5.5. 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 notPerl 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. 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, 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; # in 17 days!
This is truncated, not rounded; we'll see how to round off a number in a moment.
In Perl, printf is most often used for columnar data since most formats accept a field width. If the data won't fit, the field will generally be expanded as needed:
printf "%6d\n", 42; # output like 42 (the symbol stands for a space) printf "%2d\n", 2e3 + 1.95; # 2001
The %s conversion means a string, so it effectively interpolates the given value as a string but with a given field width:
printf "%10s\n", "wilma"; # looks like wilma
A negative field width is left-justified (in any of these conversions):
printf "%-15s\n", "flintstone"; # looks like flintstone
The %f conversion (floating-point) rounds off its output as needed and lets you request a certain number of digits after the decimal point:
printf "%12f\n", 6 * 7 + 2/3; # looks like 42.666667 printf "%12.3f\n", 6 * 7 + 2/3; # looks like 42.667 printf "%12.0f\n", 6 * 7 + 2/3; # looks like 43
To print a real percent sign, use %%, which is special in that it uses no element from the list:[*]
printf "Monthly interest rate: %.2f%%\n", 5.25/12; # the value looks like "0.44%"
5.5.1. Arrays and printf
Generally, you won't use an array as an argument to printf. That's because an array may hold any number of items, and a given format string will work with only a certain fixed number of items: if there are three conversions in the format, there will have to be exactly three items.
But there's no reason you can't whip up a format string on the fly since it may be any expression. This can be tricky to get right, so it may be handy (especially when debugging) to store the format into a variable:
my @items = qw( wilma dino pebbles ); my $format = "The items are:\n" . ("%10s\n" x @items); ## print "the format is >>$format<<\n"; # for debugging printf $format, @items;
This uses the x operator (which we learned about in Chapter 2) to replicate the given string a number of times given by @items (which is being used in a scalar context). In this case, that's 3 since there are three items, so the resulting format string is the same as if we had written it as "The items are:\n%10s\n%10s\n%10s\n." And the output prints each item on its own line, right-justified in a ten-character column, under a heading line. Pretty cool, huh? But not cool enough, because you can even combine these:
printf "The items are:\n".("%10s\n" x @items), @items;
Here we have @items being used once in a scalar context to get its length and once in a list context to get its contents. Context is important.