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5.5.2 Format-Control Letters
----------------------------

A format specifier starts with the character '%' and ends with a
"format-control letter"--it tells the 'printf' statement how to output
one item.  The format-control letter specifies what _kind_ of value to
print.  The rest of the format specifier is made up of optional
"modifiers" that control _how_ to print the value, such as the field
width.  Here is a list of the format-control letters:

'%a', '%A'
     A floating point number of the form ['-']'0xH.HHHHp+-DD' (C99
     hexadecimal floating point format).  For '%A', uppercase letters
     are used instead of lowercase ones.

          NOTE: The current POSIX standard requires support for '%a' and
          '%A' in 'awk'.  As far as we know, besides 'gawk', the only
          other version of 'awk' that actually implements it is BWK
          'awk'.  It's use is thus highly nonportable!

          Furthermore, these formats are not available on any system
          where the underlying C library 'printf()' function does not
          support them.  As of this writing, among current systems, only
          OpenVMS is known to not support them.

'%c'
     Print a number as a character; thus, 'printf "%c", 65' outputs the
     letter 'A'.  The output for a string value is the first character
     of the string.

          NOTE: The POSIX standard says the first character of a string
          is printed.  In locales with multibyte characters, 'gawk'
          attempts to convert the leading bytes of the string into a
          valid wide character and then to print the multibyte encoding
          of that character.  Similarly, when printing a numeric value,
          'gawk' allows the value to be within the numeric range of
          values that can be held in a wide character.  If the
          conversion to multibyte encoding fails, 'gawk' uses the low
          eight bits of the value as the character to print.

          Other 'awk' versions generally restrict themselves to printing
          the first byte of a string or to numeric values within the
          range of a single byte (0-255).  (d.c.)

'%d', '%i'
     Print a decimal integer.  The two control letters are equivalent.
     (The '%i' specification is for compatibility with ISO C.)

'%e', '%E'
     Print a number in scientific (exponential) notation.  For example:

          printf "%4.3e\n", 1950

     prints '1.950e+03', with a total of four significant figures, three
     of which follow the decimal point.  (The '4.3' represents two
     modifiers, discussed in the next node.)  '%E' uses 'E' instead of
     'e' in the output.

'%f'
     Print a number in floating-point notation.  For example:

          printf "%4.3f", 1950

     prints '1950.000', with a minimum of four significant figures,
     three of which follow the decimal point.  (The '4.3' represents two
     modifiers, discussed in the next node.)

     On systems supporting IEEE 754 floating-point format, values
     representing negative infinity are formatted as '-inf' or
     '-infinity', and positive infinity as 'inf' or 'infinity'.  The
     special "not a number" value formats as '-nan' or 'nan' (*note
     Strange values::).

'%F'
     Like '%f', but the infinity and "not a number" values are spelled
     using uppercase letters.

     The '%F' format is a POSIX extension to ISO C; not all systems
     support it.  On those that don't, 'gawk' uses '%f' instead.

'%g', '%G'
     Print a number in either scientific notation or in floating-point
     notation, whichever uses fewer characters; if the result is printed
     in scientific notation, '%G' uses 'E' instead of 'e'.

'%o'
     Print an unsigned octal integer (*note Nondecimal-numbers::).

'%s'
     Print a string.

'%u'
     Print an unsigned decimal integer.  (This format is of marginal
     use, because all numbers in 'awk' are floating point; it is
     provided primarily for compatibility with C.)

'%x', '%X'
     Print an unsigned hexadecimal integer; '%X' uses the letters 'A'
     through 'F' instead of 'a' through 'f' (*note
     Nondecimal-numbers::).

'%%'
     Print a single '%'.  This does not consume an argument and it
     ignores any modifiers.

     NOTE: When using the integer format-control letters for values that
     are outside the range of the widest C integer type, 'gawk' switches
     to the '%g' format specifier.  If '--lint' is provided on the
     command line (*note Options::), 'gawk' warns about this.  Other
     versions of 'awk' may print invalid values or do something else
     entirely.  (d.c.)

     NOTE: The IEEE 754 standard for floating-point arithmetic allows
     for special values that represent "infinity" (positive and
     negative) and values that are "not a number" (NaN).

     Input and output of these values occurs as text strings.  This is
     somewhat problematic for the 'awk' language, which predates the
     IEEE standard.  Further details are provided in *note POSIX
     Floating Point Problems::; please see there.