine as you had them, and *I* got myself turned around.
When I typed up your program, I was thinking of other ways I might do it,
and I produced this:
sub running_sum {
state $sum = 0;
state @numbers;
push @numbers, @_;
$sum += $_ foreach @numbers; ## WRONG - should be foreach @_
say "The sum of (@numbers) is $sum";
}
I thought it would be easier to push the whole @_ array into @numbers at
once and add everything into $sum in a one-liner as well. But I did it
wrong.
The problem was that I added the entire @numbers array to $sum, but from
from call to call, I only want to add the new items (the ones in the @_
array). That's what I get for trying to answer a question and rewrite the
example all at the same time.
As for 'say' it's a new feature in 5.10 (like the state variables) which
allows you to print without explicitly asking for the "\n" - it looks like
it's officially introduced in Learning Perl 5e on page 90.
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Re: Newbie question about variables, arrays and where they all go
On Jan 18, 7:54 pm, telemac...@arpinum.org (Telemachus) wrote: > On Sun Jan 18 2009 @ 10:59, dolphin_sonar wrote: > > > 1 # When calling 'running_sum(5, 6);' the variable 'state @numbers' > > receives those two > > 2 # parameters, (5 and 6), right? Then, the @numbers array also > > copies/stores (5 and 6) > > 3 # into the special '( @_ )' variable as well, right? Also, line > > 13 pushes '$number' into > > 4 # the '@numbers' array each time through the foreach loop, > > right? > > Maybe I'm not understanding what you mean, but I think you're confused. The > arguments to a subroutine go into the @_ array. The @numbers array is > empty until you load it up in the foreach loop. Having 5 and 6 as arguments > does not automatically put those items into @numbers, nor does @numbers > copy anything into @_. > > Also, I understand that you may just be testing out persistent variables in > 5.10, but this program is confusing. > > Consider what happens if you call running sum a second time (say as > running_sum( 2, 7 );) If I add that call, here's my output: > > telemachus ~ $ perl sum > The sum of (5 6) is 11 > The sum of (5 6 2 7) is 31 > > That appears to say that perl has added 5, 6, 2, and 7 up to 31. What > actually happened was that you added 5, 6, 2 and 7 (20) to 11 (the sum from > the previous call to running_sum - which was saved). To put this another way > around: maybe you want to keep a running sum, but the print statement in the > subroutine is very confusing. > > Hope this helps, T T! Again, you answered my question. You should be teaching Perl as your responses are very clear and that's not easy for people to do...give clear, concise responses. Regardless, I now understand exactly what your saying about the state @numbers declaration. I was confused about that and that helps me understand a lot of what is going on, that was important for me to understand. The example code actually had two more calls to running sum: running_sum( 1..3 ); and running_sum( 4 ); (see the link to the code and the out below). Here is a link to the example code in a much better format, you can see the code better than using Gmail IMO. http://sial.org/pbot/34575 # The output I get each time is this: The sum of (5 6) is 11 The sum of (5 6 1 2 3 ) is 17 The sum of (5 6 1 2 3 4 ) is 21 This example comes from page 68 O'Reilly Learning Perl 5th edition. They also used 'say' for the first time in the book instead of print as well, which was kind of confusing to me. -- To unsubscribe, e-mail: beginners-unsubscr...@perl.org For additional commands, e-mail: beginners-h...@perl.org http://learn.perl.org/
Re: Newbie question about variables, arrays and where they all go
From: Telemachus Date: Sun, 18 Jan 2009 20:17:27 -0500 To: beginners@perl.org Subject: Re: Newbie question about variables, arrays and where they all go On Sun Jan 18 2009 @ 7:54, Telemachus wrote: > The arguments to a subroutine go into the @_ array. The @numbers array is > empty until you load it up in the foreach loop. Edit: I should have said that the @numbers array is empty before the *first* run. Since it's persistent, after that it already contains all the items from previous calls. So on a second call, it already has 5 and 6. (I should also consider spelling 'understand' with a d.) -- To unsubscribe, e-mail: beginners-unsubscr...@perl.org For additional commands, e-mail: beginners-h...@perl.org http://learn.perl.org/
Re: Newbie question about variables, arrays and where they all go
On Sun Jan 18 2009 @ 10:59, dolphin_sonar wrote: >1 # When calling 'running_sum(5, 6);' the variable 'state @numbers' > receives those two >2 # parameters, (5 and 6), right? Then, the @numbers array also > copies/stores (5 and 6) >3 # into the special '( @_ )' variable as well, right? Also, line > 13 pushes '$number' into >4 # the '@numbers' array each time through the foreach loop, > right? Maybe I'm not understaning what you mean, but I think you're confused. The arguments to a subroutine go into the @_ array. The @numbers array is empty until you load it up in the foreach loop. Having 5 and 6 as arguments does not automatically put those items into @numbers, nor does @numbers copy anything into @_. Also, I understand that you may just be testing out persistent variables in 5.10, but this program is confusing. Consider what happens if you call running sum a second time (say as running_sum( 2, 7 );) If I add that call, here's my output: telemachus ~ $ perl sum The sum of (5 6) is 11 The sum of (5 6 2 7) is 31 That appears to say that perl has added 5, 6, 2, and 7 up to 31. What actually happened was that you added 5, 6, 2 and 7 (20) to 11 (the sum from the previous call to running_sum - which was saved). To put this another way around: maybe you want to keep a running sum, but the print statement in the subroutine is very confusing. Hope this helps, T -- To unsubscribe, e-mail: beginners-unsubscr...@perl.org For additional commands, e-mail: beginners-h...@perl.org http://learn.perl.org/
