Need help understanding this Perl syntax

new_user3085 · 09-19-2018, 09:58 AM

Hi Everyone,

I'm trying to understand the below syntax from a perl script that is part of a subroutine checking for free space. I know "$fs" is a variable and have a vague idea that inplace variable is being done here (please correct me if this is wrong) but have no clue what "[3]" means. I request the Gurus to explain with an example. Thanks in advance.

Code:

($fs) = (split /\s+/, $fs)[3];

astrogeek · 09-19-2018, 01:15 PM

No guru needed!

First you need to understand how the split operator works. From man perlrequick:

Code:

   The split operator
       "split /regex/, string" splits "string" into a list of substrings and returns that list.  The regex
       determines the character sequence that "string" is split with respect to.  For example, to split a
       string into words, use

           $x = "Calvin and Hobbes";
           @word = split /\s+/, $x;  # $word[0] = 'Calvin'
                                     # $word[1] = 'and'
                                     # $word[2] = 'Hobbes'

       ...

Next, think of the structure of that line of code and try to figure out what it is doing. Here is a very simple give-away, give it a try!

Code:

my $fs = 'one two three four five';
$fs = (split /\s+/, $fs)[3];
print $fs;

When in doubt, writing a quick script to demonstrate what it does is often faster than looking for an explanation!

Good luck!

individual · 09-19-2018, 01:52 PM

Just to add on to what astrogeek was saying, the purpose of writing split as

Code:

(split /\s+/, $fs)[3]

is to put it into what is known as list context. If you try to write

Code:

split(/\s+/, $fs)[3]

you will get a syntax error. [3] simply means get the value of the list at the third index. Since Perl uses 0-based indices, that will give you the fourth element in your list.

keefaz · 09-19-2018, 05:37 PM

Also $fs is surrounded with ( and ) as split is expected to return an array
If you ommit the parenthesis it will assign the number of elements from array returned by split (eg: 1), it's perl normal behaviour in scalar context.
By using parenthesis, you explicitly set a list context for the assignement

[edit]
Just tested, I was wrong, parenthesis are not needed here as the returned value is already a scalar extracted from the split array by using [n] array index selector

I guess the programmer used parenthesis as an habit of style working with split assignement

individual · 09-19-2018, 09:20 PM

Quote:

Originally Posted by keefaz

Just tested, I was wrong, parenthesis are not needed here as the returned value is already a scalar extracted from the split array by using [n] array index selector

I guess the programmer used parenthesis as an habit of style working with split assignement

Good job spotting your mistake and correcting.

igadoter · 09-26-2018, 11:07 AM

Run this code

Code:

#!/usr/bin/perl

@foo = (1, "lama", "camel",4);

($x, $y)= @foo;

print "$x, $y";

it should clarify things for you and you will understand (as me) the '($fs)' on the left-hand side.