### Re^3: Dereferencing arrays

by BrowserUk (Pope)
 on Mar 19, 2014 at 20:37 UTC ( #1078982=note: print w/replies, xml ) Need Help??

in reply to Re^2: Dereferencing arrays
in thread <SOLVED>Dereferencing arrays

Compare this code (which works) with your version and ask questions about any differences you do not understand:

```#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

my @unsortedData = ( 5, 4, 7, 2, 1, 3, 6, 9, 8, 10 );

# show the original data to screen
for( @unsortedData ) {
print "\$_  ";
}
print "\n";

my \$sortedData = mergeSort( \@unsortedData );

# show the new data to screen
for( @\$sortedData ) {
print "\$_  ";
}
print "\n";

### dereferencing of arrays isnt working in below method

sub mergeSort {
my ( \$data ) = @_;

if( @\$data < 2 ) { return \$data; }  # if there is less then two it
+ems it's already sorted!!

use integer;
#split the array into two parts
my \$middle = @\$data / 2;
print "middle: \$middle\n";

my @leftSide = ();
print "LeftSide:\n";
for( my \$i = 0; \$i < \$middle; \$i++ ) {
\$leftSide[\$i] = @\$data[\$i];
print "\$leftSide[\$i] ";
}
print "\n";
my @rightSide = ();
print "RightSide:\n";
for( my \$i = \$middle; \$i < @\$data; \$i++ ) {
\$rightSide[\$i - \$middle] = @\$data[\$i];
print "\$rightSide[\$i - \$middle] ";
}
print "\n";
mergeSort( \@leftSide );
mergeSort( \@rightSide );
merge( \@leftSide, \@rightSide, \$data );
return \$data;
}

sub merge {
my ( \$groupA, \$groupB, \$dataRef ) = @_;
my \$i = 0; my \$j = 0; my \$k = 0;

while( \$i < @\$groupA && \$j < @\$groupB ) {
if( \$groupA->[\$i] <= \$groupB->[\$j] ) {
@\$dataRef[\$k] = \$groupA->[\$i];
\$i++;
}
else {
@\$dataRef[\$k] = \$groupB->[\$j];
\$j++;
}
\$k++;
}
while( \$i < @\$groupA ) {
@\$dataRef[\$k] = \$groupA->[\$i];
\$i++;
\$k++;
}
while( \$j < @\$groupB ) {
@\$dataRef[\$k] = \$groupB->[\$j];
\$j++;
\$k++;
}
}

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

Replies are listed 'Best First'.
Re^4: Dereferencing arrays
by divitto (Novice) on Mar 19, 2014 at 20:43 UTC

i understand that you made the merge method take references but i am confused to why. could you explain that to me?

Because you cannot pass arrays to subroutines; you can only pass a (single) list.

This reality stems from this:

```( @a, @b ) = (1..10, 'a'..'b');

print @a;;
1 2 3 4 5 6 7 8 9 10 a b

print @b;;

There is nothing to delimit the two different lists on the right hand side; so everything -- the numbers and the letters -- gets assigned to @a; nothing to @b;

Similarly, when you do this:

```someFunction( @a, @b, \$c );

All the items from both arrays and the single scalar get concatenated into a single list, which inside the function you address as the array @_.

When you assign @_ to two arrays and a scalar inside your function:

```    my ( @groupA, @groupB, \$dataRef ) = @_;

The information about which elements of @_ came from which argument has been lost; so everything gets assigned to @groupA and nothing to the other two variables.

Hence if you want to pass multiple arrays (or hashes; or combination thereof) to a function, you have to pass references to them and dereference internally.

It might sound like a limitation, but once you start using your function to sort arrays of any size, you'll be glad of the efficiency that results.

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

Thank you very much, this clarifies everything. I wasn't fully aware of how @_ worked. I will do more reading on it for sure. Thanks again!

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