Last modified on 16 December 2014, at 10:22

C++ Programming/Code/Standard C Library/Functions/printf

printfEdit

Syntax
#include <cstdio>
int printf( const char *format, ... );
cout<<printf;

The printf() function prints output to stdout, according to format and other arguments passed to printf(). The string format consists of two types of items - characters that will be printed to the screen, and format commands that define how the other arguments to printf() are displayed. Basically, you specify a format string that has text in it, as well as "special" characters that map to the other arguments of printf(). For example, this code

char name[20] = "Bob";
int age = 21;
printf( "Hello %s, you are %d years old\n", name, age );

displays the following output:

 Hello Bob, you are 21 years old

The %s means, "insert the first argument, a string, right here." The %d indicates that the second argument (an integer) should be placed there. There are different %-codes for different variable types, as well as options to limit the length of the variables and whatnot.

Control Character Explanation
 %c a single character
 %d a decimal integer
 %i an integer
 %e scientific notation, with a lowercase "e"
 %E scientific notation, with an uppercase "E"
 %f a floating-point number
 %g use %e or %f, whichever is shorter
 %G use %E or %f, whichever is shorter
 %o an octal number
 %x unsigned hexadecimal, with lowercase letters
 %X unsigned hexadecimal, with uppercase letters
 %u an unsigned integer
 %s a string
 %x a hexadecimal number
 %p a pointer
 %n the argument shall be a pointer to an integer into which is placed the number of characters written so far
 %% a percent sign

A field-length specifier may appear before the final control character to indicate the width of the field:

  • h, when inserted inside %d, causes the argument to be a short int.
  • l, when inserted inside %d, causes the argument to be a long.
  • l, when inserted inside %f, causes the argument to be a double.
  • L, when inserted inside %d or %f, causes the argument to be a long long or long double respecively.

An integer placed between a % sign and the format command acts as a minimum field width specifier, and pads the output with spaces or zeros to make it long enough. If you want to pad with zeros, place a zero before the minimum field width specifier:

  %012d

You can also include a precision modifier, in the form of a .N where N is some number, before the format command:

 %012.4d

The precision modifier has different meanings depending on the format command being used:

  • With %e, %E, and %f, the precision modifier lets you specify the number of decimal places desired. For example, %12.6f will display a floating number at least 12 digits wide, with six decimal places.
  • With %g and %G, the precision modifier determines the maximum number of significant digits displayed.
  • With %s, the precision modifier simply acts as a maximum field length, to complement the minimum field length that precedes the period.

All of printf()'s output is right-justified, unless you place a minus sign right after the % sign. For example,

 %-12.4f 

will display a floating point number with a minimum of 12 characters, 4 decimal places, and left justified. You may modify the %d, %i, %o, %u, and %x type specifiers with the letter l and the letter h to specify long and short data types (e.g. %hd means a short integer). The %e, %f, and %g type specifiers can have the letter l before them to indicate that a double follows. The %g, %f, and %e type specifiers can be preceded with the character '#' to ensure that the decimal point will be present, even if there are no decimal digits. The use of the '#' character with the %x type specifier indicates that the hexidecimal number should be printed with the '0x' prefix. The use of the '#' character with the %o type specifier indicates that the octal value should be displayed with a 0 prefix.

Inserting a plus sign '+' into the type specifier will force positive values to be preceded by a '+' sign. Putting a space character ' ' there will force positive values to be preceded by a single space character.

You can also include constant escape sequences in the output string.

The return value of printf() is the number of characters printed, or a negative number if an error occurred.

Related topics
fprintf - puts - scanf - sprintf