Re: Reading files n lines a time

[choroba]

in reply to Reading files n lines a time

wont work anymore

What happens? What error message do you get?

I experimented with reading multiple lines through map, this works for me, but is a bit ugly. Maybe someone else knows a cleaner way?

while(( my @lines = map $_ = <>, 1 .. 4 )[0]) {
    print @lines;
    print "\n";
}

[naturalsciences]

Hmm looked through my test script and yes it seems that the while(<>){$nextline=<>;$thirdline=<>} can actually be extended indefinetly. It is ugly but gets the job done.

Why my scripts crashed seems to be of the undefined variables you get when your text files line number is not divisible by the number of your $nextlines. Then in the end you will get undefined variables for your $nextlines

I need to get some control element that would terminate the script (or loop if I want to continiue with script) nicely. I think akin to - if any variables undefined exit loop, in the beginning of the loop. Should stop it from panicking with unfavorable Eof situation.

[naturalsciences]

More on the undefined variables problem. Would adding something like this do a loop be correct.

while ($line0=<>) {$line1=<>;$line2=<>;$line3=<>;
last if not defined $line0;
last if not defined $line1;
last if not defined $line2;
last if not defined $line3;
do_some_stuff;
}

Could I use some OR statements to get these last if-s on a single line. Or can or and and statements used only between two values. I was thinking like

 last if not defined $line0or$line1or$line2or$line3

or

 last if not defined $line0||$line1||$line2||$line3

[choroba]

Using

while(( my @lines = map $_ = <>, 1 .. 4 )[-1]) {

should stop if the last line is not read as well. (You might need to add defined.)

[tobyink]

Perhaps marginally nicer, the kite operator:

while(( my @lines = map ~~<>, 1 .. 4 )[0]) {
    print @lines;
    print "\n";
}

[mbethke]

Considering the OP deals with multi-GB files I'd prefer the slightly uglier assignment to $_ because the kite's tail is not optimized away so all the strings would actually be shoved through the binary negation twice.

I was surprised by the result BTW when I looked at the optree; I'm almost completely clueless about what Perl can and cannot optimize but it even a fairly trivial peephole optimizer as in early C compilers could catch this.

[DrWhy]

Don't you need to check for defined rather than true for the first element of your array in the while loop?

[choroba]

Probably yes. I indicated that deeper in a reply.