> ### Section 3.1
> Blocks, Loops, and Branches

> THE ABILITY OF A COMPUTER TO PERFORM complex tasks is built on just a few ways of combining simple commands into control structures. In Java, there are just six such structures -- and, in fact, just three of them would be enough to write programs to perform any task. The six control structures are: the block, the while loop, the do..while loop, the for loop, the if statement, and the switch statement. Each of these structures is considered to be a single "statement," but each is in fact a structured statement that can contain one or more other statements inside itself.

> The block is the simplest type of structured statement. Its purpose is simply to group a sequence of statements into a single statement. The format of a block is:

>                  {
>                      statements
>                  }

> That is, it consists of a sequence of statements enclosed between a pair of braces, "{" and "}". (In fact, it is possible for a block to contain no statements at all; such a block is called an empty block, and can actually be useful at times. An empty block consists of nothing but an empty pair of braces.) Block statements usually occur inside other statements, where their purpose is to group together several statements into a unit. However, a block can be legally used wherever a statement can occur. There is one place where a block is required: As you might have already noticed in the case of the `main` subroutine of a program, the definition of a subroutine is a block, since it is a sequence of statements enclosed inside a pair of braces.

> I should probably note at this point that Java is what is called a free-format language. There are no syntax rules about how the language has to be arranged on a page. So, for example, you could write an entire block on one line if you want. But as a matter of good programming style, you should lay out your program on the page in a way that will make its structure as clear as possible. In general, this means putting one statement per line and using indentation to indicate statements that are contained inside control structures. This is the format that I will generally use in my examples.

> Here are two examples of blocks:

>             {
>                System.out.print("The answer is ");
>                System.out.println(ans);
>             }

>             {  // This block exchanges the values of x and y
>                int temp;      // A temporary variable for use in this block.
>                temp = x;      // Save a copy of the value of x in temp.
>                x = y;         // Copy the value of y into x.
>                y = temp;      // Copy the value of temp into y.
>             }

> In the second example, a variable, `temp`, is declared inside the block. This is perfectly legal, and it is good style to declare a variable inside a block if that variable is used nowhere else but inside the block. A variable declared inside a block is completely inaccessible and invisible from outside that block. When the computer executes the variable declaration statement, it allocates memory to hold the value of the variable. When the block ends, that memory is discarded (that is, made available for reuse). The variable is said to be local to the block. There is a general concept called the "scope" of an identifier. The scope of an identifier is the part of the program in which that identifier is valid. The scope of a variable defined inside a block is limited to that block, and more specifically to the part of the block that comes after the declaration of the variable.

> The block statement by itself really doesn't affect the flow of control in a program. The five remaining control structures do. They can be divided into two classes: loop statements and branching statements. You really just need one control structure from each category in order to have a completely general-purpose programming language. More than that is just convenience. In this section, I'll introduce the `while` loop and the `if` statement. I'll give the full details of these statements and of the other three control structures in later sections.

> A while loop is used to repeat a given statement over and over. Of course, its not likely that you would want to keep repeating it forever. That would be an infinite loop, which is generally a bad thing. (There is an old story about computer pioneer Grace Murray Hopper, who read instructions on a bottle of shampoo telling her to "lather, rinse, repeat." As the story goes, she claims that she tried to follow the directions, but she ran out of shampoo. (In case you don't get it, this is a joke about the way that computers mindlessly follow instructions.))

> To be more specific, a `while` loop will repeat a statement over and over, but only so long as a specified condition remains true. A `while` loop has the form:

>                  while (boolean-expression)
>                       statement

> Since the statement can be, and usually is, a block, many `while` loops have the form:

>                  while (boolean-expression) {
>                      statements
>                  }

> The semantics of this statement go like this: When the computer comes to a `while` statement, it evaluates the boolean-expression, which yields either `true` or `false` as the value. If the value is `false`, the computer skips over the rest of the `while` loop and proceeds to the next command in the program. If the value of the expression is `true`, the computer executes the statement or block of statements inside the loop. Then it returns to the beginning of the `while` loop and repeats the process. That is, it re-evaluates the boolean-expression, ends the loop if the value is `false`, and continues it if the value is `true`. This will continue over and over until the value of the expression is `false`; if that never happens, then there will be an infinite loop.

