Revisiting the printf() Function

Revisiting the printf() Function

The printf() function is the first C library function you used in this book to print out messages on the screen. printf() is a very important function in C, so it's worth it to spend more time on it.

The syntax for the printf() function is

#include <stdio.h>
int printf(const char *format-string, ...);

Here const char *format-string is the first argument that contains the format specifier(s); ... indicates the expression section that contains the expression(s) to be formatted according to the format specifiers. The number of expressions is determined by the number of the format specifiers inside the first argument. The function returns the numbers of expressions formatted if it succeeds. It returns a negative value if an error occurs.

const char * is explained later in this book. For the time being, consider the first argument to the printf() function as a series of characters surrounded with double quotes with some format specifiers inside. For instance, you can pass "The sum of two integers %d + %d is: %d.\n" to the function as the first argument, if needed.

Figure 5.1 shows the relationship between the format string and expressions. Note that the format specifiers and the expressions are matched in order from left to right.

Figure 5.1. The relation between the format string and the expressions in printf().

Please remember that you should use exactly the same number of expressions as the number of format specifiers within the format string.

The following are all the format specifiers that can be used in printf():

%c     The character format specifier.
%d     The integer format specifier.
%i     The integer format specifier (same as %d).
%f     The floating-point format specifier.
%e     The scientific notation format specifier (note the lowercase e).
%E     The scientific notation format specifier (note the uppercase E).
%g     Uses %f or %e, whichever result is shorter.
%G     Uses %f or %E, whichever result is shorter.
%o     The unsigned octal format specifier.
%s     The string format specifier.
%u     The unsigned integer format specifier.
%x     The unsigned hexadecimal format specifier (note the lowercase x).
%X     The unsigned hexadecimal format specifier (note the uppercase X).
%p     Displays the corresponding argument that is a pointer.
%n     Records the number of characters written so far.
%%     Outputs a percent sign (%).

Among the format specifiers in this list, %c, %d, %f, %e, and %E have been introduced so far. Several others are explained later in this book. The next section shows you how to convert decimal numbers to hexadecimal numbers by using %x or %X.