Formatted Input and Output

printf

• See also the detailed online documentation from cplusplus.com
• The normal method of printing data from a C program is to use printf.
• The syntax of printf is:

			printf( "format string " [ , expression ] ... );

• where:
  • The format string is a quoted string, optionally containing one or more format specifiers, beginning with % symbols.
  • After the format string come zero or more expressions to be printed.
  • Specifically there must be exactly as many expressions following the format string as there are format specifiers in the string.
  • Expressions to be printed may be any combination of constants and variables that match the data types of the format specifiers.

printf format specifiers

• printf format specifiers take the form of

			%[flags][width][.precision][length]specifier

• where:
  • specifier is the most important and only required part, indicating the type of data to be printed. e.g. "%f"
    • See below for a complete table
  • width specifies the total number of spaces to use to print the number, filling in with blanks if necssary
    • Example: "%15f" would print a float in a space 15 characters wide
    • If an asterisk ( * ) is used, then one of the data expressions is used to determine the width used.
  • .precision indicates how many digits of a number to print, or sometimes the number to the right of the decimal point.
    • Examples:
      • "%15.3f" would print 10 * pi as 31.416, with three digits to the right of the decimal point.
      • %15.3e" would print 10 * pi as 3.14e1, with three total digits of precision
    • If a dot asterisk ( .* ) is used, then one of the data expressions is used to determine the width used.
  • [flags] are optional, and can include::
    • A minus sign ( - ) to left justify the output. Default is right justified.
    • A plus sign ( + ) to force a sign to be printed.
    • ( space ) to inhibit printing of the sign
    • zero ( 0 ) to left pad the number with zeros instead of spaces
    • hash ( # ) serves different purposes depending on the specifier used.
      • With octal or hexidecimal output, specifies a leading 0 or 0x for non-zero data values
      • With floating point types, requires printing a decimal point even if there are no digits following it.
  • [length] is a subspecifier for unsigned, short, long, etc. types. See table below

printf specifiers

Note: CS 107 students are not responsible for knowing the a, A, n, or p specifiers.

specifier	Output	Example
d or i	Signed decimal integer	392
u	Unsigned decimal integer	7235
o	Unsigned octal	610
x	Unsigned hexadecimal integer	7fa
X	Unsigned hexadecimal integer (uppercase)	7FA
f	Decimal floating point, lowercase	392.65
F	Decimal floating point, uppercase	392.65
e	Scientific notation (mantissa/exponent), lowercase	3.9265e+2
E	Scientific notation (mantissa/exponent), uppercase	3.9265E+2
g	Use the shortest representation: %e or %f	392.65
G	Use the shortest representation: %E or %F	392.65
a	Hexadecimal floating point, lowercase	-0xc.90fep-2
A	Hexadecimal floating point, uppercase	-0XC.90FEP-2
c	Character	a
s	String of characters	sample
p	Pointer address	b8000000
n	Nothing printed. The corresponding argument must be a pointer to a signed int. The number of characters written so far is stored in the pointed location.
%	A % followed by another % character will write a single % to the stream.	%

printf length sub-specifiers

Note: CS 107 students are not responsible for knowing j, z, or t length specifiers, or wide character types

	specifiers
length	d i	u o x X	f F e E g G a A	c	s	p	n
(none)	int	unsigned int	double	int	char*	void*	int*
hh	signed char	unsigned char					signed char*
h	short int	unsigned short int					short int*
l	long int	unsigned long int		wint_t	wchar_t*		long int*
ll	long long int	unsigned long long int					long long int*
j	intmax_t	uintmax_t					intmax_t*
z	size_t	size_t					size_t*
t	ptrdiff_t	ptrdiff_t					ptrdiff_t*
L			long double

 

scanf

• See also the detailed online documentation from cplusplus.com
• The normal method of reading data into a C program is to use scanf.
• The syntax of scanf is:

			printf( "format string " [ , expression ] ... );

• where:
  • The format string is a quoted string, normally consisting only of one or more format specifiers, beginning with % symbols.
  • After the format string come zero or more expressions to be printed.
  • Specifically there must be exactly as many expressions following the format string as there are format specifiers in the string.
  • Expressions in scanf need to be memory addresses of where the scanned data is to be stored, which are usually generated by applying the "address of" operator, &, to a variable name, for example, &X yields the address of X, i.e. the location of X in memory.

Scanf format specifiers

• scanf format specifiers take the form of

			%[*][width][length]specifier

• where:
  • specifier is the most important and only required part, indicating the type of data to be scanned. e.g. "%f"
    • See below for a complete table
  • Asterisk, *, indicates to read in the data from the input stream, but not store it. ( I.e. read and discard )
  • width specifies the maximum number of characters to read in
  • length is a modifier for unsigned, short, long,etc data types.
    • See below for a complete table

scanf specifiers

Note: CS 107 students are not required to know the a,p, [ ], [^ ], or n specifiers

specifier	Description	Characters extracted
i, u	Integer	Any number of digits, optionally preceded by a sign (+ or -). Decimal digits assumed by default (0-9), but a 0 prefix introduces octal digits (0-7), and 0x hexadecimal digits (0-f).
d	Decimal integer	Any number of decimal digits (0-9), optionally preceded by a sign (+ or -).
o	Octal integer	Any number of octal digits (0-7), optionally preceded by a sign (+ or -).
x	Hexadecimal integer	Any number of hexadecimal digits (0-9, a-f, A-F), optionally preceded by 0x or 0X, and all optionally preceded by a sign (+ or -).
f, e, g	Floating point number	A series of decimal digits (or hexadecimal, if preceded by 0x or 0X), optionally containing a decimal point, optionally preceeded by a sign (+ or -) and optionally followed by the e or E character and a decimal integer (or p or P and an hexadecimal integer). Or any other sequence supported by strtod.
a
c	Character	The next character. If a width other than 1 is specified, the function reads exactly width characters and stores them in the successive locations of the array passed as argument. No null character is appended at the end.
s	String of characters	Any number of non-whitespace characters, stopping at the first whitespace character found. A terminating null character is automatically added at the end of the stored sequence.
p	Pointer address	A sequence of characters representing a pointer. The particular format used depends on the system and library implementation, but it is the same as the one used to format %p in fprintf.
[characters]	Scanset	Any number of the characters specified between the brackets. A dash (-) that is not the first character may produce non-portable behavior in some library implementations.
[^characters]	Negated scanset	Any number of characters none of them specified as characters between the brackets.
n	Count	No input is consumed. The number of characters written so far is stored in the pointed location.
%	%	A % followed by another % matches a single %.

scanf length specifiers

Note: CS 107 students are not required to know the j, z, or t specifiers, or wide ( long ) character types

	specifiers
length	d i	u o x	f e g a	c s [] [^]	p	n
(none)	int*	unsigned int*	float*	char*	void**	int*
hh	signed char*	unsigned char*				signed char*
h	short int*	unsigned short int*				short int*
l	long int*	unsigned long int*	double*	wchar_t*		long int*
ll	long long int*	unsigned long long int*				long long int*
j	intmax_t*	uintmax_t*				intmax_t*
z	size_t*	size_t*				size_t*
t	ptrdiff_t*	ptrdiff_t*				ptrdiff_t*
L			long double*

Related functions

• fprintf and fscanf are used for formatted data transfer into and out of files respectively.
• sprintf and sscanf are used for formatted data transfer into and out of memory respectively, e.g. "printng" into a character array.
• getc,putc, getchar and putchar transfer individual characters, faster and at a more controlled level than printf( "%c", c ) and scanf( "%c", c ) respectively.