The C Programming Language, Ansi C
is executed if none of the other cases are satisfied. A
default
is
optional; if it isn't there and if none of the cases match, no action at all
takes place. Cases and the default clause can occur in any order.
In Chapter 1 we wrote a program to count the occurrences of each digit, white space, and all other characters, using a sequence of if ... else if ... else. Here is the same program with a switch:
main() /* count digits, white space, others */
{
int c, i, nwhite, nother, ndigit[10];
nwhite = nother = 0;
for (i = 0; i < 10; i++)
ndigit[i] = 0;
while ((c = getchar()) != EOF) {
switch (c) {
case '0': case '1': case '2': case '3': case '4':
case '5': case '6': case '7': case '8': case '9':
ndigit[c-'0']++;
break;
case ' ':
case 'n':
case 't':
nwhite++;
break;
default:
nother++;
break;
}
}
printf("digits =");
for (i = 0; i < 10; i++)
printf(" %d", ndigit[i]);
printf(", white space = %d, other = %dn",
nwhite, nother);
return 0;
}
The
break
statement causes an immediate exit from the
switch
.
Because cases serve just as labels, after the code for one case is done,
execution
falls through
to the next unless you take explicit action to
escape.
break
and
return
are the most common ways to leave a
switch
. A
break
statement can also be used to force an immediate
exit from
while
,
for
, and
do
loops, as will be
discussed later in this chapter.
Falling through cases is a mixed blessing. On the positive side, it allows several cases to be attached to a single action, as with the digits in this example. But it also implies that normally each case must end with a break to prevent falling through to the next. Falling through from one case to another is not robust, being prone to disintegration when the program is modified. With the exception of multiple labels for a single computation, fall-throughs should be used sparingly, and commented.