HP 3000 Manuals HP 3000 Manuals
The if Statement
The if statement executes a statement depending on the evaluation of an
expression.
Syntax
if (expression ) statement
if (expression ) statement else statement
Description
The if statement is for testing values and making decisions. An if
statement can optionally include an else clause. For example:
if (j<1)
func(j);
else
{
j=x++;
func(j);
}
The first statement is executed only if the evaluated expression is true
(i.e., evaluates to a nonzero value). The expression may be of any
scalar type. Note that expressions involving relational expressions
actually produce a result and may therefore be used in an if statement.
If you include the else clause, the statement after the else is executed
only if the evaluated expression is false (i.e., evaluates to a zero
value). Under no circumstances are both statements in an if-else
statement executed (unless you include a goto statement from one
substatement to the other).
If the first substatement is entered as the result of a goto to a label,
the second substatement (if provided) is not executed.
The "dangling else" problem associated with if statements of this form is
resolved by associating the else with the last lexically preceding if
(without an else) that is in the same block, but not in an enclosed
block.
The else-if construction is useful to include more than one alternative
to the if statement. The following is an example of a three-way branch
using the else-if chain:
if(a==b)
k = 1;
else if(a==c)
k = 2;
else if(a==d)
k = 4;
Regardless of the relationships between the variables a, b, and c, only
one statement assigning a value to k is executed. You should use the
else-if chain in place of the switch statement when the controlling
expressions are not constant expressions. However, nesting too many
else-if statements can make a program cumbersome.
The tests are each executed in order until successful or until the end of
the selection statement is reached. In the previous example, if a is
equal to d, all three comparisons would be executed. On the other hand,
if a is equal to c, only the first two comparisons are executed.
Therefore, conditions that are most likely to be true should be tested
first in an else-if chain. The switch statement, however, may execute
only one comparison (depending on efficiency tradeoffs). Use the switch
statement where possible to make a program more readable and efficient
(See "The switch Statement" below).