Learning Perl, Fourth 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^[131] 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. The example above has 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 the most common ones. Of course, the full details are available in the perlfunc manpage.

To print a number, generally use %g,^[132] 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^[133] integer, truncated as needed:

    printf "in %d days!\n", 17.85;  # in 17 days!

This is truncated, ...