Let’s take
our example from earlier, which checks whether a value entered is
negative. Using an if-else structure we can extend our code so that it prints out a
statement if the value is non-negative.
The modified (partial) program would look like this: Scanner
s = new Scanner(System.in); System.out.print(“Enter a number: “); double
num = s.nextDouble(); //
execute block only if num is negative if (num
< 0.0) System.out.println(“You entered a negative value.”); else System.out.println(“You entered a non-negative value.”); // the
following statement will execute regardless of the //
value entered System.out.println(“Done.”); Notice that
since each of the blocks had a single statement, it was possible to omit the
curly braces. A common error
encountered is to add a statement to the if block without add the
curly braces. Consider this code: if (num
< 0.0) System.out.println(“You entered a negative value.”); System.out.println(“The compiler will give an error
here”); else System.out.println(“You entered a non-negative value.”); The second println statement will cause a compiler error since it expects the else block to start immediately after the end of the if block. Negating cell color One of the
effects that we can easily create with conditional statements is to negate
the color of cells in a Board object. While
this concept can be applied to cells of any color, we’ll limit ourselves for
now to cells that are either white or black.
Given a Board object with a number of cells that are on (we don’t know
which), we’d like to turn those off, while we turn on the cells that are off. To do that, we need some way of determining
if a cell at a given coordinate is on or not.
This can be accomplished with the help of a method called isOn which evaluates (returns) to true if a
designated cell is on (has a Color attribute), false otherwise. The x and y
coordinate of the cell are sent as arguments to the method. For instance, given a Board object
referenced b, the statement System.out.println(b.isOn(0,0)); outputs true if the top righthand cell is on, false otherwise. Since the
method returns a Boolean value, which is the same as what a logical statement
evaluates to, we can use the method invocation as the condition in an if-statement. Here’s a
code segment that negates the color of cell (0,0): if (b.isOn(0,0)) b.turnOff(0,0); else b.turnOn(0,0); In order to
negate all the cells in a Board object, we need to apply this operation to
each cell as we traverse the board row by row (or column by column) using a
nested loop structure. But to better appreciate
this effect, it would be a good idea to start with a board that has a few
cells turned on as opposed to a blank one.
We could use the Random class to select random cells that we
will turn on before we attempt to negate the board. In the code below, we create a 10x10 board
with 10 cells initially turned on. import java.util.Random; public class NegateBoard { public static void main(String[] args) { int rows = 10, cols
= 10; Random generator = new Random(); Board b = new Board(rows, cols); // turn on up to 10 randomly selected
cells for (int i = 0; i < 10; i++) { // select random x and y coordinates int xrand = generator.nextInt(cols); int yrand = generator.nextInt(rows); b.turnOn(xrand, yrand); } // pause to see the effect b.pause(10000); // traverse the board cell by cell and
negate cells for (int y = 0;
y < rows; y++) { for (int x = 0;
x < cols; x++) { if (b.isOn(x,
y)) b.turnOff(x,
y); else b.turnOn(x,
y); } } } } Here’s the
board object before and after the effect:
Finally, a
note on negating arbitrary colors. For
a color object with intensities (r, g, b), its negative is defined as the difference
between the maximum color intensity and the color component intensities. For instance the negative of color (10, 20,
30) is (255-10, 255-20, 255-30), which is the color with RGB values (245,
235, 225). Notice that this is
consistent with our understanding of black and white in that the negative of the
color black (0, 0, 0) is (255 – 0, 255 – 0, 255 – 0), which corresponds to
the RGB values of white. |
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