A printf format reference page (cheat sheet)

Submitted by alvin on October 4, 2008 - 6:22pm

tags:

I've created a printf format specifier cheat sheet, and thought I would share it here (i.e., a "printf reference" page, or "cheat sheet").

A cool thing about the printf formatting syntax is that the specifiers you can use are very similar, if not identical, between several different languages, including C, C++, Java, Perl, and Ruby, so your knowledge is reusable, which I like.

printf formatting with Perl and Java

In this cheat sheet I'm going to show all the examples using Perl, but I thought at first it might help to one printf example using both Perl and Java. So, here's a simple Perl printf example to get us started:

printf("the %s jumped over the %s, %d times", "cow", "moon", 2);

And here are three different Java printf examples, using different methods that are available to you in the Java programming language:

System.out.format("the %s jumped over the %s, %d times", "cow", "moon", 2);
System.err.format("the %s jumped over the %s, %d times", "cow", "moon", 2);
String result = String.format("the %s jumped over the %s, %d times", "cow", "moon", 2);

As you can see in that last String.format example, that line of code doesn't print any output, while the first line prints to standard output, and the second line prints to standard error.

In the remainder of this document I'm going to use Perl examples, but again, the actual format specifier strings can be used in many different languages.

printf format specifiers - summary

Here's a quick summary of the available printf format specifiers:

%c	character
%d	decimal (integer) number (base 10)
%e	exponential floating-point number
%f	floating-point number
%i	integer (base 10)
%o	octal number (base 8)
%s	a string of characters
%u	unsigned decimal (integer) number
%x	number in hexadecimal (base 16)
%%	print a percent sign
\%	print a percent sign

Controlling printf integer width

The "%3d" specifier means a minimum width of three spaces, which, by default, will be right-justified. (Note: the alignment is not currently being displayed properly here.)

printf("%3d", 0);	0
printf("%3d", 123456789);	123456789
printf("%3d", -10);	-10
printf("%3d", -123456789);	-123456789

Left-justifying printf integer output

To left-justify those previous printf examples, just add a minus sign (-) after the % symbol, like this:

printf("%-3d", 0);	0
printf("%-3d", 123456789);	123456789
printf("%-3d", -10);	-10
printf("%-3d", -123456789);	-123456789

The printf zero-fill option

To zero-fill your printf integer output, just add a zero (0) after the % symbol, like this:

printf("%03d", 0);	000
printf("%03d", 1);	001
printf("%03d", 123456789);	123456789
printf("%03d", -10);	-10
printf("%03d", -123456789);	-123456789

printf integer formatting

Here is a collection of printf examples for integer printing. Several different options are shown, including a minimum width specification, left-justified, zero-filled, and also a plus sign for positive numbers.

Description	Code	Result
At least five wide	printf("'%5d'", 10);	' 10'
At least five-wide, left-justified	printf("'%-5d'", 10);	'10 '
At least five-wide, zero-filled	printf("'%05d'", 10);	'00010'
At least five-wide, with a plus sign	printf("'%+5d'", 10);	' +10'
Five-wide, plus sign, left-justified	printf("'%-+5d'", 10);	'+10 '

printf - floating point numbers

Here are several examples showing how to print floating-point numbers with printf.

Description	Code	Result
Print one position after the decimal	printf("'%.1f'", 10.3456);	'10.3'
Two positions after the decimal	printf("'%.2f'", 10.3456);	'10.35'
Eight-wide, two positions after the decimal	printf("'%8.2f'", 10.3456);	' 10.35'
Eight-wide, four positions after the decimal	printf("'%8.4f'", 10.3456);	' 10.3456'
Eight-wide, two positions after the decimal, zero-filled	printf("'%08.2f'", 10.3456);	'00010.35'
Eight-wide, two positions after the decimal, left-justified	printf("'%-8.2f'", 10.3456);	'10.35 '
Printing a much larger number with that same format	printf("'%-8.2f'", 101234567.3456);	'101234567.35'

printf string formatting

Here are several printf formatting examples that show how to format string output with printf format specifiers.

Description	Code	Result
A simple string	printf("'%s'", "Hello");	'Hello'
A string with a minimum length	printf("'%10s'", "Hello");	' Hello'
Minimum length, left-justified	printf("'%-10s'", "Hello");	'Hello '

Summary of special printf characters

The following character sequences have a special meaning when used as printf format specifiers:

\a	audible alert
\b	backspace
\f	form feed
\n	newline, or linefeed
\r	carriage return
\t	tab
\v	vertical tab
\\	backslash

As you can see from that last example, because the backslash character itself is treated specially, you have to print two backslash characters in a row to get one backslash character to appear in your output.

Here are a few examples of how to use this special characters:

Description	Code	Result
Insert a tab character in a string	printf("Hello\tworld");	Hello world
Insert a newline character in a string	printf("Hello\nworld");	Hello world
Typical use of the newline character	printf("Hello world\n");	Hello world
A DOS/Windows path with backslash characters	printf("C:\\Windows\\System32\\");	C:\Windows\System32\