Roughly:

• loop through array
• split array element on white-space
• check if first part of this one matches the previous
• print header if it doesn't
• store first part for next round
• end loop

Yes, I am aware of the logic of my question. What I an hung on is the third bullet. How Do I iterate through the array, yet compare the next item against the former?

How Do I iterate through the array, yet compare the next item against the former?

You use an index, or you keep a stack

The index approach, its missing boundary checking ( ; beware of off-by-one errors

my @stuff = 0 .. 9;
for my $ix ( 0 .. $#stuff ){
  my $item = $stuff[$ix];
  my $prev = $stuff[$ix-1];
  my $next = $stuff[$ix+1];
}
[download]

The stack approach

my @stuff = 0 .. 9;
my $lastitem = "";
for my $item ( @stuff ){
  my $next = $item;
  if( $item eq $lastitem ){
    print "they measure up\n";
  }
  $lastitem = $item;
}
[download]

Oh, but you $lastitem is not an array, right

my @stuff = 0 .. 9;
my @lastitem ;
for my $item ( @stuff ){
  my $next = $item;
  if( @lastitem and  $item eq $lastitem[-1] ){
    print "they measure up\n";
  }
  push @lastitem, $item;
  shift @lastitem while 3 == @lastitem; ### keep at most 3 last items
}
[download]

G'day jtucker,

Welcome to the monastery.

As you haven't shown any code, it's difficult to know where you're getting stuck with this. For instance, your earlier comment "What I an hung on is the third bullet." [sic] is meaningless to me: it's the first and only time you've mentioned bullets; there are no bulleted lists; how does the 3rd bullet (not shown) differ from the 1st and 2nd bullets (also not shown). In order to get a better answer when you post here, you'd do well to read these guidelines: How do I post a question effectively?

The following skeleton code may help resolve whatever problem you're experiencing. If not, please post any follow-up questions in line with the guidelines I've linked to above.

$ perl -Mstrict -Mwarnings -E '
    my @data = (
        q{1:1 word1},
        q{1:1 word2},
        q{1:1 word3},
        q{1:1 word4},
        q{1:2 word1},
        q{1:2 word2},
        q{1:2 word3},
        q{1:3 word1},
        q{1:3 word2},
        q{1:3 word3}
    );

    my $old_head = q{};

    for (@data) {
        my $new_head = (split)[0];

        if ($new_head ne $old_head) {
            say "> $new_head";
            $old_head = $new_head;
        }

        say;
    }
'
> 1:1
1:1 word1
1:1 word2
1:1 word3
1:1 word4
> 1:2
1:2 word1
1:2 word2
1:2 word3
> 1:3
1:3 word1
1:3 word2
1:3 word3
[download]

To clarify, my comment regarding the bullets was in conjunction to another's response. It probably would have been best to read the comment as a response, since it appears, at least on my display, indented to the aforesaid comment.

I appreciate the response. I am aware of the forum guidelines. But since my question was a very limited subset of data, namely an array, I was just needing a prompt in how to compare a scalar item to the previous item on the stack. The fence post error served as a necessary reminder to debug, as mentioned by the previous comments.

So my code was


@line = split /\r/m, $file;
for my $line (@line) {
my $header = ((split(/\t/, $line))[0]);

##stuck here as I was unsure how to compare $header to previously seen
+ header
}
[download]

So, thanks for your response too. It showed me another way to resolve the issue. Much appreciated.