printf and scanf format codes

printf and scanf key letters

The first argument to printf() or scanf() is a format string. In printf(), most things are output directly except for format codes beginning with a '%'. Key letters show data type AND formatting.

The most important key letters are d, g (usually used as "lg"), and s.

code	type	format
d	int	decimal (base ten) number
o	int	octal number (no leading '0' supplied in printf)
x or X	int	hexadecimal number (no leading '0x' supplied in printf; accepted if present in scanf) (for printf, 'X' makes it use upper case for the digits ABCDEF)
ld	long	decimal number ('l' can also be applied to any of the above to change the type from 'int' to 'long')
u	unsigned	decimal number
lu	unsigned long	decimal number
c	char [footnote]	single character
s	char pointer	string
f	float [footnote]	number with six digits of precision
g	float [footnote]	number with up to six digits of precision
e	float [footnote]	number with up to six digits of precision, scientific notation
lf	double [footnote]	number with six digits of precision
lg	double [footnote]	number with up to six digits of precision
le	double [footnote]	number with up to six digits of precision, scientific notation

Footnote: In printf(), the rvalue type promotions are expected. Thus %c actually corresponds to a parameter of type int and %f and %g actually correspond to parameters of type double. Thus in printf() there is no difference between %f and %lf, or between %g and %lg. However, in scanf() what is passed is a pointer to the variable so no rvalue type promotions occur or are expected. Thus %f and %lf are quite different in scanf, but the same in printf.

Personally, I used to use %lg routinely for doubles in both printf and scanf, but this is out of favour these days and indeed gcc will give you a warning message for using %lg in printf. The usual procedure these days is to use %g for double in printf and %lg for double in scanf. It doesn't matter which you use for printf because the printf library function treats them as synonymous, but it's crucial to get it right for scanf.

 

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printf format modifiers

Modifiers appear between the '%' and the key letter.

• a number is a field width
• '.' and a number is a "precision"
  • Example: printf("%6.3f", 2.8) yields _2.800 (with a space before the '2')
  • Note that that "6" includes the 3 decimal places and the 1 '.' -- 6 characters total. Thus 6-3-1 = 2 characters to the left of the decimal point.
• 0 (the digit zero) means pad with zeroes to field width (usually used only with integers)
• l (the letter) means "long", e.g. %ld to format a long int in decimal

Example of '0':

int dollars = 2;


int cents = 3;   /* $2.03 */


printf("  ?? what goes here ??    ", dollars, cents);

If format is	then output is
"$%d.%d"	$2.3
"$%d.%2d"	$2. 3    (one space between "." and "3")
"$%d.%02d"	$2.03

In "%02d", the 0 is not part of the field width. It is a modifier character. And when you have a field width, you must write it such that it does not start with a zero (and then you can prepend a zero as a modifier character if you want that).