Hello,=0A=0Afor the following program, what is the function of lines 12 and=
14 ???=0A=0A 1 #!/usr/bin/perl=0A 2 =0A 3 use strict;=0A 4 use warning=
s;=0A 5 use 5.010;=0A 6 use Spreadsheet::XLSX;=0A 7 =0A 8 my $excel =3D=
Spreadsheet::XLSX -> new ('Datos RCP 4_2_11-v2.xlsx');=0A 9 =0A 10 foreac=
h my $sheet ( [at] {$excel -> {Worksheet}}) {=0A 11 printf("Sheet: %s\n",=
$sheet->{Name});=0A 12 $sheet -> {MaxRow} ||=3D $sheet -> {MinRow};=
=0A 13 foreach my $row ($sheet -> {MinRow} .. $sheet -> {MaxRow}) {=
=0A 14 $sheet -> {MaxCol} ||=3D $sheet -> {MinCol};=0A 15 =
foreach my $col ($sheet -> {MinCol} .. $sheet -> {MaxCol}) {=
=0A 16 my $cell =3D $sheet -> {Cells} [$row] [$col];=
=0A 17 if ($cell) {=0A 18 =
printf("( %s , %s ) =3D> %s\n", $row, $col, $cell =0A-> {Val});=0A 19 =
}=0A 20 }=0A 21 }=0A 22 }=0A=0A=0A=


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>> 12 $sheet -> {MaxRow} ||=3D $sheet -> {MinRow};=0A>=0A>Line 12 ca=
n be written as:=0A>$sheet->{'MaxRow'} =3D $sheet->{'MaxRow'} || $sheet->{'=
MinRow'};=0A=0A=0Athen that I don't understand is the program logic :-(=0A=
=0Awhat's the purpose of lines 12 and 14?? =0A=0A=0A=0A

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--20cf3054a4d5dba41b049c05d54f
Content-Type: text/plain; charset=ISO-8859-1

It's the If $sheet->{MaxRow} is false[0], then the value of $sheet->{MinRow}
is assigned to that variable; If it's true, nothing happens.

Same deal with line 14; you canr ead more about the logical-or and other
operators in perldoc perlop[1].

Brian.

[0] http://perldoc.perl.org/perlsyn.html#Truth-and-Falsehood
[1] http://perldoc.perl.org/perlop.html

--20cf3054a4d5dba41b049c05d54f--

On Fri, Feb 11, 2011 at 5:38 AM, mailing lists <listas.correo [at] yahoo.es> wro=
te:
>>> 12 =C2=A0 =C2=A0 =C2=A0 =C2=A0$sheet -> {MaxRow} ||=3D $sheet -> {MinRo=
w};
>>
>>Line 12 can be written as:
>>$sheet->{'MaxRow'} =3D $sheet->{'MaxRow'} || $sheet->{'MinRow'};
>
>
> then that I don't understand is the program logic :-(
>
> what's the purpose of lines 12 and 14??

$sheet->{'MaxRow'} and $sheet->{'MinRow'} are used in the range in
line 13. Line 13 goes through a loop for every $row from MinRow to
MaxRow. If MaxRow isn't defined, the range operator in line 13 will
warn about uninitialized values. And if MaxRow isn't true
Spreadsheet::XLSX will probably complain since there is no row 0 in
Excel.

So line 12 checks to make sure that MaxRow is true. If MaxRow is true,
the ||=3D operator does nothing, but if MaxRow is false, the ||=3D
operator assigns the value of MinRow to it. This ensures that MaxRow
is always true (unless MinRow is also undef, which would be a bigger
problem) and that line 13 functions as expected. If MinRow =3D=3D MaxRow
(i.e. there is only one row), the loop only executes once.

HTH,

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On 11/02/2011 10:38, mailing lists wrote:
>>> 12 $sheet -> {MaxRow} ||= $sheet -> {MinRow};
>>
>> Line 12 can be written as:
>> $sheet->{'MaxRow'} = $sheet->{'MaxRow'} || $sheet->{'MinRow'};
>
>
> then that I don't understand is the program logic :-(
>
> what's the purpose of lines 12 and 14??

Perl has no proper boolean values. Instead, the boolean operators treat
zero, undef, and the null string '' all as false. Anything else is true.

So

'' || 'xx'

evaluates to true, because although the left operand is false, the right
one is true and so the 'or' of the two is true.

