But here comes a short explanation:

printf take a pattern string and a list of value, to fill into the pattern.
So the basic form is:

printf "pattern", value, value, value;

This pattern can hold any kind of text, including newlines, if you put it between doubles.
The pattern contains a placeholder for each value in the list.

The placeholders look like %n.mt, where the % is the begin of the placeholder, n is the length, m is the decimals if you have a number and t is the type of value.

n and m are optional, t can be one of a long list of which are s,d,f some examples.
s is for strings, d is for integers and f is for floats.
The complete list and further info can be found here

Transforming your example to this gives:

printf "%18s %18s %18s\n", $ip_date_time[$i]{ip}, $ip_date_time[$i]{date}, $ip_date_time[$i]{time};

update: added link to perlfunc:sprintf

update 2:there is a tutorial in the tutorial section (where else :-): Using (s)printf()

printf "%18s %18s %18s", ($ip_date_time[$i]{ip}, $ip_date_time[$i]{dat
+e}, $ip_date_time[$i]{time});
[download]

Read Appendix A of Effective Perl Programming. (Or say "man 3 printf" in a Linux/Unix shell ... might also help.)

Update: If you need to get the output into a text format (e.g., for redirecting into a file), do this (thanks to jmcnamara for the tip):

man 3 printf | col -b > printf.txt
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The "col()" command will filter out all the terminal escapes for you.

Update: Why "man 3 printf"? Because there is a printf command, too, and you will get its man page when you just say "man printf". But you want the manpage on the printf C library call, so you have to use "man 3 printf" to indicate you want the printf entry from section 3 of the manpages, which describes (surprise) library calls. See "man man" for more details on the surprisingly useful man command.

Christian Lemburg
Brainbench MVP for Perl
http://www.brainbench.com

And if you want to print out a nice troff formatted version of a man page (looks great on most printers), you can use the nifty -t switch:

man -t printf | lpr
[download]

(You need troff installed, but that should be there on any *nix-like OS worth having =)

perl -e 'print "How sweet does a rose smell? "; chomp ($n = <STDIN>); 
+$rose = "smells sweet to degree $n"; *other_name = *rose; print "$oth
+er_name\n"'
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Other mighty monks have already said all that there is to say. I'll just add some comments about legibility and nice formatting.

The printf function takes a format as first argument, and then a list of values. The first argument being special, I'm fond of using the => operator to let it stand out from the rest:

Now, we use three times the $ip_data_time[$i]: if you translate that into English, it reads: "print out the ip of the i^th element of $ip_date_time, the date of the i^th element of $ip_date_time and the time of the i^th element of $ip_date_time". I guess it would sound more natural to say "print out the ip, date and time of the i^th element of $ip_date_time". Several ways to say that in perl:

# a map
print "%18s %18s %18s\n" => map {$ip_date_time[$i]{$_}} qw/ip date tim
+e/;
# a hash slice
print "%18s %18s %18s\n" => @{$ip_date_time[$i]}{qw/ip date time/};
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Voilà, that was just my €0.02.

--bwana147

xoxo,
Andy
--
Throw down the gun and tiara and come out of the float!