Format Specifiers in C

In this C programming language tutorial we take another look at the printf
function. We will look at how to use format specifiers to print formatted output
onto the screen. The topics covered are; a little printf background, format
specifiers and conversions, formatting of different types and format conversions
of strings.

printf Background

The printf function is not part of the C language, because there is no input or
output defined in C language itself. The printf function is just a useful
function from the standard library of functions that are accessible by C
programs. The behavior of printf is defined in the ANSI standard. If the
compiler that you’re using conforms to this standard then all the features and
properties should be available to you.

Format Specifiers

There are many format specifiers defined in C. Take a look at the following
list:

%i or %d	int
%c	char
%f	float
%lf	double
%s	string

Note: %lf stands for long
float.

Let’s take a look at an example of printf formatted output:

 #include<stdio.h>

 main()
 {
  int a,b;
  float c,d;

  a = 15;
  b = a / 2;
  printf("%d\n",b);
  printf("%3d\n",b);
  printf("%03d\n",b);

  c = 15.3;
  d = c / 3;
  printf("%3.2f\n",d);
 }

Output of the source above:

7
   7
007
5.10

As you can see in the first printf statement we print a decimal. In the second
printf statement we print the same decimal, but we use a width (%3d) to say that
we want three digits (positions) reserved for the output.

The result is that two “space characters” are placed before printing the
character. In the third printf statement we say almost the same as the previous
one. Print the output with a width of three digits, but fill the space with 0.

In the fourth printf statement we want to print a float. In this printf
statement we want to print three position before the decimal point (called
width) and two positions behind the decimal point (called precision).

The \n used in the printf statements is called an escape sequence. In this case
it represents a newline character. After printing something to the screen you
usually want to print something on the next line. If there is no \n then a next
printf command will print the string on the same line. Commonly used escape
sequences are:

• 
  \n (newline)
• 
  \t (tab)
• 
  \v (vertical tab)
• 
  \f (new page)
• 
  \b (backspace)
• 
  \r (carriage return)
• 
  \n (newline)

Let’s take another look at a printf formatted output in a more application like
example:

#include<stdio.h>

main()
{
 int Fahrenheit;

 for (Fahrenheit = 0; Fahrenheit <= 300; Fahrenheit = Fahrenheit + 20)
  printf("%3d %06.3f\n", Fahrenheit, (5.0/9.0)*(Fahrenheit-32));
}

Output of the source above:

  0 -17.778
 20 -6.667
 40 04.444
 60 15.556
 80 26.667
100 37.778
120 48.889
140 60.000
160 71.111
180 82.222
200 93.333
220 104.444
240 115.556
260 126.667
280 137.778
300 148.889

As you can see we print the Fahrenheit temperature with a width of 3 positions.
The Celsius temperature is printed with a width of 6 positions and a precision
of 3 positions after the decimal point. Let’s recap:

• 
  %d (print as a decimal integer)
• 
  %6d (print as a decimal integer with a width of at least 6 wide)
• 
  %f (print as a floating point)
• 
  %4f (print as a floating point with a width of at least 4 wide)
• 
  %.4f (print as a floating point with a precision of four characters after
  the decimal point)
• 
  %3.2f (print as a floating point at least 3 wide and a precision of 2)

Formatting other Types

Until now we only used integers and floats, but there are more types you can
use. Take a look at the following example:

 #include<stdio.h>

 main()
 {
  printf("The color: %s\n", "blue");
  printf("First number: %d\n", 12345);
  printf("Second number: %04d\n", 25);
  printf("Third number: %i\n", 1234);
  printf("Float number: %3.2f\n", 3.14159);
  printf("Hexadecimal: %x\n", 255);
  printf("Octal: %o\n", 255);
  printf("Unsigned value: %u\n", 150);
  printf("Just print the percentage sign %%\n", 10);
 }

Output of the source example:

The color: blue
First number: 12345
Second number: 0025
Third number: 1234
Float number: 3.14
Hexadecimal: ff
Octal: 377
Unsigned value: 150
Just print the percentage sign %

Note: In the last printf
statement only the percentage sign is printed.

The number 10 in this statement doesn’t matter; it’s not used in the output. So
if you want to print a percentage number you would use something like this:
printf(“%2d%%\n”, 10); (The output will be 10%)

Formatting Strings

By now you have seen most of the format conversion possible, but there is one
type that is a little different

and that are string format conversions. Take a look at the following example:

 #include<stdio.h>

 main()
 {
  printf(":%s:\n", "Hello, world!");
  printf(":%15s:\n", "Hello, world!");
  printf(":%.10s:\n", "Hello, world!");
  printf(":%-10s:\n", "Hello, world!");
  printf(":%-15s:\n", "Hello, world!");
  printf(":%.15s:\n", "Hello, world!");
  printf(":%15.10s:\n", "Hello, world!");
  printf(":%-15.10s:\n", "Hello, world!");
 }

The output of the example above:

:Hello, world!:
:  Hello, world!:
:Hello, wor:
:Hello, world!:
:Hello, world!  :
:Hello, world!:
:     Hello, wor:
:Hello, wor     :

As you can see, the string format conversion reacts very different from number
format conversions.

• 
  The printf(“:%s:\n”, “Hello, world!”); statement prints the string (nothing
  special happens.)
• 
  The printf(“:%15s:\n”, “Hello, world!”); statement prints the string, but
  print 15 characters. If the string is smaller the “empty” positions will be
  filled with “whitespace.”
• 
  The printf(“:%.10s:\n”, “Hello, world!”); statement prints the string, but
  print only 10 characters of the string.
• 
  The printf(“:%-10s:\n”, “Hello, world!”); statement prints the string, but
  prints at least 10 characters. If the string is smaller “whitespace” is
  added at the end. (See next example.)
• 
  The printf(“:%-15s:\n”, “Hello, world!”); statement prints the string, but
  prints at least 15 characters. The string in this case is shorter than the
  defined 15 character, thus “whitespace” is added at the end (defined by the
  minus sign.)
• 
  The printf(“:%.15s:\n”, “Hello, world!”); statement prints the string, but
  print only 15 characters of the string. In this case the string is shorter
  than 15, thus the whole string is printed.
• 
  The printf(“:%15.10s:\n”, “Hello, world!”); statement prints the string, but
  print 15 characters.
  
  If the string is smaller the “empty” positions will be filled with “whitespace.”
  But it will only print a maximum of 10 characters, thus only part of new
  string (old string plus the whitespace positions) is printed.
• 
  The printf(“:%-15.10s:\n”, “Hello, world!”); statement prints the string,
  but it does the exact same thing as the previous statement, accept the
  “whitespace” is added at the end.

A little warning!

The printf function uses its first argument to determine how many arguments will
follow and of what types they are. If you don’t use enough arguments or if they
are of the wrong type than printf will get confuses, with as a result wrong
answers.

That’s all for this C tutorial. Just make some examples of your own, they are
easy to make. This is the only way to learn and see how the format conversions
reacts.