6.5. 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 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; # in 17 days!
Note that 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
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 ‘‘‘‘‘
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%"
6.5.1. 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 must 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, though, 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, "Scalar Data") 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;
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