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CONDITIONAL, SWITCH, BREAK and OTHER STATEMENTS.
There are four conditional statements in C, namely:
The if....else statement and three loop constructs:
The while loop
The for loop
The do...while loop
All are based on the Boolean logic of true (1) or false (0).
The if...else statement.
The generic format of this statement is:
if(value)
statement1;
else
statement2;
where 'value' is any expression that resolves to an integer value. If
'value' is nonzero (true) then statement1 is executed, otherwise statement2
is executed.
In some cases the alternative is not relevant and thus the 'else' and
'statement2' can be omitted, so that if value = 0 (false), then no
statement is executed.
Statement1 and statement2 can be replaced by a compound statement, which
consists of
a left brace ( { )
a number of statements, each ending with a semicolon.
a right brace ( } )
The if statement is illustrated in the programs KEYCHECK.C and the
associated LIMITKEY.H and in STRUCTUR.C, whilst the if...else
statement is used in the programs BINTREE.C and TWOLINKS.C
The while loop.
The while loop is the most general loop and can replace the other two,
which are provided only for convenience. The format is:
while (expression)
statement;
where expression resolves to a zero or nonzero value and statement is
either a single or compound statement. The expression is evaluated and if
true the statement is executed and the process repeated until expression
false.
The for loop.
The for loop is available in most programming languages, but in C it is
very flexible and very powerful. This for loop allows execution of a set
of statements some fixed number of times while a variable, known as the
index variable, steps through a range of values.
The generic format of the for loop statement is:
for (exp1; exp2; exp3)
statement;
where exp1 is usually an assignment to the index variable
exp2 is a test for loop continuation
exp3 is usually some modification of the index variable
and statement can be a compound statement.
Examples of for loops can be found in many of the demonstration programs,
including the simple program EXAMPLE.C and also on pages 175-6 of the
User's Guide (version 2.0).
The do...while loop.
The generic format of the do...while loop is:
do statement while (exp);
This is different from the while loop because the statements in the
do...while loop always execute at least once.
There is some similarity with the repeat...until loop of Turbo Pascal,
except that the repeat loop executes until its condition is true, whereas
the do...while loop executes while its condition is true.
The switch statement.
Repeated if..else if..else if.. constructs can be replaced by the switch
statement, which has the format:
switch (expression) {
case const-expr: statements;
case const-expr: statements;
...
default: statements;
}
Each case is labeled by one or more integer-valued constants or constant
expressions. If a case matches the expression value, execution starts at
that case. The case labeled 'default' is executed if none of the other
cases are satisfied, but if default is not present then no statement is
executed.
Because cases serve as labels only, execution falls through to the next
case unless explicit action to escape is taken. Break and return are the
most common ways to leave a switch and must be used unless it is required
to deliberately proceed to the statements of the next case.
The break statement.
The break statement can be used with all three loop statements and with the
switch statement, but not with the if...else statement. It permits the
quick and easy exit from a loop, before the end is reached and the exit
from a switch statement.
The return statement.
There are two major uses of the return statement. First, if a function
returns a value, then return must be used to pass the value back to the
calling routine. The following simple example ilustrates this:
int imax(int a, int b);
{
if (a > b)
return(a);
else
return(b);
The second usage is to exit a function at some other point than its end, as
when a condition is detected that requires termination. If the function is
of type 'void' then return can be used on its own without a value passed
back. An example in the middle of a function might be:
...
if (size <=0)
return(-1);
...
The continue statement.
Sometimes it is necessary to skip the rest of a loop and start again at the
top. The continue statement allows this. See page 201 of the User's Guide.