The if 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 (in other words, 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 (in other words, 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" ambiguity associated with if statements of this form is resolved by associating the else with the last lexically preceding else-less if 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 ”.)