Today in my college a teacher asked me a question. He wrote this code on the paper and said
"What will be the output of this code?"
use warnings;
for (1 .. 20)
{
print ".";
}
I found it easy and said that it will loop 20 times and at each iteration it will print a dot (.) and hence total 20 dots will be the output.
He said you are right and then he made some changes in the code. The code was:
use warnings;
for (1 .. 20)
{
print ".";
sleep 1;
}
He said the what will be the output now? I didn't know about the sleep function, I guessed that at each iteration it will print the dot (.) and then it will wait for 1 second (because of the sleep function) and then again it will iterate and then again it will print (.) then it will wait for 1 second and so on...
The teacher told me to check it at home. I tried it at home and I came to know that the second code waits for 20 seconds and then it prints all dots (20 dots) at once. I want to know how this happened? Why isn't the dot (.) is getting print on each iteration?
The real issue has nothing to do with sleep, but rather that............
You are Suffering from Buffering. The link provided takes you to an excellent article from The Perl Journal circa 1998 from Marc Jason Dominus (the author of Higher-Order Perl). The article may be over a decade old, but the topic is as relevant today as it was when he wrote it.
Others have explained the $| = 1; technique. I would add to those comments that in the predominant thinking of the Perl community seems to be that $| = 1 is preferable over $|++ simply because it is clearer in its meaning. I know, autoincrement is pretty simple too, but does everyone who will ever look at your code know $|'s behavior when ++ or -- are applied (without looking it up in perlvar). I happen to also prefer to localize any modification of Perl's "special variables" so that the effects are not washing over into other portions of code that may not play nice with a particular change to default Perl behavior. So that being the case, I would write it as:
use strict;
use warnings;
{
local $| = 1;
for ( 1 .. 20 ) {
print '.';
sleep 1;
}
}
Perl, and many other programs, line-buffer output by default. You can set $| to 1 if you need unbuffered output.
It's not clearing the buffer. If there is a newline at the end of the print statement it will do that for you automatically:
use warnings;
for (1 .. 20) {
print ".\n";
sleep 1;
}
If you don't want the newline (I don't imagine you do) you can use the special autoflush variable $|. Try setting it to 1 or incrementing it.
use warnings;
$|++;
for (1 .. 20) {
print ".";
sleep 1;
}
Related
How I can echo a progress bar while an external process is executing and capture its STDOUT when it's done, using only standard modules. And not using fork?
Run external process, something like: #array = `ls -l`;
While it executing, do printing progress bar, like: print '.';
Capture STDOUT of the process into array, when it done
Continue works main script
I'm reading about IPC::Open2, IPC::Open3, but I don't understand how to use them for this task. Maybe it's not the right direction?
What do you have so far? If you have having trouble with the interprocess communication, forget about the progress bar for the moment and ask just about that.
You can't really have a progress bar for something that has an indeterminate end. If you don't know how much input you will read, you don't know what fraction of it you have read. People tend to think of progress bars as a representation of fraction of work done, just not activity. That is, unless you use macOS and understand that "less than one minute" means "more than three hours". ;)
I tend to do something simple, where I output a dot every so often. I don't know how many dots I'll see, but I know that I'll see new ones.
$|++; # unbuffer stdout to see dots as they are output
while( <$fh> ) {
print '.' if $. % $chunk_size; # $. is the line number
print "\n[$.] " if $. % $chunk_size * $row_length;
...
}
That $fh can be anything that you want to read from, including a pipe. perlopentut has examples of reading from external processes. Those are doing a fork, though. And, the other modules will fork as well. What's the constraint that makes you think you can't use fork?
You can get more fancy with your display by using curses and other things (a carriage return is handy :), but I'm not inclined to type those out.
Perhaps OP is looking for something of next kind just to indicate that external process is running.
Define a handler for $SIG{ALRM} and set alarm 1 to run handler every second. Once process complete reset alarm 0 to turn off alarm handler.
use strict;
use warnings;
use feature 'say';
use Data::Dumper;
my $ls_l; # variable to store output of external command
$| = 1; # unbuffered output
$SIG{ALRM} = \&handler;
alarm 1; # run handler every second
say 'Running sig_alarm_sleep';
$ls_l=`./sig_alarm_sleep`;
say ' done';
alarm 0;
my #fields = qw(rwx count user group size month day time name);
my #lines = split("\n",$ls_l);
my(#array);
for( #lines ) {
my $x->#{#fields} = split(' ',$_);
push #array, $x;
}
say Dumper(\#array);
exit 0;
sub handler {
print '.';
$SIG{ALRM} = \&handler;
alarm 1;
}
Bash script sig_alarm_sleep sample
#!/usr/bin/bash
sleep 20
ls -al
I have pretty big perl script executed quite frequently (from cron).
Most executions require pretty short & simple tests.
How to split single file script into two parts with "part two" compiled based on "part 1" decision?
Considered solution:
using BEGIN{ …; exit if …; } block for trivial test.
two file solution with file_1 using require to compile&execute file_2.
I would prefer single file solution to ease maintenance if the cost is reasonable.
