Learning Perl, 5th Edition

Formatted Output with printf

You may wish to have a little more control with your output than print provides. In fact, you may be accustomed to the formatted output of C’s printf function. Fear not—Perl provides a comparable operation with the same name.

The printf operator takes a format string followed by a list of things to print. The format^[†] string is a fill-in-the-blanks template showing the desired form of the output:

printf "Hello, %s; your password expires in %d days!\n",
  $user, $days_to_die;

The format string holds a number of so-called conversions; each conversion begins with a percent sign (%) and ends with a letter. (As we’ll see in a moment, there may be significant extra characters between these two symbols.) There should be the same number of items in the following list as there are conversions; if these don’t match up, it won’t work correctly. In the example above, there are two items and two conversions, so the output might look something like this:

Hello, merlyn; your password expires in 3 days!

There are many possible printf conversions, so we’ll take time here to describe just the most common ones. Of course, the full details are available in the perlfunc manpage.

To print a number in what’s generally a good way, use %g,^[*] which automatically chooses floating-point, integer, or even exponential notation as needed:

printf "%g %g %g\n", 5/2, 51/17, 51 ** 17;  # 2.5 3 1.0683e+29

The %d format means a decimal^[†] integer, truncated as needed:

printf "in %d days!\n", 17.85; ...