On Sat, Jul 9, 2011 at 1:33 AM, J. S. John <phillyj...@gmail.com> wrote:
> Hi all,
> I'm teaching myself perl. Right now, I am stuck with this script. I
> don't understand how it works. I know what it does and how to do it by
> hand.
>
> $n = 1;
> while ($n < 10) {
> $sum += $n;
> $n += 2;
> }
> print "The sum is $sum.\n"
>
> I know $sum is initially 0 (undef). I see that $sum becomes 1, then $n
> becomes 3. The loop goes back and then I don't understand. Now $sum is
> 1+3?
>
> Thanks,
> JJ
>
> --
> To unsubscribe, e-mail: beginners-unsubscr...@perl.org
> For additional commands, e-mail: beginners-h...@perl.org
> http://learn.perl.org/
>
>
>
First of all it is a really bad thing to not use warnings and strict in your
scripts just start your script with:
use warnings;
use strict;
and this script will end up screaming that you have to declare your
variables.
So your script would end up looking like this:
use warnings;
use strict;
my $n = 1;
my $sum; # Should actually be my $sum = 0; even if it is only for
readability but we shall let that slide for now ;-)
while ($n < 10) {
$sum += $n;
$n += 2;
}
print "The sum is $sum.\n"
So what does the script do?
Well literally it says while $n is smaller then 10 execute what is in side
the { } brackets. Inside you say $sum = $sum + $n; and then $n = $n + 2;
So on the second loop you look at the value of $n which now is 1 + 2 = 3. So
then we take $sum which is 1 and add $n to that so 1 + 3 = 4 and we increase
$n by 2 ending up at 5.
The next loop (5 is smaller then 10 after all) $sum becomes 4 + 5 = 9 and $n
becomes 5 + 2 = 7
The next loop (7 is smaller then 10 after all) $sum becomes 9 + 7 = 16 and
$n becomes 7 + 2 = 9
The next loop (9 is smaller then 10 after all) $sum becomes 16 + 9 = 25 and
$n becomes 9 + 2 = 11
Then 11 is not smaller then 10 anymore an the loop ends the next thing you
do is print the value of $sum which should by now be 25.
To see if I am right why not modify the script a little so it will print the
inner values:
use warnings;
use strict;
my $n = 1;
my $sum = 0;
while ($n < 10) {
print '$sum = ' . $sum . ' $n = ' . $n;
print '$sum += $n';
$sum += $n;
print '$sum = ' . $sum;
$n += 2;
print '$n + 2 = ' . $n;
}
print "The sum is $sum.\n"
Now it should show you exactly what it is doing inside the loop :-)
Regards,
Rob
