validate if output of command is numeric or not

kaka_2 has asked for the wisdom of the Perl Monks concerning the following question:

Dear Monks

in my perl script, I am running few commands which result 0 or < 0 which will be later used for an arithmetic calculation.

but in some cases command may throw error and arithmetic calculation may give an result which is not expected.

so I would like to validate if return output from command is numeric or not.

I see many people recommended if section in below code.


use warnings;
my $cmd = "cmd";
my $value = qx($cmd);

my $cmd1 = "cmd1";
my $value1 = qx($cmd1);

if (($value+0) eq $value) {
print $value . " is numeric";
}
else
{
print $value . " is not numeric";
}

if (($value1+0) eq $value1) {
print $value1 . " is numeric";
}
else
{
print $value1 . " is not numeric";
}

my $total = $value + $value1;
print $total;   // this gives me total of two values.
[download]

I tried to use the same but always result is not numeric which it could be but then i would expect warning while performing the arithmetic operation but i get correct result there so i believe $value is numeric but validation code has some problem.

any pointers?

Comment on validate if output of command is numeric or not Download Code

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Re: validate if output of command is numeric or not by Discipulus (Canon) on Jan 08, 2016 at 08:44 UTC
Hello, why do you not just print the result? like in `print "DEBUG \$value is: [$value]\n";` Do the same for the other value so you'll be sure of the content of them. Remember that is possible the returned value is a multiline string, which probaly is not exactly a 'numeric value' (see qx at Quote like Operators). Having done the above you can grab the numeric part with a regex (multiline switch?) and do te equality test: `if ($num1 eq $num2){ ... }` [download] HtH L* UPDATE: If the command return an error it is probably sent to STDERR that is not affected by qx operator. See the yet mentioned doc to see how workaround that issue. You must first check if the command executed correctly: perlvars contains infos about error variables, anyway try something like `perl -e "$ret = qx/error_returning_command/;print qq([$ret] [$^E] [$ +{^CHILD_ERROR_NATIVE}])"` [download] to see what your commands return. There are no rules, there are no thumbs.. Reinvent the wheel, then learn The Wheel; may be one day you reinvent one of THE WHEELS.	[reply] [d/l] [select]
Re: validate if output of command is numeric or not (numify) by Anonymous Monk on Jan 08, 2016 at 08:47 UTC
To see if you have a number perldoc -f number -> perlfaq4 -> How do I determine whether a scalar is a number/whole/integer/float? -> http://perldoc.perl.org/Scalar/Util.html#looks_like_number To quiet not-numeric warning, don't numify empty strings, they're false, so promote them to zero `$num \|\|= 0;` Instead of numbered variables ($cmd1...) use an array, @commands Exit code is often used to communicate failure, so you probably don't have to guess based on whats a program prints. my preferred way to qx is with Capture::Tiny `#!/usr/bin/perl -- use strict; use warnings; use Capture::Tiny qw/ capture /; my @cmd = ( 'python', '-V' ); my( $stdout, $stderr, $exit ) = capture { system { $cmd[0] } @cmd; };; $exit and die "it failed with $exit and $stderr ";` [download]	[reply] [d/l] [select]
Re: validate if output of command is numeric or not by BillKSmith (Monsignor) on Jan 08, 2016 at 14:03 UTC
You can have either of two types of problems. Test for each separately because you probably want to respond to each differently. Choose one of the suggested methods to verify that the command ran without error. If successful, use a regex to verify that the result is "reasonable" for your calculations. Bill	[reply]
Re: validate if output of command is numeric or not by Anonymous Monk on Jan 09, 2016 at 09:37 UTC
The try of mathematics is nice, but flames your brain. You may much faster through using regex "\d+". Please note by this time, that "eq" is not the same than "==" inside of perl! `sub check { my $value=""; if ($_[0]=~ m/\d+/) { $value=$_[0]; print qq~$_[0] is numeric\n~; } el +se { $value=""; print qq~$_[0] is not numeric\n~; } return($value); } my $in=0815; my $out=&check($in); # have phun!` [download]	[reply] [d/l]
Re^2: validate if output of command is numeric or not by worik (Sexton) on Feb 28, 2016 at 22:42 UTC
Again. `sub _isNumeric($){ my $t = shift; defined($t) or confess; no warnings; my $a = $t; $a+=0; return $a eq $t; }` [download] This should be built in! Doing $a+=0 issues a warning. Unless you a a very careless programmer your code must issue no warnings for valid input. So if you are testing if input is numeric (where it may or may not be, both cases valid) then you have to jump through hoops like this. That code above may not work for all cases but it is enough for my purposes. Yet another reason to hate Perl. Fix it Larry!	[reply] [d/l]
Re^3: validate if output of command is numeric or not by Anonymous Monk on Feb 29, 2016 at 19:01 UTC
Your `_isNumeric` functions nearly the same as Scalar::Util's `looks_like_number`, mentioned by the other anon elsewhere in this thread. Scalar::Util is a core module, and exposes Perl's internal `looks_like_number` function, so it pretty much is built in, and your `_isNumeric` is just using it in a kind of roundabout way. Sorry, but this is a hoop you've built for yourself!	[reply] [d/l] [select]