askucins askucins - 2 months ago 17
Perl Question

Why does smartmatch return true when comparing array slices that should be different?

The following script smart-matches slices of two arrays. At the start, both arrays are the same and I'm getting reasonable results. Then I change one of the arrays and smart-match two new slices, but it still says that the slices are identical. However, when I copy the slices into arrays, smart-matching the arrays shows that they are indeed different.

The script:

#!/usr/bin/perl
use warnings;
use strict;
use diagnostics;

my @x = qw (one two);
my @y = qw (one two);
my @x_s;
my @y_s;

print "Before change: values are the same:\n";
@x_s = @x[0,1];
@y_s = @y[0,1];
print "\@x_s: @x_s\n";
print +(@x[0,1] ~~ @y[0,1]) ? "equal\n" : "not equal\n";
print +(@x_s ~~ @y_s) ? "equal\n" : "not equal\n";

$x[0]='three';

print "After change: values should be different:\n";
@x_s = @x[0,1];
@y_s = @y[0,1];
print "\@x_s: @x_s\n";
print +(@x[0,1] ~~ @y[0,1]) ? "equal\n" : "not equal\n";
print +(@x_s ~~ @y_s) ? "equal\n" : "not equal\n";


The output:

Before change: values are the same:
@x_s: one two
equal
equal
After change: values should be different:
@x_s: three two
equal
not equal


I'm using Perl 5.10.1, and this happens for both array slices and hash slices. Why does this happen?

Answer

Looks like smart matching works in scalar context with slices.

Consider the following code pieces:

Your case:

#!/usr/bin/perl

my @foo = (1,2);
my @bar = (3,4);
print @foo[1,2] ~~ @bar[1,2] ? "Equal\n" : "Not equal\n";

That's probably what you need:

#!/usr/bin/perl

my @foo = (1,2);
my @bar = (3,4);
print [ @foo[1,2] ] ~~ [ @bar[1,2] ] ? "Equal\n" : "Not equal\n";