6: Controlling Program Flow with Loops in Java

Print Code-39 in Java 6: Controlling Program Flow with Loops

Repeating a Certain Number of Times (Java for Statements)

Write I will not talk in class on the blackboard 100 times What your teacher really meant was, Set the count to 0 As long as the count is less than 100, Write I will not talk in class on the blackboard, Add 1 to the count Fortunately, you didn t know about loops and counters at the time If you pointed out all this stuff to your teacher, you d have gotten into a lot more trouble than you were already in One way or another, life is filled with examples of counting loops And computer programming mirrors life or is it the other way around When you tell a computer what to do, you re often telling the computer to print three lines, process ten accounts, dial a million phone numbers, or whatever Because counting loops are so common in programming, the people who create programming languages have developed statements just for loops of this kind In Java, the statement that repeats something a certain number of times is called a for statement The use of the for statement is illustrated in Listings 6-2 and 6-3 Listing 6-2 has a rock-bottom simple example, and Listing 6-3 has a more exotic example Take your pick

Listing 6-2:

import static javalangSystemout; class Yawn { public static void main(String args[]) { for (int count = 1; count <= 10; count++) { outprint( The value of count is ); outprint(count); outprintln( ); } outprintln( Done! ); } }

Figure 6-3 shows you what you get when you run the program of Listing 6-2 (You get exactly what you deserve) The for statement in Listing 6-2 starts by setting the count variable equal to 1 Then the statement tests to make sure that count is less than or equal to 10 (which it certainly is) Then the for statement dives ahead and executes the printing statements between the curly braces (At this early stage of the game, the computer prints The value of count is 1) Finally, the for statement does that last thing inside its parentheses it adds 1 to the value of count

With count now equal to 2, the for statement checks again to make sure that count is less than or equal to 10 (Yes, 2 is smaller than 10) Because the test turns out okay, the for statement marches back into the curly braced statements and prints The value of count is 2 on the screen Finally, the for statement does that last thing inside its parentheses it adds 1 to the value of count, increasing the value of count to 3 And so on This whole thing keeps being repeated over and over again until, after 10 iterations, the value of count finally reaches 11 When this happens, the check for count being less than or equal to 10 fails, and the loop s execution ends The computer jumps to whatever statement comes immediately after the for statement In Listing 6-2, the computer prints Done! The whole process is illustrated in Figure 6-4

After the word for, you always put three things in parentheses The first of these three things is called an initialization, the second is an expression, and the third thing is called an update for ( initialization ; expression ; update )

Figure 6-4: The action of the for loop in Listing 6-2

Each of the three items in parentheses plays its own distinct role: The initialization is executed once, when the run of your program first reaches the for statement The expression is evaluated several times (before each iteration) The update is also evaluated several times (at the end of each iteration) If it helps, think of the loop as if its text is shifted all around: int count = 1 for count <= 10 { outprint( The value of count is ); outprint(count); outprintln( ); count++ } You can t write a real for statement this way Even so, this is the order in which the parts of the statement are executed If you declare a variable in the initialization of a for loop, you can t use that variable outside the loop For instance, in Listing 6-2, you get an error message if you try putting outprintln(count) after the end of the loop Anything that can be done with a for loop can also be done with a while loop Choosing to use a for loop is a matter of style and convenience, not necessity