Originally Posted by durden_tyler

I don't quite understand the last paragraph of your 2nd post, but I'll explain the working of the final loop of your code.

Code:

...
sub induced {
my (@z)=@_;
for my $QT (\@z ){
    #print Dumper $QT;
    for my $triplet ( @trip ){
        my %Pie;
        undef @Pie{@$QT};
        delete @Pie{ @$triplet };
        print "@$triplet\n" if keys(%Pie) <= ( @$QT - @$triplet ) ;
        return (@$triplet);
    }
}}
my @C;
my $d;
my $p=$#subgraphs+1;
for ($d=$p; $d >=1; $d--)
  { 
    print "component $d = @{ $subgraphs[$d-1] }\n";
    my ($qw)=join(induced(@{ $subgraphs[$d-1] }));
    print "induced=$qw";

}

Notice the following about the subroutine "induced":
(1) it has a "print" statement in it based on an "if" condition being true.
(2) it returns an array. $triplet is an array ref, and @$triplet is the array it references.

Also notice the following about the final "for" loop:

(3) The "join" function has not been provided a "join-string" i.e. the first argument. So what happens if you use join without the join-string?

Code:

$
$ perl -le '@x = qw(a b c); $y = join(@x); print $y'

$

It doesn't print the array elements. Once you do supply the join-string, then:

Code:

$
$ perl -le '@x = qw(a b c); $y = join("~", @x); print $y'
a~b~c
$
$

it works fine.

(4) the final print function that prints the value of $qw does not have a newline.

Now, before the "for" loop, the value of $p is 2. In the first iteration:

(a) $d is 2 and the first print function prints the expected values.
(b) "join" returns nothing and hence $qw is a zero-length string.
(c) the second print function prints "induced=" and does **NOT** go to the next line due to the absence of a newline.

So after iteration 1, you have this printed:

Code:

component 2 = e d
induced=

In iteration # 2, the following happens:

(a) $d is 1 and the first print function prints the expected values (c, a, b)
(b) the "induced" subroutine, when invoked, prints the value "@$triplet\n" because apparently the "if" condition evaluates to true. Hence you see "b c a" printed. The "join" function, as usual, returns nothing and hence $qw is a zero-length string.
(c) the final print statement prints the constant string "induced=" followed by nothing.

So the output of **only** iteration 2 is as follows:

Code:

component 1 = c a b
b c a
induced=

And the overall output of both iterations is as follows:

Code:

component 2 = e d
induced=component 1 = c a b
b c a
induced=

Based on this understanding of the code, you may decide what to do next. You probably want to -
(a) fix the "join" function
(b) ensure that the final print function has a newline, and maybe
(c) suppress the "print" function inside the subroutine "induced"

Thus, if the final portion of your program looks like this:

Code:

...sub induced {
my (@z)=@_;
for my $QT (\@z ){
    #print Dumper $QT;
    for my $triplet ( @trip ){
        my %Pie;
        undef @Pie{@$QT};
        delete @Pie{ @$triplet };
        #print "@$triplet\n" if keys(%Pie) <= ( @$QT - @$triplet ) ;
        return (@$triplet);
    }
}}
my @C;
my $d;
my $p=$#subgraphs+1;
for ($d=$p; $d >=1; $d--)
{
  print "component $d = @{ $subgraphs[$d-1] }\n";
  my $qw = join " ", induced(@{ $subgraphs[$d-1] });
  print "induced = $qw\n";
}

then the final part of the output would be like this -

Code:

...
component 2 = e d
induced = b c a
component 1 = c b a
induced = b c a

I am not sure if you are expecting the return value of "induced" subroutine to be the same each time, or if you simply want to display the distinct return values. If that's the case, then set the variable $qw as a hash key, and print all keys of the hash after the final loop.

tyler_durden