The reason code like

$x = $x || $y

or the equivalent

$x ||= $y

are useful is because of the way Perl evaluates the expression. It is
called short-circuit evaluation, and is described here

<http://www.perlmonks.org/?node_id=301355>
and
<http://en.wikipedia.org/wiki/Short-circuit_evaluation>

Essentially,

$x ||= $y

is equivalent to

if ($x) {
$x = $x;
}
else {
$x = $y;
}

and, clearly, assigning $x to itself has no effect, so we have

if (not $x) {
$x = $y;
}

or

$x = $y if not $x;

So line 12 of your code can be replaced with

$sheet->{MaxRow} = $sheet->{MinRow} if not $sheet->{MaxRow};

which you may prefer, and I would find it hard not to agree with you.

HTH,

Rob

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>>>>> "RD" == Rob Dixon <rob.dixon [at] gmx.com> writes:

RD> On 11/02/2011 10:38, mailing lists wrote:
>>>> 12 $sheet -> {MaxRow} ||= $sheet -> {MinRow};
>>>
>>> Line 12 can be written as:
>>> $sheet->{'MaxRow'} = $sheet->{'MaxRow'} || $sheet->{'MinRow'};
>>
>>
>> then that I don't understand is the program logic :-(
>>
>> what's the purpose of lines 12 and 14??

RD> Perl has no proper boolean values. Instead, the boolean operators
RD> treat zero, undef, and the null string '' all as false. Anything else
RD> is true.

to be pedantic, '0' is also false. it isn't exactly the same as 0.

uri

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Hi,

> 12 $sheet -> {MaxRow} ||= $sheet -> {MinRow};

Line 12 can be written as:
$sheet->{'MaxRow'} = $sheet->{'MaxRow'} || $sheet->{'MinRow'};

For example:
$variable_1 ||= $variable_2 is equivalent to $variable_1 = $variable_1
|| $variable_2.

The same applies to:

**= += *= &= <<= &&=
-= /= |= >>= ||=
.= %= ^= //=
x=

Regards,
Alan Haggai Alavi.
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Hi,

> then that I don't understand is the program logic :-(

It is a logical OR. Quoting `perldoc perlop`:

C-style Logical Or
Binary "||" performs a short-circuit logical OR operation. That
is, if the left operand is true, the right operand is not even
evaluated. Scalar or list context propagates down to the right
operand if it is evaluated.

$sheet->{'MaxRow'} = $sheet->{'MaxRow'} || $sheet->{'MinRow'};

Equivalent to:

unless ( $sheet->{'MaxRow'} ) {
$sheet->{'MaxRow'} = $sheet->{'MinRow'};
}

In English:
If $sheet->{'MaxRow'} has a value that evaluates to false, make
$sheet->{'MaxRow'} equal to $sheet->{'MinRow'}.

Though I have left the else clause,

By the way, it is not recommended to use `unless` for anything complex
as it can get in the way of ordinary thinking process. Use `if` and
`!`.

Regards,
Alan Haggai Alavi.
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On 2011-02-11 11:26, Alan Haggai Alavi wrote:

> $variable_1 ||= $variable_2 is equivalent to
> $variable_1 = $variable_1 || $variable_2.

Hmm, I don't buy that, I would say that $x ||= $y is equivalent to

$x = $y unless $x;

alternatively:

$x or $x = $y;

because the setting of $x only needs to happen if $x is false.

Realize that C<$variable_1 = $variable_1;> can only be optimized away if
no magic is involved.

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On Sat, 12 Feb 2011 12:13:13 +0100, Dr.Ruud wrote:
> On 2011-02-11 11:26, Alan Haggai Alavi wrote:
>
>> $variable_1 ||= $variable_2 is equivalent to $variable_1 = $variable_1
>> || $variable_2.
>
> Hmm, I don't buy that, I would say that $x ||= $y is equivalent to
>
> $x = $y unless $x;
>
> alternatively:
>
> $x or $x = $y;
>
> because the setting of $x only needs to happen if $x is false.
>
> Realize that C<$variable_1 = $variable_1;> can only be optimized away if
> no magic is involved.

No, the optimization seems to happen either way:

$ perl -E 'tie $x, "F"; tie $y, "F"; say "Orcish:"; $x ||= $y; say
"Unwrapped:"; $x = $x || $y; say "Done"; sub F::TIESCALAR{ bless [],
"F" }; sub F::AUTOLOAD{ say $F::AUTOLOAD } '
Orcish:
F::FETCH
Unwrapped:
F::FETCH
Done
F::DESTROY
F::DESTROY


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