Argument isn't numeric warning in if statement

[SSC_perl]

I just ran across something puzzling. Why are these two statements not equivalent when it comes to warnings?

if ($item->{'optionprice'}) {
$item->{'unitprice'} += $item->{'optionprice'};
}

and

$item->{'unitprice'} += $item->{'optionprice'} if ($item->{'optionprice'});

Given the following values:

$item->{'unitprice'} = '12.16';
$item->{'optionprice'} = '';

the 2nd statement returns an "Argument '' isn't numeric in addition (+)" warning, while the 1st one doesn't. I thought I read where Peal reads a statement like the 2nd one from right to left. It looks like it doesn't, since $item->{'optionprice'} is evaluated in spite of the 'if'. Am I mistaken?

Perl 5.10.1

Thanks,
Frank

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[s***@optusnet.com.au]

-----Original Message-----
From: SSC_perl
Sent: Wednesday, September 17, 2014 10:37 AM
To: Perl Beginners
Subject: Argument isn't numeric warning in if statement

Post by SSC_perl
I just ran across something puzzling. Why are these two statements not
equivalent when it comes to warnings?
if ($item->{'optionprice'}) {
$item->{'unitprice'} += $item->{'optionprice'};
}
and
$item->{'unitprice'} += $item->{'optionprice'} if
($item->{'optionprice'});
$item->{'unitprice'} = '12.16';
$item->{'optionprice'} = '';
the 2nd statement returns an "Argument '' isn't numeric in addition (+)"
warning, while the 1st one doesn't. I thought I read where Peal reads a
statement like the 2nd one from right to left. It looks like it doesn't,
since $item->{'optionprice'} is evaluated in spite of the 'if'. Am I
mistaken?
Perl 5.10.1

Can't reproduce the anomaly here on 5.10.0 and 5.12.0 - I don't have 5.10.1.

##############################
C:\_32\pscrpt>type try.pl

use warnings;

$item->{'unitprice'} = '12.16';
$item->{'optionprice'} = '';
$item->{'unitprice'} += $item->{'optionprice'} if ($item->{'optionprice'});

C:\_32\pscrpt>perl try.pl

C:\_32\pscrpt>
##############################

Are you sure you've quoted the code (that's producing the warning) correctly
?

Cheers,
Rob

[SSC_perl]

Are you sure you've quoted the code (that's producing the warning) correctly?

Yes, I did. I double-checked it just to be certain. However, I ran the code by itself and it doesn't produce that warning, so it must be something upstream that's causing it.

Thanks,
Frank

SurfShop shopping cart is now open source...

Follow us on GitHub!
https://github.com/surfshopcart/surfshop

[Lawrence Statton]

Post by SSC_perl

Are you sure you've quoted the code (that's producing the warning) correctly?

Yes, I did. I double-checked it just to be certain. However, I ran the code by itself and it doesn't produce that warning, so it must be something upstream that's causing it.
Thanks,
Frank

The code

if (' ') { ... } will execute the block.

The code

if ('') { ...} will not

Neither ' ' nor '' are numeric, so using them in an arithmetic
expression will generate a warning.

[Rob Dixon]

I just ran across something puzzling. Why are these two statements
not equivalent when it comes to warnings?
if ($item->{'optionprice'}) {
$item->{'unitprice'} += $item->{'optionprice'};
}
and
$item->{'unitprice'} += $item->{'optionprice'} if ($item->{'optionprice'});
$item->{'unitprice'} = '12.16';
$item->{'optionprice'} = '';
the 2nd statement returns an "Argument '' isn't numeric in addition
(+)" warning, while the 1st one doesn't.
I thought I read where Peal reads a statement like the 2nd one from
right to left. It looks like it doesn't, since $item->{'optionprice'}
is evaluated in spite of the 'if'.
Am I mistaken?
Perl 5.10.1

As you have presented them, those code fragments are identical in
meaning. The code I used to prove it is below, and it give the output

{ optionprice => "", unitprice => 12.16 }

which is unchanged from the initial settings.

The probable error I can see is that, even if `$item->{optionprice}` is
true, it need not be numeric, and so would raise the `not numeric` warning.

Rob



use strict;
use warnings;
use 5.010;

my $item;

$item->{unitprice} = '12.16';
$item->{optionprice} = '';

if ($item->{optionprice}) {
$item->{unitprice} += $item->{optionprice};
}

$item->{unitprice} += $item->{optionprice} if ($item->{optionprice});

use Data::Dump;
dd $item;



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[SSC_perl]

As you have presented them, those code fragments are identical in meaning.

That was my understanding as well, but the inline 'if' gave an error while the block didn't. Running the code by itself in TextWrangler does not produce the warning. However, inside of my script, it does. Strange.

This probably doesn't have anything to do with it, but I'm logging during tests with this code:

BEGIN {
use CGI::Carp qw(carpout);
open(my $log, '>>', 'temp_logs/error.log') or warn("Unable to open error.log: $! \n");
carpout($log);
}

$item->{unitprice} += $item->{optionprice} if ($item->{optionprice});

Just so I'm clear on this, am I correct in thinking that Perl evaluates an inline 'if' from right to left - meaning that if $item->{optionprice} is NOT true, then the addition will not even be seen? Or does Perl look at the entire line before performing it?

Thanks,
Frank

SurfShop shopping cart is now open source...

Follow us on GitHub!
https://github.com/surfshopcart/surfshop

[Rob Dixon]

Post by SSC_perl

As you have presented them, those code fragments are identical in meaning.

That was my understanding as well, but the inline 'if' gave an error
while the block didn't. Running the code by itself in TextWrangler
does not produce the warning. However, inside of my script, it does.
Strange.
This probably doesn't have anything to do with it, but I'm logging
BEGIN {
use CGI::Carp qw(carpout);
open(my $log, '>>', 'temp_logs/error.log') or warn("Unable to open error.log: $! \n");
carpout($log);
}

You're mistaken. There's something else at play here that you've
overlooked.

if ($item->{optionprice}) {
$item->{unitprice} += $item->{optionprice};
}

is *identical* to

$item->{unitprice} += $item->{optionprice} if $item->{optionprice}

and is also the same as

$item->{optionprice} and $item->{unitprice} += $item->{optionprice}

Post by SSC_perl

$item->{unitprice} += $item->{optionprice} if ($item->{optionprice});

Just so I'm clear on this, am I correct in thinking that Perl
evaluates an inline 'if' from right to left - meaning that if
$item->{optionprice} is NOT true, then the addition will not even be
seen? Or does Perl look at the entire line before performing it?

I'm not clear what you mean by an *inline* if statement. If you mean the
post-fixed if, as in

$item->{unitprice} += $item->{optionprice} if $item->{optionprice}

then yes, the statement to the left of `if` won't be evaluated unless
the expression after it is true.

Rob


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