An alternative screencast comparing values in Perl

Given two variables $x and $y how can we compare them? Are 1, 1.0 and 1.00 equal? What about "1.00" ? What is bigger "foo" or "bar"?

Two sets of comparison operators

Perl has two sets of comparison operators. As we saw with the binary operators of addition (+), concatenation (.) and repetition (x), here too, the operator is what defines how the operands behave and how they are compared.

The two sets of operators are as follows:

examples/operators.txt

Numeric    String         Meaning
==            eq           equal
!=            ne           not equal
<             lt           less than
>             gt           greater than
<=            le           less than or equal
>=            ge           greater than or equal

The operators on the left will compare the values as numbers while the operators on the right (the middle column) will compare the values based on the ASCII table or based on the current locale.

Let's see some examples:

use strict;
use warnings;
use 5.010;

if ( 12.0 == 12 ) {
  say "TRUE";
} else {
  say "FALSE";
}

In this simple case Perl will print TRUE as the == operator compares the two numbers and Perl does not care if the number is written as an integer or as a floating point number.

A more interesting situation will be comparing

"12.0" == 12

which is also TRUE as the == operator of Perl converts the string to a number.

examples/operators_example.txt

 2  < 3  is TRUE because < compares the two numbers.

 2  lt 3 is also TRUE as 2 is before 3 in the ASCII table

12 > 3  is TRUE obviously.

12 gt 3 will give you FALSE

This might be surprising for some people at first but if you think about it, the way Perl compares the two strings is character by character. So it compares "1" to "3" and as they are different and "1" is before "3" in the ASCII table Perl decides at this point that 12 as a string is smaller than 3 as a string.

You have to make sure you compare things as you really want them!

"foo"  == "bar" will be TRUE

It will also give you two warnings if(!) you enabled warnings by use warnings. The reason for the warning is that you are using two strings as numbers in the numerical == comparison and that's what generates the warnings. As mentioned in the previous part Perl will look at the left hand side of each string and convert them to the numbers it sees there. As both strings start with a letter they will be both converted to 0. 0 == 0 so that's why we get true.

OTOH:

"foo"  eq "bar"  FALSE

So you have to make sure you compare values as you want them to be compared!

The same happens when you compare

"foo"  == "" will be TRUE

and

"foo"  eq "" will be FALSE

This table might be handy to see the results:

examples/comparision.txt

 12.0   == 12    TRUE
"12.0"  == 12    TRUE
"12.0"  eq 12    FALSE
  2     <   3    TRUE
  2    lt   3    TRUE
 12     >   3    TRUE
 12    gt   3    FALSE ! (look out, might not be obvious at first)
"foo"  ==  ""    TRUE  ! (You get warnings if you used the "warnings" pragmata)
"foo"  eq  ""    FALSE
"foo"  == "bar"  TRUE  ! (You get warnings if you used the "warnings" pragmata)
"foo"  eq "bar"  FALSE

Finally an example where people can fall in a trap is when you get some input from the user and after carefully chomping off the newline from the end you try to check if the given string is empty.

examples/incorrect_use_of_equal.pl

use strict;
use warnings;
use 5.010;

print "input: ";
my $name = <STDIN>;
chomp $name;

if ( $name == "" ) {   # wrong! you need to use eq instead of == here!
  say "TRUE";
} else {
  say "FALSE";
}

If you run this script and type in "abc" you will get that it is TRUE, as if Perl thought "abc" is the same as the empty string

Comments

what if I want to Print value in between, example ,

cat /proc/partitions | tail -n+3 | grep sd. | perl -lane 'if ($F[2] > 900000000) { print "/dev/".$F[3] }' | sort === here it will print all above 900000000 blocks disks

but what signs will be there if I want to print blocks between value 900000000 and 4007018584 ?