Step 7: Drop the bomb

Key presses

So far in Forest Bomber, there is not much for the player to do other than sit back and watch the plane fly across and down the screen until it eventually disappears off the bottom right-hand corner. Not much of a game! We want our player to be able to interact with Forest Bomber so that when they hit the spacebar, a bomb is dropped.

Pygame has a way of capturing or storing any key presses.

The code will look as in Figure 4-1.

• The first line assigns any key presses to a variable key_pressed.

• The second line checks to see if the key_pressed variable holds a specific key, in this case the spacebar.

Let’s build the key press code into Forest Bomber. Insert code listing 10 (Figure 4-2) at line 96.

We already know that lines 97 and 98 check to see if the spacebar was pressed. Lines 99–102 is the code which will run when the spacebar is pressed.

Let’s examine line 99 more closely. It is another if statement . This statement checks for three things. It checks if

• bomb_dropped is False

• level_cleared is False

• plane_exploded is False

In Chapter 3, we learned about Boolean variables that can store either True or False. Let’s look again at the if statement to work out exactly what we are checking for:

• bomb_dropped is False

Is there a bomb already dropping? If so, we do not want to allow another bomb to be dropped because it would make the game too easy. We have to wait until any previous bombs have exploded or disappear off-screen before allowing the player to drop the next one.

• level_cleared is False

We can’t drop a bomb if the level has cleared because the game is waiting to level up.

• plane_exploded is False

We can’t drop a bomb if the plane has exploded because it is game over.

We need all three of these statements to be False before we can drop a bomb. To check that they are all False, we place and between each statement.

If all three statements are False, then lines 100 and 102 are executed:

• Line 100 sets the Boolean variable bomb_dropped to True. This will prevent another bomb being dropped until bomb_dropped is set back to False again.

• Line 101 sets the x coordinate of the bomb to the value of the x coordinate of the plane, plus 15. This means that the bomb will be displayed horizontally in the middle of the plane.

• Line 102 sets the y coordinate of the bomb to the value of the y coordinate of the plane plus 10. This means that the bomb will be displayed 10 pixels below the plane.

We now need some code to display the bomb on screen. Remember how we wrote the code to display the plane by using the blit command? We will use very similar code to display the bomb.

Insert code listing 11 (Figure 4-3) at line 135.

Key learning

pygame.key.get_pressed() checks any key presses.

The and keyword can be used to join multiple conditions within a single if statement.

When and is used, all the statements have to work out as being true for the if statement to execute.

Now run the program and try hitting the spacebar. A bomb should appear where the plane is – it just doesn’t move yet.

Move the bomb

In Chapter 3, we learned how to move the plane by changing its coordinates and then redrawing the screen. We will move the bomb in exactly the same way.

Add code listing 12 (Figure 4-4) at line 117.

Test the game by running it. If we press the spacebar, the bomb drops. The only problem is it keeps going off the screen and we can never launch another bomb. This is because the bomb_dropped Boolean variable is set to True when the bomb is launched and never changed back to False.

Remember the code that tests if a bomb can be dropped (Figure 4-5)?

A bomb can only be launched if bomb_dropped is False. Let’s fix our program so that when a bomb disappears off-screen, bomb_dropped is reset to False. Insert code listing 13 (Figure 4-6) at line 122 (note that line 122 has two indentations – eight spaces).

If the bomb has disappeared off the bottom, or off the right of the screen, bomb_dropped will be set to False which means we can now use the spacebar to drop a new bomb.

Let’s take a closer look at line 122. It compares the y coordinate of the bomb (bomb.y) with the SCREEN_HEIGHT . But how does the program know what SCREEN_HEIGHT is?

Look back to lines 18 and 19 of the program. SCREEN_WIDTH is set to 640 and SCREEN_HEIGHT is set to 480. We don’t have to do this, but it makes our code easier to read. And if we decided we wanted a bigger screen for our game, say 800 × 600, then we would just have to set SCREEN_WIDTH and SCREEN_HEIGHT to 800 and 600, respectively, at the start of the program.

You may be also wondering why we use capital letters for some variable names, like SCREEN_WIDTH and SCREEN_HEIGHT. This is because they are constants. A constant is assigned a value at the start of the program, and it doesn’t change. Python’s naming convention suggests we should use capital letters to indicate the use of constants. That way we know that we are dealing with a constant value, and therefore its value should not be changed by the program.

Run the program to test that a new bomb can be dropped once the previous bomb has disappeared off the edge of the screen.

Exploding trees

The code to blow up a tree is quite complex, and given we are only just beginning to learn how to code, we will only focus on a small part of it. Let’s begin by adding code listing 14 (Figure 4-7) at line 128.

We will learn how to display the score and level in the next chapter.

The code between lines 142 and 147 uses a timer to display the burning tree image for a series of ten frames before removing it from the game altogether. The code is beyond the scope of this chapter, so we will skip past it.

Test the game again. You should find that if the bomb hits a tree, it explodes.

Ground level

Our game of Forest Bomber is starting to take shape. The big problem now is that when our plane reaches the ground level, it flies through the trees and off the screen.

Code listing 15 (Figure 4-8) addresses this. Insert the code at line 149.

Test the game. Now the plane will come to a halt if the player manages to clear all of the trees from the forest, or it will explode if it hits a tree.

Step 8: Game over/level up

If the plane hits a tree, then it is game over.

If the plane reaches the right-hand side of the screen on the ground level, then the game needs to move on to the next level.

In either case, we will display a message to the user to tell them what has happened and invite them to hit the return key to continue.

We will use these variables to determine what to do next.

Add code listing 16 (Figure 4-9) at line 104.

Notice the use of the keyword elif at line 105 (Figure 4-10).

Also notice that the way in which we test if the return key is pressed is just the same as the way in which we tested if the space key was pressed.

Looking at the first part of line 108 (Figure 4-11), we are checking to see if the plane has exploded.

If the plane has exploded, then we know it’s game over. Lines 109–117 reset all the variables back to their start-of-game values.

There is a second part to line 108 (Figure 4-12).

This checks to see if we have cleared the level and also if the current level is equal to TOTAL_LEVELS , which is 4. In other words, if we have cleared level 4, we go back to the beginning of the game and start again at level 1.

Now that we have dealt with the game over functionality, let’s add the functionality to move up a level. Add code listing 17 (Figure 4-13) at line 119 (making sure to check the indentation very carefully).

Let’s take a closer look at some of the code used to level up (Figures 4-14, 4-15 and 4-16).

All that’s missing now is a scoreboard and some feedback for the user at the end of each level and the end of the game.