Which C++ structure

What is the structure that causes a statement or a set of statements to execute repeatedly?

One ship drives east and another drives west With the selfsame winds that blow.'Tis the set of the sails and not the gales Which tells us the way to go. --Ella Wheeler Wilcox

This chapter shows you how to structure the flow of control through a PL/SQL program. You learn how statements are connected by simple but powerful control structures that have a single entry and exit point. Collectively, these structures can handle any situation. Their proper use leads naturally to a well-structured program.

Major TopicsOverviewConditional Control: IF StatementsIterative Control: LOOP and EXIT StatementsSequential Control: GOTO and NULL Statements

Overview

According to the structure theorem, any computer program can be written using the basic control structures shown in Figure 3-1. They can be combined in any way necessary to deal with a given problem.

Figure 3-1 Control Structures

The selection structure tests a condition, then executes one sequence of statements instead of another, depending on whether the condition is true or false. A condition is any variable or expression that returns a Boolean value [TRUE or FALSE]. The iteration structure executes a sequence of statements repeatedly as long as a condition holds true. The sequence structure simply executes a sequence of statements in the order in which they occur.

Conditional Control: IF Statements

Often, it is necessary to take alternative actions depending on circumstances. The IF statement lets you execute a sequence of statements conditionally. That is, whether the sequence is executed or not depends on the value of a condition. There are three forms of IF statements: IF-THEN, IF-THEN-ELSE, and IF-THEN-ELSIF.

IF-THEN

The simplest form of IF statement associates a condition with a sequence of statements enclosed by the keywords THEN and END IF [not ENDIF], as follows:

IF condition THEN sequence_of_statements END IF;

The sequence of statements is executed only if the condition is true. If the condition is false or null, the IF statement does nothing. In either case, control passes to the next statement. An example follows:

IF sales > quota THEN compute_bonus[empid]; UPDATE payroll SET pay = pay + bonus WHERE empno = emp_id; END IF;

You might want to place brief IF statements on a single line, as in

IF x > y THEN high := x; END IF;

IF-THEN-ELSE

The second form of IF statement adds the keyword ELSE followed by an alternative sequence of statements, as follows:

IF condition THEN sequence_of_statements1 ELSE sequence_of_statements2 END IF;

The sequence of statements in the ELSE clause is executed only if the condition is false or null. Thus, the ELSE clause ensures that a sequence of statements is executed. In the following example, the first UPDATE statement is executed when the condition is true, but the second UPDATE statement is executed when the condition is false or null:

IF trans_type = 'CR' THEN UPDATE accounts SET balance = balance + credit WHERE ... ELSE UPDATE accounts SET balance = balance - debit WHERE ... END IF;

The THEN and ELSE clauses can include IF statements. That is, IF statements can be nested, as the following example shows:

IF trans_type = 'CR' THEN UPDATE accounts SET balance = balance + credit WHERE ... ELSE IF new_balance >= minimum_balance THEN UPDATE accounts SET balance = balance - debit WHERE ... ELSE RAISE insufficient_funds; END IF; END IF;

IF-THEN-ELSIF

Sometimes you want to select an action from several mutually exclusive alternatives. The third form of IF statement uses the keyword ELSIF [not ELSEIF] to introduce additional conditions, as follows:

IF condition1 THEN sequence_of_statements1 ELSIF condition2 THEN sequence_of_statements2 ELSE sequence_of_statements3 END IF;

If the first condition is false or null, the ELSIF clause tests another condition. An IF statement can have any number of ELSIF clauses; the final ELSE clause is optional. Conditions are evaluated one by one from top to bottom. If any condition is true, its associated sequence of statements is executed and control passes to the next statement. If all conditions are false or null, the sequence in the ELSE clause is executed. Consider the following example:

BEGIN ... IF sales > 50000 THEN bonus := 1500; ELSIF sales > 35000 THEN bonus := 500; ELSE bonus := 100; END IF; INSERT INTO payroll VALUES [emp_id, bonus, ...]; END;

If the value of sales is larger than 50000, the first and second conditions are true. Nevertheless, bonus is assigned the proper value of 1500 because the second condition is never tested. When the first condition is true, its associated statement is executed and control passes to the INSERT statement.

