printf()

The printf() function was originally part of C but it's included in C++ as well. You don't even have to #include <iostream> to use it, as it's declared in the <stdio.h> header used by C.

Parameters

1. A C string that contains the text to be written to stdout

2. (optional) embedded format specifiers, which will be replaced by the values specified in subsequent additional arguments and formatted as requested.

Format Specifiers

A format specifier follows the prototype as follows: %[flags][width][.precision][length]specifier

The format specifier can also contain sub-specifiers:

1. flags
2. width
3. .precision
4. modifiers

Width

Precision

• Format: .precision, where precision depends on the flag in use, explained below:

Modifiers

• The length sub-specifier modifies the length of the data type.

• This is a chart showing the types used to interpret the corresponding arguments with and without length specifier

• The first column denotes the length specifiers, which indicate the size of the underlying data type to be formatted.

Additional Arguments

• Depending on the format string, the function may expect a sequence of additional arguments, each containing a value to be used to replace a format specifier in the format string (or a pointer to a storage location, for n)

• There should be at least as many of these arguments as the number of values specified in the format specifiers. Additional arguments are ignored by the function.

Return Value

• On success, the total number of characters written is returned.

• If a writing error occurs, the error indicator ferror is set and a negative number is returned.

• If a multibyte character encoding error occurs while writing wide characters, errno is set to EILSEQ and a negative number is returned.

Escape Sequences

printf() allows you to specify certain characters by escaping them. The most common example is \n which is used to represent a newline. Here is a table of characters, their escape sequence, and their corresponding hex value. That hex value corresponds with the character's location in the ASCII table.

Argument Index Specifiers

The printf command allows for the reuse of arguments, which can come in handy when you're trying to save space in a command. To use an argument specifier, the must be the very next character that comes after the % character, and must have a $ character appended after it. e.g. %1$s would print the first argument.

printf '%1$s has been moved to %1$s.old\n' file.txt

Format Specifiers

The printf command's real utility is in its format specifiers, which allow you to replace sections of the format string with the argument strings that follow it.

Examples

C / C++

• Examples using printf() (C/C++)

  printf ("Characters: %c %c \n", 'a', 65);
    /* Characters: a A */
  printf ("Decimals: %d %ld\n", 1977, 650000L);
    /* Decimals: 1977 650000 */
  printf ("Preceding with blanks: %10d \n", 1977);
    /* Preceding with blanks:       1977 */
  printf ("Preceding with zeros: %010d \n", 1977);
    /* Preceding with zeros: 0000001977 */
  printf ("Some different radices: %d %x %o %#x %#o \n", 100, 100, 100, 100, 100);
    /* Some different radices: 100 64 144 0x64 0144 */
  printf ("floats: %4.2f %+.0e %E \n", 3.1416, 3.1416, 3.1416);
    /* floats: 3.14 +3e+000 3.141600E+000 */
  printf ("Width trick: %*d \n", 5, 10);
    /* Width trick:    10 */
  

• Examples representing Hex/Unicode characters (C/C++)

  /* Print a decimal in its hex representation */
  printf("The decimal 10 in hex is %x\n", 10);
  // => "The decimal 10 in hex is a"
  
  
  /* Print an ASCII character according to its hexadecimal notation */
  printf("The hex number 0x40 is the character: \x40\n");
  // => "The hex number 0x40 is the character A"
  
  
  /* Print a 2-byte character according to its Unicode value */
  printf("Water in chinese is \u6c34\n");
  // => "Water in chinese is 水"
  
  
  /* Print a 4-byte character according to its Unicode value */
  printf("That's so funny! \U0001f602\n");
  // => "That's so funny! 😂
  

zsh

• Setting up a number to take up 10 columns, but only print 3 decimal places (zsh):

  printf '| %10.3f | \n | %10.3f | \n' 3.1234 5.6
  # =>    |      3.123 |
  # =>    |      5.600 |
  

• More examples of modifiers (zsh):

  printf '| %10s | %20s | %10s | \n' 'Value1' 'Value2' 'Value3'
  # =>    |     Value1 |               Value2 |     Value3 |
  
  printf '| %-10s | %-20s | %-10s | \n' 'Value1' 'Value2' 'Value3'
  # =>    | Value1     | Value2               | Value3     |
  
  printf 'Number % d\nNumber % d\n' 123 -123
  # =>    Number  123
  # =>    Number -123
  
  printf 'Number %+d\nNumber %+d\n' 123 -123
  # =>    Number +123
  # =>    Number -123
  
  printf 'The letter %c is ASCII hex number %#x\n' 'A' 65
  # =>    The letter A is ASCII hex number 0x41
  
  printf 'My GPA is certainly not a %#f\n' 4
  # =>    My GPA is certainly not a 4.000000
  
  printf 'Hex number %#x\n' 95
  # =>    Hex number 0x5f
  
  printf '%-10s%-+#10.2f\n%-10s%-+#10.2f\n' 'Positive:' 2 'Negative:' -2
  # =>    Positive: +2.00
  # =>    Negative: -2.00
  

• More examples of printf (zsh):

  printf 'My name is %s\n' 'Austin'
  # =>    My name is Austin
  
  printf 'This includes %b in the argument string\n' '\nNewline values'
  # =>    This includes
  # =>    Newline values in the argument string
  
  printf 'The first letter of the alphabet is %c\n' 'a'
  # =>    The first letter of the alphabet is a
  
  printf 'The first number is the number %d\n' 1
  # =>    The first number is the number 1
  
  printf 'The first 3 digits of pi are %f\n' 3.14
  # =>    The first 3 digits of pi are 3.140000
  
  printf 'The decimal %g has its trailing zeroes truncated\n' 31.200000
  # =>    The decimal 31.2 has its trailing zeroes truncated
  
  printf 'Avogadros number is %e\n' 602214000000000000000000
  # =>    Avogadros number is 6.022140e+23
  
  printf 'The number 10 in hex is %x\n' 10
  # =>    The number 10 in hex is a
  
  printf 'This is %d%% luck, %d%% skill, %d%% concentrated power of will\n' 10 20 15
  # =>    This is 10% luck, 20% skill, 15% concentrated power of will
  
  printf '/%s\ ' {1..5}
  # => /1\/2\/3\/4\/5\
  
  printf '/%.s\' {1..5}
  # => /\/\/\/\/\
  

• Printing uint8_t in base-10 and base-16

  uint8_t value = 9;
  printf("Dec\t%hhu\n", value);
  printf("Dec\t%hhx\n", value);
  

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