The if-else Statement – Control Flow

The if-else Statement

The if-else statement is used to decide between two actions, based on a condition. It has the following syntax:

if (
condition
)
statement
1
else
statement
2

The condition is evaluated first. If its value is true (or unboxed to true), statement1 (the if block) is executed and then execution continues with the rest of the program. If the value is false (or unboxed to false), statement2 (the else block) is executed and then execution continues with the rest of the program. In other words, one of two mutually exclusive actions is performed. The else clause is optional; if omitted, the construct is equivalent to the simple if statement. The semantics are illustrated by the activity diagram in Figure 4.1b.

In the following examples of the if-else statement, it is assumed that all variables and methods have been appropriately defined:

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if (emergency)
  operate();
else
  joinQueue();
if (temperature > critical)
  soundAlarm();
else
  businessAsUsual();
if (catIsAway()) {
  getFishingRod();
  goFishing();
} else
  playWithCat();

Since actions can be arbitrary statements, the if statements can be nested.

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if (temperature >= upperLimit) {        // (1)
  if (danger)                           // (2) Simple if.
    soundAlarm();
  if (critical)                         // (3)
    evacuate();
  else                                  // Goes with if at (3).
    turnHeaterOff();
} else                                  // Goes with if at (1).
    turnHeaterOn();

The use of block notation, {}, can be critical to the execution of if statements. The if statements (A) and (B) in the following examples do not have the same meaning. The if statements (B) and (C) are the same, with extra indentation used in (C) to make the meaning evident. Leaving out the block notation in this case could have catastrophic consequences: The heater could be turned on when the temperature is above the upper limit.

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// (A):
if (temperature > upperLimit) {          // (1) Block notation.
  if (danger) soundAlarm();              // (2)
} else                                   // Goes with if at (1).
  turnHeaterOn();
// (B):
if (temperature > upperLimit)            // (1) Without block notation.
  if (danger) soundAlarm();              // (2)
else turnHeaterOn();                     // Goes with if at (2).
// (C):
if (temperature > upperLimit)            // (1)
  if (danger)                            // (2)
    soundAlarm();
  else                                   // Goes with if at (2).
    turnHeaterOn();

The rule for matching an else clause is that an else clause always refers to the nearest if that is not already associated with another else clause. Block notation and proper indentation can be used to make the meaning obvious.

Cascading of if-else statements comprises a sequence of nested if-else statements where the if block of the next if-else statement is joined to the else clause of the previous if-else statement. The decision to execute a block is then based on all the conditions evaluated so far.

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if (temperature >= upperLimit) {                           // (1)
  soundAlarm();
  turnHeaterOff();
} else if (temperature < lowerLimit) {                     // (2)
  soundAlarm();
  turnHeaterOn();
} else if (temperature == (upperLimit-lowerLimit)/2) {     // (3)
  doingFine();
} else                                                     // (4)
  noCauseToWorry();

The block corresponding to the first if condition that evaluates to true is executed, and the remaining if statements are skipped. In the preceding example, the block at (3) will execute only if the conditions at (1) and (2) are false and the condition at (3) is true. If none of the conditions is true, the block associated with the last else clause is executed. If there is no last else clause, no actions are performed.