6: Controlling Your Program s Flow in Java

Generation ECC200 in Java 6: Controlling Your Program s Flow

So far, the statements you ve seen fall into two categories: simple statements and compound statements. Function calls, such as calls to printf(), and assignment statements are called simple statements. Always place a semicolon at the end of a simple statement, even if it is broken over several lines, like this:

printf( "%d%d%d%d", var1, var2, var3, var4 );

Statements made up of several parts, including, possibly, other statements, are called compound statements. Compound statements obey some pretty strict rules of syntax. The if statement, for example, always looks like this:

Notice that there are no semicolons in this definition. The statement part of the if can be a simple statement or a compound statement. If the statement is simple, follow the semicolon rules for simple statements and place a semicolon at the end of the statement. If the statement is compound, follow the semicolon rules for that particular type of statement. Notice that using curly braces, or curlies, to build a superstatement or block out of smaller statements does not require the addition of a semicolon.

Guess what A single semicolon qualifies as a statement, albeit a somewhat lonely one. For example, this code fragment

if ( bored ) ;

is a legitimate (and thoroughly useless) if statement. If bored is true, the semicolon statement gets executed. The semicolon by itself doesn t do anything but fill the bill where a statement is needed. There are times where the semicolon by itself is exactly what you need.

The if statement uses the value of an expression to decide whether to execute or skip over a statement. If the statement is executed, it is executed just once. Another type of statement, the while statement, repeatedly executes a statement as long as a specified expression is true. The while statement follows this pattern:

The while statement is also known as the while loop, because once the statement is executed, the while loops back to reevaluate the expression. Here s an example of the while loop in action:

int i=0; while ( ++i < 3 ) printf( "Looping: %d\n", i ); printf( "We are past the while loop." ); i;

This example starts by declaring a variable, i, to be of type int. i is then initialized to 0. Next comes the while loop. The first thing the while loop does is evaluate its expression. The while loop s expression is

Before this expression is evaluated, i has a value of 0. The prefix notation used in the expression (++i) increments the value of i to 1 before the remainder of the expression is evaluated. The evaluation of the expression results in true, since 1 is less than 3. Since the expression is true, the while loop s statement, a single printf() is executed. Here s the output after the first pass through the loop:

Looping: 1

Next, the while loops back and reevaluates its expression. Once again, the prefix notation increments i, this time to a value of 2. Since 2 is less than 3, the expression evaluates to true, and the printf() is executed again. Here s the output after the second pass through the loop:

Looping: 1 Looping: 2

Once the second printf() completes, it s back to the top of the loop to reevaluate the expression. Will this never end Once again, i is incremented, this time to a value of 3. Aha! This time, the expression evaluates to false, since 3 is not less than 3. Once the expression evaluates to false, the while loop ends, and control passes to the next statement, the second printf() in our example:

printf( "We are past the while loop." );