What is eval?

[Mallik]

Can anybody explain the functionality of eval in brief?

Thanks in advance,
Mallik.

[James Edward Gray II]

On Jan 29, 2004, at 6:26 AM, Mallik wrote:

> Can anybody explain the functionality of eval in brief?

perldoc -f eval

James

[Bradley A. Brown]

[url]http://www.perldoc.com/perl5.8.0/pod/func/eval.html[/url]

Bradley

-----Original Message-----
From: Mallik [mailto:mallikarjunk@softima.com]
Sent: Thursday, January 29, 2004 7:26 AM
To: Perl Beginners
Subject: What is eval?


Can anybody explain the functionality of eval in brief?

Thanks in advance,
Mallik.

--
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[Jeff 'Japhy' Pinyan]

On Jan 29, Mallik said:

>Can anybody explain the functionality of eval in brief?

For starters, please read 'perldoc -f eval'. All Perl functions are
explained in detail in the standard documentation.

eval() has two forms. The first takes a string, and treats it as Perl
code. It compiles and executes it, returning whatever value is
appropriate. If there is a fatal error, it sets the $@ variable.

my $string = "make the vowels capitals";
my $from = "aeiou";
my $to = "AEIOU";

eval "\$string =~ tr/$from/$to/";
# now $string is mAkE thE vOwEls cApItAls

We had to use eval() here, because tr/// doesn't interpolate variables.
If we'd just done

$string =~ tr/$from/$to/;

we'd be changing $ to $, f to t, r to o, o to o, and m to o, and the
string would be "oake the vowels capitals".

The other form is eval { CODE }. This executes a block of code in a
fail-safe environment. If there are any fatal errors, the $@ is set, and
the block is exited. A common use for this is:

print "Give me a number: ";
chomp(my $n = <STDIN>);
my $result = eval { 100 / $n };
if ($@) { print "You tried dividing by zero.\n" }

Basically, if we didn't use eval { } here, the program would die if the
user entered the number 0.

--
Jeff "japhy" Pinyan [email]japhy@pobox.com[/email] [url]http://www.pobox.com/~japhy/[/url]
RPI Acacia brother #734 [url]http://www.perlmonks.org/[/url] [url]http://www.cpan.org/[/url]
<stu> what does y/// stand for? <tenderpuss> why, yansliterate of course.
[ I'm looking for programming work. If you like my work, let me know. ]

[Rob Hanson]

It evaluates the code that you give it. It can be used when you need to
create code on the fly, like this...

my $cmd = 'print';
my $arg = 'Hello World';
eval("$cmd '$arg'");

This is useful for allowing a user to pass code to the program (for whatever
reason).

The other use it to trap errors.

my $this = 1;
my $that = 2;
eval {
if ($this != $that) {
die "They don't match!";
}
};
print $@;

In this case the "die" doesn't terminate the program, it only terminates the
eval. And $@ stores the error message that was "thrown" in the eval. So
this prints the error message without abnormally terminating the program.
This is akin to the try/catch blocks in other languages (the Errors gives
you try/catch functionality by using eval).

Hope that helps.

Rob

-----Original Message-----
From: Mallik [mailto:mallikarjunk@softima.com]
Sent: Thursday, January 29, 2004 7:26 AM
To: Perl Beginners
Subject: What is eval?


Can anybody explain the functionality of eval in brief?

Thanks in advance,
Mallik.

--
To unsubscribe, e-mail: [email]beginners-unsubscribe@perl.org[/email]
For additional commands, e-mail: [email]beginners-help@perl.org[/email]
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[Steve Grazzini]

Mallik wrote:

> Can anybody explain the functionality of eval in brief?

There are two different flavors of eval().

The "string" version looks like this:

eval $code;

And it treats its argument as a string containing Perl source code,
which it compiles and executes.

The "block" version looks like this:

eval { $obj->might_die() };

And its argument is just an ordinary block (like the body of an
"if" statement). Both forms will trap fatal errors and place the
error in $@.

There's a more complete explanation in "perlfunc"

% perldoc -f eval

--
Steve

[Rob Dixon]

James Edward Gray II wrote:

>
> On Jan 29, 2004, at 6:26 AM, Mallik wrote:
>

> > Can anybody explain the functionality of eval in brief?

>
> perldoc -f eval

How succinct!

/R

[ashishgupta62@gmail.com]

Hi,

I am writing some script here i have noticed that first character of Japanese string (when used in eval function) is replaced with result1 or result2 variable.
Here is the script that i am using

#!/usr/bin/perl
my $string1 = "<japanese string>";
my $string2 = "Type";
my $result1 = "S";
my $result2 = "G";
my $length = 8;

print "EXPECTED\n";
print "$string2: $result1\n";
print "$string1: $result1\n";
print "$string1: $result2\n";

print "\n";

print "ACTUAL\n";
eval (
"format TEST = \n"
. $string2 . ':' . ' ' x ($length - length($string2)) .' @' . "\n"
. '$result1'."\n"
. $string1 . ':' . ' ' x ($length - length($string1)) .' @' . "\n"
. '$result1'."\n"
. $string1 . ':' . ' ' x ($length - length($string1)) .' @' . "\n"
. '$result2'."\n"
. ".\n");

$�~ = 'TEST';
write;


Output of script:

EXPECTED
Type: S
^Cv: S
^Cv: G

ACTUAL
Type: S
SCv:
GCv:

Can someone please explain me why eval function not abale to give result as expected

Thanks in advance