In Section 2.11 we crafted a logical expression that evaluates to true when a dart lands in the centermost circle of a dart game, a circle centered at (300, 300) with radius of 60, and a second expression that evaluates to true when a dart lands within the next circular band (green) between circles with radii 60 and 120.
Let’s continue to develop our game by tracking a player’s total score. When the first expression is true, we add 4 to the total score. When the second expression is true, we add 3 to the total score. This pattern continues for the third (blue) and fourth (yellow) circular bands.
Figure5.1.1.Dart Game Board
To accomplish this, we need to execute a block of code depending upon the value computed by each of the test expressions given the (x, y) dart’s landing coordinates. If an expression evaluates to true, the total score is incremented. If false, the code block is skipped. We need the ability to execute a block of code conditionally based on whether or not an expression evaluates to true (execute the code) or false (skip the code).
This behavior is called branching, and is achieved in one way using an if-statement. Branching is our first program flow control technique. Figure 5.1.2 illustrates the parts of an if-statement. Pay close attention to the way the if-statement is constructed. Most of the statements we will encounter follow the same pattern.
Figure5.1.2.Parts of an if-statement
An if-statement starts with the keyword if followed by a pair or parentheses (…), and a pair of curly braces{…}. Within the parentheses is the boolean expression that must evaluate to true or false. The code between the brackets is executed if the expression evaluates to true and skipped if the expression evaluates to false.
Let’s pull these concepts together and write a Java method that tests the landing location of a dart and awards the appropriate number of points to the dart thrower.
// Award points to the player's total depending upon where the dart lands
public static void awardPoints(double x1, double y1) {
// Circle parameters
double xc = 300.0, yc = 300.0;
// Compute distance
double dist = Math.sqrt( Math.pow(x1 - xc, 2) + Math.pow(y1 - yc, 2) );
// Branch depending on distance from center
if (dist < 60) {
// In center red circle. Add 4 points.
total = total + 4;
}
if (dist >= 60 && dist < 120) {
// In green circle. Add 3 points.
total = total + 3;
}
// More to do...
}
Listing5.1.3.awardPoints() Method
Note that the two if-statements in Listing 5.1.3 are mutually exclusive. There are no values for (x1, y1) that will cause both if-statement blocks to execute. This is important. Note that both if-statements are independent of one another and so both conditions are tested when the method runs. For this method, we never want the code blocks of both if-statements to execute. That would award too many points to the player and our game would have a bug. Writing several if-statements in succession, where only one block of code in the group should execute, is a strategy that we’ll call mutually exclusive conditionals. The conditions in each if-statement must be written carefully so that only one will evaluate to true at any time.
When writing multiple sequential if-statements with the intention executing only one block, the test conditions for each must be mutually exclusive -- both conditionals should never evaluate to true at the same time.
Activity5.1.1.
Our game has two more circles that must be tested and points awarded. Finish the above method by adding two more if-statements that award points when the dart lands in the blue and yellow circular bands. Make sure that each condition is written in a way that ensures it is exclusive of all other conditions.