> Here is an example of a `while` loop that simply prints out the numbers 1, 2, 3, 4, 5:

>             int number;   // The number to be printed.
>             number = 1;   // Start with 1.
>             while ( number < 6 ) {  // Keep going as long as number is < 6.
>                 System.out.println(number);
>                 number = number + 1;  // Go on to the next number.
>             }
>             System.out.println("Done!");

> The variable `number` is initialized with the value 1. So the first time through the `while` loop, when the computer evaluates the expression "number < 6", it is asking whether 1 is less than 6, which is `true`. The computer therefor proceeds to execute the two statements inside the loop. The first statement prints out "1". The second statement adds 1 to `number` and stores the result back into the variable `number`; the value of `number` has been changed to 2. The computer has reached the end of the loop, so it returns to the beginning and asks again whether `number` is less than 6. Once again this is true, so the computer executes the loop again, this time printing out 2 as the value of `number` and then changing the value of `number` to 3. It continues in this way until eventually `number` becomes equal to 6. At that point, the expression "number < 6" evaluates to `false`. So, the computer jumps past the end of the loop to the next statement and prints out the message "Done!". Note that when the loop ends, the value of `number` is 6, but the last value that was printed was 5.

> By the way, you should remember that you'll never see a `while` loop standing by itself in a real program. It will always be inside a subroutine which is itself defined inside some class. As an example of a `while` loop used inside a complete program, here is a little program that computes the interest on an investment over several years. This is an improvement over examples from the previous chapter that just reported the results for one year:

>         public class Interest3 {

>            /*
>               This class implements a simple program that
>               will compute the amount of interest that is
>               earned on an investment over a period of
>               5 years.  The initial amount of the investment
>               and the interest rate are input by the user.
>               The value of the investment at the end of each
>               year is output.
>            */

>            public static void main(String[] args) {

>               double principal;  // The value of the investment.
>               double rate;       // The annual interest rate.

>               /* Get the initial investment and interest rate from the user. */

>               TextIO.put("Enter the initial investment: ");
>               principal = TextIO.getlnDouble();

>               TextIO.put("Enter the annual interest rate: ");
>               rate = TextIO.getlnDouble();

>               /* Simulate the investment for 5 years. */

>               int years;  // Counts the number of years that have passed.

>               years = 0;
>               while (years < 5) {
>                  double interest;  // Interest for this year.
>                  interest = principal * rate;
>                  principal = principal + interest;     // Add it to principal.
>                  years = years + 1;    // Count the current year.
>                  System.out.print("The value of the investment after ");
>                  System.out.print(years);
>                  System.out.print(" years is $");
>                  System.out.println(principal);
>               } // end of while loop

>            } // end of main()

>         } // end of class Interest3

> And here is the applet which simulates this program:

> Sorry, your browser doesn't
> support Java.

> You should study this program, and make sure that you understand what the computer does step-by-step as it executes the `while` loop.

> An if statement tells the computer to take one of two alternative courses of action, depending on whether the value of a given boolean-valued expression is true or false. It is an example of a "branching" or "decision" statement. An `if` statement has the form:

>                    if ( boolean-expression )
>                        statement
>                    else
>                        statement

> When the computer executes an `if` statement, it evaluates the boolean expression. If the value is `true`, the computer executes the first statement and skips the statement that follows the "`else`". If the value of the expression is `false`, then the computer skips the first statement and executes the second one. Note that in any case, one and only one of the two statements inside the `if` statement is executed. The two statements represent alternative courses of action; the computer decides between these courses of action based on the value of the boolean expression.

> In many cases, you want the computer to choose between doing something and not doing it. You can do this with an `if` statement that omits the `else` part:

>                    if ( boolean-expression )
>                        statement

> To execute this statement, the computer evaluates the expression. If the value is `true`, the computer executes the statement that is contained inside the `if` statement; if the value is `false`, the computer skips that statement.

> Of course, either or both of the statement's in an `if` statement can be a block, so that an `if` statement often looks like:

>                    if ( boolean-expression ) {
>                        statements
>                    }
>                    else {
>                        statements
>                    }

> or:

>                    if ( boolean-expression ) {
>                        statements
>                    }

> As an example, here is an `if` statement that exchanges the value of two variables, `x` and `y`, but only if `x` is greater than `y` to begin with. After this `if` statement has been executed, we can be sure that the value of `x` is definitely less than or equal to the value of `y`:

>             if ( x > y ) {
>                 int temp;      // A temporary variable for use in this block.
>                 temp = x;      // Save a copy of the value of x in temp.
>                 x = y;         // Copy the value of y into x.
>                 y = temp;      // Copy the value of temp into y.
>             }

> Finally, here is an example of an `if` statement that includes an `else` part. See if you can figure out what it does, and why it would be used:

>             if ( years > 1 ) {  // handle case for 2 or more years
>                 System.out.print("The value of the investment after ");
>                 System.out.print(years);
>                 System.out.print(" years is $");
>             }
>             else {  // handle case for 1 year
>                 System.out.print("The value of the investment after 1 year is $");
>             }  // end of if statement
>             System.out.println(principal);  // this is done in any case

> I'll have more to say about control structures later in this chapter. But you already know the essentials. If you never learned anything more about control structures, you would already know enough to perform any possible computing task. Simple looping and branching are all you really need!

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