Programming in C

Initializing Structures
181
sets the time1 variable to the same initial values as shown in the previous example.The
statement
struct date today = { .year = 2004 };
sets just the year member of the date structure variable today to 2004.
Compound Literals
You can assign one or more values to a structure in a single statement using what is
know as compound literals.For example, assuming that today has been previously declared
as a struct date variable, the assignment of the members of today as shown in
Program 9.1 can also be done in a single statement as follows:
today = (struct date) { 9, 25, 2004 };
Note that this statement can appear anywhere in the program; it is not a declaration
statement.The type cast operator is used to tell the compiler the type of the expression,
which in this case is struct date, and is followed by the list of values that are to be
assigned to the members of the structure, in order.These values are listed in the same
way as if you were initializing a structure variable.
You can also specify values using the .member notation like this:
today = (struct date) { .month = 9, .day = 25, .year = 2004 };
The advantage of using this approach is that the arguments can appear in any order.
Without explicitly specifying the member names, they must be supplied in the order in
which they are defined in the structure.
The following example shows the dateUpdate function from Program 9.4 rewritten
to take advantage of compound literals:
// Function to calculate tomorrow's date – using compound literals
struct date dateUpdate (struct date today)
{
struct date tomorrow;
int numberOfDays (struct date d);
if ( today.day != numberOfDays (today) )
tomorrow = (struct date) { today.month, today.day + 1, today.year };
else if ( today.month == 12 ) // end of year
tomorrow = (struct date) { 1, 1, today.year + 1 };
else
// end of month
tomorrow = (struct date) { today.month + 1, 1, today.year };
}
return tomorrow;