printf and scanf format codes

printf and scanf key letters

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

Footnote: In printf(), the expression type promotions are expected -- in an expression, char and short are converted to int, and float is converted to double. 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 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.

printf format modifiers

• 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);

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).