To find the matched and unmatched values in perl - perl

Question

I am a newbie to programming and I hope someone can explain this to me:
So I have two text files i.e. Scan1.txt and Scan2.txt that are stored in my computer. Scan1.txt contains:
Tom
white
black
mark
john
ben
Scan2.txt contains:
bob
ben
white
gary
tom
black
patrick
I have to extract the matched values of these two files and the unmatched values and print them separately. I somehow found the solution for this which works fine. But can someone please explain how exactly the match happens here. Looks like somehow just this line:
$hash{$matchline}++ in the code does the matching and increments the value of hash when the match is found. I understand the logic but I do not understand how this match actually happens. Can someone help me understand this?
Thank you in advance!
Here is the code:
open (F1, "Scan1.txt");
open (F2, "Scan2.txt");
%hash=();
while ($matchline= <F1> ){
$hash{$matchline}=1;
}
close F1;
while( $matchline= <F2> ){
$hash{$matchline}++;
}
close F2;
foreach $matchline (keys %hash){
if ($hash{$matchline} == 1){
chomp($matchline);
push(#unmatched, $matchline);
}
else{
chomp($matchline);
push (#matched, $matchline);
}
}
print "Matched Entries are >>\n";
print "```````````````````````\n";
print join ("\n", #matched) . "\n";
print "```````````````````````\n";
print "Unmatched Entries are >>\n";
print "```````````````````````\n";
print join ("\n", #unmatched) . "\n";
print "```````````````````````\n";

Answer 1

The code you mention above will give you a false result if a given word exists more than one time in the second file and not exists in the first.
this line:
$hash{$matchline}++
increments a different counter for each different word.
in the first loop it sets to 1 for the words in the first file.
so if a word exists in each file the counter will be at least 2.
the $hash itself is a set of counters.

Answer 2

A more generalized version of your problem is that of computing the set union or intersection between two sets. This link gives a very good treatment of the problem in general.
In your case, the set is nothing but the list of values from each file. The logic is, if a certain value was present in both files then $hash{matchline} == 2, because the value will be incremented in both the while loops. However, if the line was present in only one of the files, the value of $hash{matchline} == 1, since only one while loop will increment the value and not the other.
Also, Lajos Veres raises a very important point: if a certain word, say "Tom" is present twice in the same file, then the algorithm will fail. It is a subtle detail, which can be resolved in many ways- removing duplicates beforehand, using two hashes, etc.
Hope this helps.