CODEWITHSUNDEEP
perl
iridescent
iridescent

Reputation: 305

Default value of $#ArrayName when there is no element (Perl)

The code below is to display the number of arguments entered in the command line.

#!/usr/bin/perl –w
$myVar = $#ARGV + 1;
print "Hi " , $ARGV[0] , "\n";
print "You have $myVar arguments\n";

From the perlintro, $#ARGV is a special variable which tells you the index of the last element of an array.

If this is the case, when I don't enter any value in the command line, how does $myVar value end up with 0 ?

Is it because when there is no element in the array, the index of "no element" is -1 ? As -1 + 1 = 0.

Upvotes: 1

Views: 242

Answers (4)

Matthias
Matthias

Reputation: 1005

Is it because when there is no element in the array, the index of "no element" is -1 ? As -1 + 1 = 0

Almost. It is not "the index of 'no element'" but rather the following rule applies:

perldata

The following is always true:

scalar(@whatever) == $#whatever + 1;

Upvotes: 0

Zaid
Zaid

Reputation: 37146

You are right.

$#ARGV is scalar @ARGV - 1, as squiguy points out.

But there are less-noisier alternatives to count the number of arguments passed to your program that you should consider using instead:

my $count = scalar @ARGV;  # Explicit using of 'scalar' function
my $count = 0+@ARGV;       # Implicitly enforce scalar context
my $count = @ARGV;         # Since the context is already set by LHS

Upvotes: 2

rjp
rjp

Reputation: 1958

$#ARGV means "the index of the last element of ARGV" - not just any array as the perlintro sentence seems to imply.

For any array, if it's empty, $#array will be -1 and scalar @array will be 0.

CAVEAT: If someone has modified $[ ("Index of first element"), that'll change $# as well. You should probably always use scalar @array if you're after the length, and $array[-1] to get the last element.

> cat demo.pl
my @array = ();
print "Size=", scalar @array, " items, last=", $#array, "\n";
$[ = 2;
print "Size=", scalar @array, " items, last=", $#array, "\n";
> perl demo.pl
Size=0 items, last=-1
Size=0 items, last=1

Upvotes: 5

squiguy
squiguy

Reputation: 33370

According to the perlvar page:

@ARGV The array @ARGV contains the command-line arguments intended for the script. $#ARGV is generally the number of arguments minus one, because $ARGV[0] is the first argument, not the program's command name itself. See $0 for the command name.

Upvotes: 2

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