Guidelines

Avoid clumsy IF statements like those in the following example:

DECLARE ... overdrawn BOOLEAN; BEGIN ... IF new_balance < minimum_balance THEN overdrawn := TRUE; ELSE overdrawn := FALSE; END IF; ... IF overdrawn = TRUE THEN RAISE insufficient_funds; END IF; END;

This code disregards two useful facts. First, the value of a Boolean expression can be assigned directly to a Boolean variable. So, you can replace the first IF statement with a simple assignment, as follows:

overdrawn := new_balance < minimum_balance;

Second, a Boolean variable is itself either true or false. So, you can simplify the condition in the second IF statement, as follows:

IF overdrawn THEN ...

When possible, use the ELSIF clause instead of nested IF statements. That way, your code will be easier to read and understand. Compare the following IF statements:

These statements are logically equivalent, but the first statement obscures the flow of logic, whereas the second statement reveals it.

Iterative Control: LOOP and EXIT Statements

LOOP statements let you execute a sequence of statements multiple times. There are three forms of LOOP statements: LOOP, WHILE-LOOP, and FOR-LOOP.

LOOP

The simplest form of LOOP statement is the basic [or infinite] loop, which encloses a sequence of statements between the keywords LOOP and END LOOP, as follows:

LOOP sequence_of_statements END LOOP;

With each iteration of the loop, the sequence of statements is executed, then control resumes at the top of the loop. If further processing is undesirable or impossible, you can use an EXIT statement to complete the loop. You can place one or more EXIT statements anywhere inside a loop, but nowhere outside a loop. There are two forms of EXIT statements: EXIT and EXIT-WHEN.

EXIT

The EXIT statement forces a loop to complete unconditionally. When an EXIT statement is encountered, the loop completes immediately and control passes to the next statement. An example follows:

LOOP ... IF credit_rating < 3 THEN ... EXIT; -- exit loop immediately END IF; END LOOP; -- control resumes here

The next example shows that you cannot use the EXIT statement to complete a PL/SQL block:

BEGIN ... IF credit_rating < 3 THEN ... EXIT; -- illegal END IF; END;

Remember, the EXIT statement must be placed inside a loop. To complete a PL/SQL block before its normal end is reached, you can use the RETURN statement. For more information, see "Using the RETURN Statement".

EXIT-WHEN

The EXIT-WHEN statement allows a loop to complete conditionally. When the EXIT statement is encountered, the condition in the WHEN clause is evaluated. If the condition is true, the loop completes and control passes to the next statement after the loop. An example follows:

LOOP FETCH c1 INTO ... EXIT WHEN c1%NOTFOUND; -- exit loop if condition is true ... END LOOP; CLOSE c1;

Until the condition is true, the loop cannot complete. So, a statement inside the loop must change the value of the condition. In the last example, if the FETCH statement returns a row, the condition is false. When the FETCH statement fails to return a row, the condition is true, the loop completes, and control passes to the CLOSE statement.

The EXIT-WHEN statement replaces a simple IF statement. For example, compare the following statements:

IF count > 100 THEN | EXIT WHEN count > 100; EXIT; | END IF; |

These statements are logically equivalent, but the EXIT-WHEN statement is easier to read and understand.

Loop Labels

Like PL/SQL blocks, loops can be labeled. The label, an undeclared identifier enclosed by double angle brackets, must appear at the beginning of the LOOP statement, as follows:

LOOP sequence_of_statements END LOOP;

Optionally, the label name can also appear at the end of the LOOP statement, as the following example shows:

LOOP ... END LOOP my_loop;

When you nest labeled loops, you can use ending label names to improve readability.

With either form of EXIT statement, you can complete not only the current loop, but any enclosing loop. Simply label the enclosing loop that you want to complete. Then, use the label in an EXIT statement, as follows:

LOOP ... LOOP ... EXIT outer WHEN ... -- exit both loops END LOOP; ... END LOOP outer;

Every enclosing loop up to and including the labeled loop is exited.

WHILE-LOOP

The WHILE-LOOP statement associates a condition with a sequence of statements enclosed by the keywords LOOP and END LOOP, as follows:

WHILE condition LOOP sequence_of_statements END LOOP;

Before each iteration of the loop, the condition is evaluated. If the condition is true, the sequence of statements is executed, then control resumes at the top of the loop. If the condition is false or null, the loop is bypassed and control passes to the next statement. An example follows:

WHILE total