First, you should measure how long the compilation really takes, to see if this "optimization" is even necessary. If it does happen to be, then since you said you'd prefer a one-file solution, one possible solution is using the __DATA__ section for code like so:
use warnings;
use strict;
# measure compliation and execution time
use Time::HiRes qw/ gettimeofday tv_interval /;
my $start;
BEGIN { $start = [gettimeofday] }
INIT { printf "%.06f\n", tv_interval($start) }
END { printf "%.06f\n", tv_interval($start) }
my $condition = 1; # dummy for testing
# conditionally compile and run the code in the DATA section
if ($condition) {
eval do { local $/; <DATA>.'; 1' } or die $#;
}
__DATA__
# ... lots of code here ...
I see two ways of achieving what you want. The simple one would be to divide the script in two parts. The first part will do the simple tests. Then, if you need to do more complicated tests you may "add" the second part. The way to do this is using eval like this:
<first-script.pl>
...
eval `cat second-script.pl`;
if( $# ) {
print STDERR $#, "\n";
die "Errors in the second script.\n";
}
Or using File::Slurp in a more robust way:
eval read_file("second-script.pl", binmode => ':utf8');
Or following #amon suggestion and do:
do "second-script.pl";
Only beware that do is different from eval in this way:
It also differs in that code evaluated with do FILE cannot see lexicals in the enclosing scope; eval STRING does. It's the same, however, in that it does reparse the file every time you call it, so you probably don't want to do this inside a loop.
The eval will execute in the context of the first script, so any variables or initializations will be available to that code.
Related to this, there is this question: Best way to add dynamic code to a perl application, which I asked some time ago (and answered myself with the help of the comments provided and some research.) I took some time to document everything I could think of for anyone (and myself) to refer to.
The second way I see would be to turn your testing script into a daemon and have the crontab bit call this daemon as necessary. The daemon remains alive so any data structures that you may need will remain in memory. On the down side, this will take resources in a continuos way as the daemon process will always be running.
I'm new to PERL but have picked it up rather quickly as I work in C.
My question seems simple, but for the life of me I cant find a simple answer. Basically I want to print something on the same line as user input
Example
print "Please Enter Here: ";
my $input = <STDIN>;
chomp $input;
print " - You Entered: $input";
Output
Please Enter Here: 123
- You Entered: 123
This is undesired as I want all of this on one line in the terminal window. At the moment it prints the second string on the line below once the user has pressed the enter key. I guess what i'm going to need to do is something with STDIN like ignore the carriage return or newline but I'm not sure.
Desired Output
Please Enter Here: 123 - You Entered: 123
I don't know why this seems to be a complicated thing to google but I just haven't fathomed it, so any help would be appreciated.
Ta
Well, this is interesting...
First, you'd have to turn off terminal echoing, using something like IO::Stty. Once you do that, you could use getc. Note that the getc perldoc page has a sample program that could be used. You can loop until you get a \n character as input.
You can also try Term::ReadKey.
I have never done this myself. I'll have to give it a try, and if I succeed, I'll post the answer.
The Program
Ended up I already had Term::ReadKey installed:
#! /usr/bin/env perl
#
use warnings;
use strict;
use feature qw(say);
use Term::ReadKey;
ReadMode 5; # Super Raw mode
my $string;
print "Please Enter Here: ";
while ( my $char = getc ) {
last if ord( $char ) < 32; # \r? \n? \l?
$string .= $char;
print "$char";
}
ReadMode 0;
say qq( - You entered "$string".);
I realized that, depending how the terminal is setup, it's hard to know exactly what is returned when you press <RETURN>. Therefore, I punted and look for any ASCII character that's before a space.
One more thing: I didn't handle what happens if the user hit a backspace, or other special characters. Instead, I simply punt which may not be what you want to do.
In the end I did:
"\033[1A\033[45C
It uses the code functions for line up and 45 indent and it works. Thanks for all the help though.
I'm working on a new version of an already released code of perl, and found the line:
$|++;
AFAIK, $| is related with pipes, as explained in this link, and I understand this, but I cannot figure out what the ++ (plus plus) means here.
Thank you in advance.
EDIT: Found the answer in this link:
In short: It forces to print (flush) to your console before the next statement, in case the script is too fast.
Sometimes, if you put a print statement inside of a loop that runs really really quickly, you won’t see the output of your print statement until the program terminates. sometimes, you don’t even see the output at all. the solution to this problem is to “flush” the output buffer after each print statement; this can be performed in perl with the following command:
$|++;
[update]
as has been pointed out by r. schwartz, i’ve misspoken; the above command causes print to flush the buffer preceding the next output.
$| defaults to 0; doing $|++ thus increments it to 1. Setting it to nonzero enables autoflush on the currently-selected file handle, which is STDOUT by default, and is rarely changed.
So the effect is to ensure that print statements and the like output immediately. This is useful if you're outputting to a socket or the like.
$| is an abbreviation for $OUTPUT_AUTOFLUSH, as you had found out. The ++ increments this variable.
$| = 1 would be the clean way to do this (IMHO).
It's an old idiom, from the days before IO::Handle. In modern code this should be written as
use IO::Handle;
STDOUT->autoflush(1);
It increments autoflush, which is most probably equivalent to turning it on.
I have a computationally expensive task in perl, and would like to inform the user that computation is ongoing by printing out a period after each portion of the computation is completed. Unfortunately, until I print a "\n", none of my periods are printed. How can I address this?
You need to set autoflush for STDOUT. Example:
use IO::Handle;
STDOUT->autoflush(1);
foreach (1..20) {
print '.';
sleep(1);
}
set $|=1 before you start printing. Eg.
perl -e ' $|=1; foreach (1..10) { print "$_ "; sleep(1); }'
An excellent article you should read: Suffering from Buffering?
See the FAQ How do I flush/unbuffer an output filehandle? Why must I do this? and note:
Besides the $| special variable, you can use binmode to give your filehandle a :unix layer, which is unbuffered ...
For the general problem, you might want to look at Time::Progress:
%b
%B
progress bar which looks like:
##############......................
What worked for me was to put the line
STDOUT->autoflush(1);
before my line
print ".";
inside my existing loop. Didn't use the sleep for fear of slowing the things down even more.