Remember previous element in a loop

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

Hi... I need some urgent help please!

How can I remember the previous element in a loop... Here's a sample of a for loop looping through a result set of a DBI query..

for($x=0; $x<$sth->rows; $x++) {
    $data = $sth->fetch_hashref;

    $this = $data->{id};
    $prev = ...
}
[download]

Thanks

2006-02-03 Retitled by g0n, as per Monastery guidelines
Original title: 'LOOP Help!!!'

Comment on Remember previous element in a loop Download Code

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Re: Remember previous element in a loop by Corion (Patriarch) on Feb 03, 2006 at 11:39 UTC
Simply remember your "previous" element before you fetch the next element. `my $data; for($x=0; $x<$sth->rows; $x++) { my $prev = $data; $data = $sth->fetch_hashref; my $this = $data->{id}; }` [download] Maybe you should use a different loop style than the C loop for iterating over your results to avoid the polling of `$sth->rows`: `my $prev; while (defined my $data = $sth->fetchrow_hashref) { my $this = $data->{id}; ... $prev = $data; };` [download]	[reply] [d/l] [select]
Re^2: Remember previous element in a loop by Anonymous Monk on Feb 03, 2006 at 11:46 UTC
Ofcourse... It's programming 101... Thanx for you help	[reply]
Re: Remember previous element in a loop by tweetiepooh (Hermit) on Feb 03, 2006 at 11:43 UTC
Just set $prev lower down and initialise outside of loop. `$prev = <some dummy value>; for (......) { $data = <get data>; $element = <get element>; ... process data allowing for dummy value in $prev ... $prev = $element; }` [download]	[reply] [d/l]
Re: Remember previous element in a loop by blazar (Canon) on Feb 03, 2006 at 13:25 UTC
Maybe I don't understand: the only thing your code seems to miss is the thing you want to assign to `$prev` which should be presumably `$x` or something obtained in connection with it. OTOH you iterate over that `$x` but appear not to use it at all. And as a side note chances are that your code is not strict safe, although you may have `my $x;` in a part of your program you do not show... but even in that case I would recommend scoping as closely as possible instead! Also, all in all perl-style `for` loops are preferrable over C-style ones, but in this case I think it's customary to do `while (my $data = $sth->fetchrow_hashref) { # ...` [download] instead.	[reply] [d/l] [select]