Re^2: Writing to File based on condition

Thank you very much, I resolved this in my next reply, let me know if we had the same idea,but for now

my %down;

open my $ifh, '<', $hostfile or die "Can't read '$hostfile: $!\n";

while(my $addy = <$ifh>) {
    chomp $addy;
    $down{$addy} = system qq{nc -w 1 $addy 22 > /dev/null 2>&1}; # 0 =
+ up, not 0 = down
}
[download]

For the above could I have said the following to put the dead ips into the hash ? Is there a difference?

@down=<$ifh>;
foreach my $line(@down)
{
push @down{$addy}, $_ ;
}
or
@down{$addy}=<$ifh>;
[download]

Also I am not understanding your logic on this line, all ips dead and alive are now in keys %down, you are assigning the value of 1 unless the ip is not apart of keys %down?

    $down{$addy} = 1 unless ! $down{$addy};
    delete $down{$addy} if exists $down{$addy} and ! $down{$addy};
}
[download]

then lastly, couldn't I just have said below, or you had to go through the hash to get \n formatting?

print $ifh sort keys %down;
[download]

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Re^3: Writing to File based on condition by shmem (Chancellor) on Feb 18, 2018 at 16:17 UTC
For the above could I have said the following to put the dead ips into the hash ? Is there a difference? Yes. There may be hosts in your down file which at that time were down, but at the current run are up again. Also I am not understanding your logic on this line, all ips dead and alive are now in keys %down, you are assigning the value of 1 unless the ip is not apart of keys %down? I am assigning the value 1 to the key in %down unless the host is not down, which implies it even exists in the hash %down and its value is not 0. So, hosts which aren't present anymore in the hosts file, but in the down file, are skipped. Then in the next line I delete the host if present in %down if it happens to be up. then lastly, couldn't I just have said below, or you had to go through the hash to get \n formatting? The below is wrong because it uses $ifh, which is input filehandle here, and because only a bare print implies `$_`, while print to an explicit filehandle does not. You have to mention `$_`. To get a newline at the end of each print operation, you have to mention that as well, unless you set `$\` (output record separator, see perlvar) @list = (a..z); { # ok for STDOUT local $\ = $/; # set output record sep to input record sep for this +block print for @list; } { # wrong open my $fh,'>', 'foo' or die "Oops: foo - $!\n"; local $\ = $/; print $fh for @list; # WRONG. This prints GLOB(0x1945bd8) or such 26 + times } # file 'foo' closed here, since $fh is out of scope { # ok open my $fh,'>', 'bar' or die "Oops: bar - $!\n"; my $fh = select $fh; # $fh now default filehandle local $\ = $/; print for @list; # OK select $fh; # restore STDOUT as default filehandle } # file 'bar' closed here, since $fh is out of scope [download] Running the above, you will have a..z printed to STDOUT, each character on a line, a file "foo" with 0 bytes and a file "bar" with 52 bytes. perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'	[reply] [d/l] [select]
Re^4: Writing to File based on condition by cbtshare (Monk) on Feb 18, 2018 at 18:05 UTC
Thank you for the explanation, but in this example I learnt it is possible to print hash keys directly to an array without doing a for loop. I think the difference is in your example its an array you want to print to a filehandle `if( keys %old > $oldsize ) # only write if new ips added { open my $out, '>', $awsLists or die "$! opening $awsLists"; ##print directly to the file the new keys difference print $out sort keys %old; # sort not needed, but nice touch :) close $out; }` [download] https://www.perlmonks.org/?node_id=1208878	[reply] [d/l]
Re^5: Writing to File based on condition by shmem (Chancellor) on Feb 18, 2018 at 18:44 UTC
`print $out sort keys %old;` [download] Try that. As is, this would print all keys concatenated together as one string. That's fine if each key has a trailing record separator attached. perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'	[reply] [d/l]