Table of Contents
Step 019  Homing missiles
In this example game, two players can fire bullets and rockets at each other or at big nasty monsters.
Bullets have a short range, but steal hitpoints if hitting a player or a monster.
There are 2 different kind of rockets:
 a “light”, weak damage, fast flying variant. Short lifetime. Can be shot down.
 a “heavy”, heavy damage, slow flying “sliding” variant. Longer lifetime. Can not be shot down, but it's flightpath can be disturbed if hit.
Both types of rockets aim automatically toward the target. While the “light” rockets move in the direction of their heading (boring), the “heavy” rockets “slide” like in open space, making more interesting flight patterns.
A player can store a maximum ammo of 32 rockets (see class Player, self.rocketsmax). The first 16 rockets are light rockets, the others are heavy rockets.
Not shooting increase the stock of missiles of each player (see the blue rocketbar above each player)
If no more monsters are present, the players can fight each other (duel mode). New monsters can be created my pressing M.
If both players are shot down by monsters, the monsters start fighting themselves (Overtime). You can press O to get more overtime and watch the monsters fighting each other.
Monster states
Each 15 seconds, (see class Monster, self.hunttime) the monster has a chance to select a new target.
Each monster has a primitive State_machine, cycling between the three states “do nothing”, “shoot bullets at target” and “shoot rockets at target”. Each “state” is exactly 3 seconds long. This is the code line to see in what “state” or “phase” a monster currently is (inside class Monster, def update):
self.phase = self.phases[int(self.alivetime) % 3 ]
Self.alivetime is a variable counting the time (in seconds) of existence of a monster. The %sign is the modulo operator. It doesn't return the result of a division, but instead the remainder.
As you can see in this example, the remainder cycles between 0 , 1 and 2 if the divisor is 3:
for x in range(31): # python2.x print x, "divided by 3 =", x/3, "remainder:", x%3,"float result:", x/3.0
output:
0 divided by 3 = 0 remainder: 0 float result: 0.0 1 divided by 3 = 0 remainder: 1 float result: 0.333333333333 2 divided by 3 = 0 remainder: 2 float result: 0.666666666667 3 divided by 3 = 1 remainder: 0 float result: 1.0 4 divided by 3 = 1 remainder: 1 float result: 1.33333333333 5 divided by 3 = 1 remainder: 2 float result: 1.66666666667 ...
classes
additional resources
source code on github
To run this example you need:
file  in folder  download 

019_homing_missiles.py  pygame  Download the whole Archive with all files from Github: https://github.com/horstjens/ThePythonGameBook/archives/master 
player_red2.png  pygame/data 

player_blue2.png  pygame/data 

xmonster_s.png  pygame/data 

xmonster_fire_s.png  pygame/data 

xmonster_left_s.png  pygame/data 

xmonster_right_s.png  pygame/data 

claws.ogg  pygame/data 

wormhole.ogg  pygame/data 

bomb.ogg  pygame/data 

shoot.ogg  pygame/data 

beep.ogg  pygame/data 

explode.ogg  pygame/data 
View/Edit/Download the file directly in Github: https://github.com/horstjens/ThePythonGameBook/blob/master/pygame/019_homing_missiles.py
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