Question

I have written a function that is a countdown timer.

I want to print this way, Starts in 00:05

Hence I did this, but it doesn’t print correctly, it overwrites my sentence. Can you help fix it?

printf("\nStarts in %02d:%02d",countdownsleep(5));

# Sub for countdown
sub countdownsleep {
    my $x = shift;
    my $t = 1 *$x;
    my ($m,$s);
    my $stoptime = time + $t;
     while((my $now = time) < $stoptime) {
   #printf( "%02d:%02d\r", ($stoptime - $now) / 60, ($stoptime - $now) % 60);
   $m = ($stoptime - $now) / 60;
   $s = ($stoptime - $now) % 60;
    select(undef,undef,undef,1);
 }
 return ($m,$s);
}

Answer 1

The problem is that you are using \r (carriage return) – it returns the carriage to the very start of string (thus overwriting the first 5 characters in the best case scenario); AND causes weird printing behavior in the absence of “\n” (thus possibly not printing anything else after 5 characters).

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To fix your problem, you need to do this in your loop inside countdownsleep ():

$prefix = "Starts in "; # could be passed in as parameter to countdownsleep()
printf( "$prefix %02d:%02d\r", ($stoptime - $now) / 60
                               , ($stoptime - $now) % 60);
# NOTE this ^^^ - now you re-print your prefix every time and not lose due to \r

And in your call:

countdownsleep(5); print "\n"; # printing is done by the loop inside already
# or if you added a $prefix parameter to it:
countdownsleep("Starts in ", 5); print "\n";

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Here is why you need to print “\n” at the VERY END

$ perl -e '{print "1234567"; printf("1\r");}'

$ perl -e '{print "1234567"; printf("8\r"); print "\n";}' # Works now
12345678

# And this is what CR really dows
$ perl -e '{print "1234567"; printf("z\r");  printf("yy\r");  print "\n";}'
yy34567z
$ perl -e '{print "1234567"; printf("z\r");  printf("yy\r");  print "zzzz";}'
zzzz

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In other words, printing a carriage return (\r) at the end of the string WITHOUT newline (\n) will effectively not print the string at all – more specifically, will erase everything that was suppose to be printed.

Printing (\r) before some other characters in the string will cause the subsequent characters to be printed from the beginning of the line, overwriting existing characters (as many as the new ones), but will keep the subsequent characters intact – with the caveat that the non-overwritten characters won’t be printed unless \n is printed at the end.

• print "$something\r"; # prints nothing

• print "$something\r$finish"; # prints $finish but not $something

  # $finish is assumed to not contain "\r"
  

• print "$something\r$finish\n";

  # * prints $something (entirely)
  # * Moves to start of the line
  # * prints $finish overwriting as many characters  from $somthing as needed
  # * prints the rest of $something if it was longer than $finish
  # * prints newline.
  

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On a different note, you should consider using existing coundown/progress CPAN modules instead of rolling your own.