On Jun 8, 5:03 pm, [EMAIL PROTECTED] (Rob Dixon) wrote: > oldgeezer wrote: > > Hi all, > > > Last week I discovered this perl.beginners group. > > Good stuff here, albeit many times hard to grasp > > the answers. But I'm learning. > > > What I would like to understand is why looping > > 2 times through 5000 lines takes less time than > > looping 5000 times through 2 lines. > > > To show what I mean, I wrote a snippet that > > does nothing with the data and yet the first > > part is 5 times faster than the second part > > (in my pc anyway). > > > Thanks. > > Rob. > > > #!/usr/bin/perl > > > use strict; > > use warnings; > > use POSIX qw(clock); > > > #just create a small array for this test > > my @few=(); > > for my $i(1...2){push @few, $i}; > > > #just create a big array for this test > > my @many=(); > > for my $i(1...500000){push @many, $i}; > > > # part 1, loop few times over much data > > my $time_1=clock(); > > foreach my $val (@few){ > > foreach my $other(@many) {} #do zilch > > } > > my $time_2=clock(); > > my $delta1=$time_2-$time_1; > > print "Delta1 $delta1\n"; > > > # part 2, loop many times over few data > > my $time_3=clock(); > > foreach my $val (@many){ > > foreach my $other(@few) {} #do zilch > > } > > my $time_4=clock(); > > my $delta2=$time_4-$time_3; > > print "Delta2 $delta2\n"; > > > exit 0; > > In addition to what others have said, it is worth doing the same experiment > for > an empty loop: > > foreach my $val (@many){ > } > > to see how much overhead there is for each iteration of the loop. > > You should also investigate the Benchmark module, which makes comparisons such > as these more convenient. > > Rob
Hi all, Sorry for the late response, I've been busy doing something else. As a newbie in this perl.beginners, I did not know that it is a good thing to tell what purpose the script has. My script is purely hobby. All my scripts are. I understand it now, I simply overlooked: >> How many times do you create a new iteration >> variable <snip> And then someone replied: >>In either case ... do not worry. I did not worry. My script works okay, I only wondered why reshuffling a loop made a big difference in time. I wasn't aware that a Benchmark module exists. Seems worth playing with it. Which I will do. The code I wrote in my question was only to give you an idea what I was talking about, not the real code because then I would have given the data that I work on as well. It is not secret data, but they are long lines and I was afraid they would wrap. But you also made me aware of a mistake I always made until now. The three periods in for (x...y) I must have inherited that error from another language. Probably from an interpreter I wrote myself some 30 years ago. Thanks for all the help. Below I give you the original lines in my script. I usually write a lot of comment lines, because I know myself. After some time I forget why I did something, and then reinvent the wheel. The script is a CGI. Data in the data base is separated with a ';' (is that called a semi-colon? I am not native english). foreach my $entry (uniq @IK){ foreach my $line(@DATA) { # Note /;$entry;/ takes longer than # /$entry;/ and that takes longer than /$entry/ # But /$entry/ also splits remarks_entries. # So I need the trailing ';' because they # are not in the remarks_entries # Appending the i, like: /$entry/i slows # down too. I don't need it. # Prepending m, like m/$entry/ # does not change anything (timewise). # For the average person /$entry;/ is fastest if($line=~ /$entry;/) { splitline($line); [EMAIL PROTECTED] changes when I splitline #next; don't do this. It makes it slower }; }; }; Swapping the first two foreach-lines make a big time difference, @DATA is the data base and is about 5000 lines, @IK varies from 1 to around 20 strings, dependent on the request POSTed to the CGI-script. 'uniq' and 'splitline' are subs. There is a trick. @IK changes in the loop, but the 'uniq @IK' is only defined once and is not changed by the loop. The changed @IK is used later again for some other reason. Oh, and those trailing ';' after all the '}' may well be superfluous. But I like to see when a } is really the end of a block and will not be followed by an else or something like that. I know: emacs does a nice indentation, but still I (personally) like to see that ';' Thanks again, Rob. -- To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] http://learn.perl.org/