ICS3C/4C/3U/4U Unit 2 Workbook Selection: If Statement

Transcription

1 Selection: If Statement Selection allows a computer to do different things based on the situation. The if statement checks if something is true and then runs the appropriate code. We will start learning how to program a video game to demonstrate this. In this example, a ball will bounce around the screen. 1 We will use an if statement to check if the ball hits the edge. If it does, the ball will change direction. Y position = 0 X position = 0 X position = Window.ClientBounds.Width Y position = Window.ClientBounds.Height Here s the UML activity diagram for when the program updates: Notice the diamond shape whenever you use an if statement the diamond allows you two directions (AND ONLY TWO!) to go in. In this case, either the ball hits the side, or it doesn t. 1 Example from Learning XNA 3.0, by Aaron Reed. Copyright 2009 Aaron Reed M c T a v i s h Page 9

2 Create a new project, use XNA Game Studio 3.1, Windows Game. Name it: U2MMDDNAMEIFGAME In Solution Explorer, right-click the Content Folder. Select Add Folder, name it Images. In Solution Explorer, right-click the Images Folder. Select Add Existing Item Add the ball.bmp file (S:\pickup\mctavish) In the Game1.cs code, find the declaration for the class Game1: public class Game1 : Microsoft.Xna.Framework.Game GraphicsDeviceManager graphics; SpriteBatch spritebatch; C# is an object-oriented programming language. A class is an object the entire game is considered an object. Graphics and spritebatch are global variables that are available to the entire program. You will be adding some more global variables. Add the following code after the SpriteBatch line: Texture2D texture; Vector2 ballposition = Vector2.Zero; float xspeed = 2f; float yspeed = 2f; The texture variable will be the graphic we will use for the ball that bounces around. ballposition is a Vector2 variable Vector2 variables have an x value and a y value. It is initialized to the value (0,0) M c T a v i s h Page 10

3 xspeed and yspeed are float variables (similar to Double variables, the f after the number indicates it is a float). In the LoadContent function you will need to add the following line of code (right after the ToDo line) texture = Content.Load<Texture2D>(@"images/ball"); In the Draw function add the following code spritebatch.begin(); spritebatch.draw(texture, ballposition, Color.White); spritebatch.end(); If you run the program now, the ball should appear in the top left corner of the screen. In the Update function, add the following code after the TODO section and before base.update(gametime); 1. ballposition.x += xspeed; 2. if (ballposition.x > Window.ClientBounds.Width - texture.width ballposition.x < 0) 3. xspeed *= -1; 4. ballposition.y += yspeed; 5. if (ballposition.y > Window.ClientBounds.Height - texture.height ballposition.y < 0) 6. yspeed *= -1; If you run the program now, the ball should bounce around the screen. Here s what is happening: 1. ballposition.x += xspeed; The x location of the ball is set to the current x location plus the xspeed value (note += is an operator that adds to the item) 2. if (ballposition.x > Window.ClientBounds.Width - texture.width ballposition.x < 0) M c T a v i s h Page 11

4 Here s the if statement, the first part checks if the location of the ball is off the right of the screen. Note texture.width takes into account the width of the ball itself. is the OR operator in the if statement if the statement to the left OR the statement to the right are true, the next line will run. ballposition.x < 0 checks if the ball is off the left. 3. xspeed *= -1; In this line *= takes the value of xspeed and then multiplies it by -1. This essentially changes the direction by changing the value from positive to negative (or negative to positive) The next three lines do the same things, just with the y values instead. 4. ballposition.y += yspeed; 5. if (ballposition.y > Window.ClientBounds.Height - texture.height ballposition.y < 0) 6. yspeed *= -1; M c T a v i s h Page 12

5 if (Boolean expression) //line of code to run; The example you just programmed used the above format. Everything in the brackets has to result in a true or false answer. If the result is true, the next line of code will run. To run multiple lines of code, use the following: if (Boolean expression) //multiple lines of code between the braces; Comparison Operators The following comparison operators can be used with any primitive data types (Int32, float, Double, BUT NOT strings). In the following examples x and y are Int32 variables x = 4, y = 3 Operator Meaning Expression Result == Equal to x == y False!= Not equal to x!=y True > Greater than x>y True < Less than x<y False >= Greater than or x>=y True equal to <= Less than or equal to x<=y False But what about strings? If you have two strings and you want to compare them, you need to use the Compare or Equals function: if (String.Compare(username,"bob",true) == 0) The above line compares the variable username with the value of bob. It ignores whether there are capital letters or not. The result is -1 if username is alphabetically before bob, 0 if username is bob and 1 if username is alphabetically after bob. ==0 checks if the words are the same. M c T a v i s h Page 13

6 Boolean Operators Boolean operators check multiple situations. In the example you just did you used the OR operator. If the ball hit the top of the screen OR it hit the bottom, it changed direction. Operator Meaning Expression Result && AND TRUE && TRUE TRUE TRUE && FALSE FALSE FALSE && TRUE FALSE FALSE && FALSE FALSE OR TRUE TRUE TRUE TRUE FALSE TRUE FALSE TRUE TRUE FALSE FALSE FALSE! NOT!(TRUE) FALSE!(FALSE) TRUE M c T a v i s h Page 14

7 Advanced if statements The following code uses a variety of if statements. private void btnrun_click(object sender, EventArgs e) //Declare Variables Int32 mark; String output; //Initialize variable //if not an integer, display error if (!(Int32.TryParse(txtInput.Text, out mark))) output = "You must enter a number"; else //Now check what mark they got if (mark >= 90) output = "You got an A"; else if (mark >= 70) output = "You got a B"; else if (mark >= 50) M c T a v i s h Page 15

8 output = "You passed."; else output = "You did not pass."; rtboutput.text = output; M c T a v i s h Page 16

9 Switch Switch statements are similar to if statements. They can be used whenever you have a small range of integers to test. In the following example a mark (1-10) is entered. If the mark is 10, 9 or 8, output is set to Level 4. The break statement is used to break out of the switch statement. switch (mark) case 10: case 9: case 8: case 7: case 6: case 5: output = "Level 4"; output = "Level 3"; output = "Level 2"; output = "Level 1"; default: output = "Below level 1"; M c T a v i s h Page 17