08.02 使用 OOP 对森林火灾建模

calendar May 5, 2026 · 2 分钟阅读 · Python OOP 建模  ·
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使用 OOP 对森林火灾建模

1%matplotlib inline
2
3import matplotlib.pyplot as plt
4import numpy as np

对森林建模

 1class Forest(object):
 2    def __init__(self, size=(150, 150), p_sapling=0.0025, p_lightning=5.e-6, name=None):
 3        self.size = size
 4        self.trees = np.zeros(self.size, dtype=bool)
 5        self.forest_fires = np.zeros(self.size, dtype=bool)
 6        self.p_sapling = p_sapling
 7        self.p_lightning = p_lightning
 8        if name is not None:
 9            self.name = name
10        else:
11            self.name = self.__class__.__name__
12
13    @property
14    def num_cells(self):
15        return self.size[0] * self.size[1]
16
17    @property
18    def tree_fraction(self):
19        return self.trees.sum() / float(self.num_cells)
20
21    @property
22    def fire_fraction(self):
23        return self.forest_fires.sum() / float(self.num_cells)
24
25    def advance_one_step(self):
26        self.grow_trees()
27        self.start_fires()
28        self.burn_trees()
29
30    def grow_trees(self):
31        growth_sites = self._rand_bool(self.p_sapling)
32        self.trees[growth_sites] = True
33
34    def start_fires(self):
35        lightning_strikes = (self._rand_bool(self.p_lightning) & 
36            self.trees)
37        self.forest_fires[lightning_strikes] = True
38        
39    def burn_trees(self):
40        fires = np.zeros((self.size[0] + 2, self.size[1] + 2), dtype=bool)
41        fires[1:-1, 1:-1] = self.forest_fires
42        north = fires[:-2, 1:-1]
43        south = fires[2:, 1:-1]
44        east = fires[1:-1, :-2]
45        west = fires[1:-1, 2:]
46        new_fires = (north | south | east | west) & self.trees
47        self.trees[self.forest_fires] = False
48        self.forest_fires = new_fires
49
50    def _rand_bool(self, p):
51        return np.random.uniform(size=self.trees.shape) < p

定义一个森林类之后，我们创建一个新的森林类对象：

1forest = Forest()

显示当前的状态：

1print forest.trees

[[False False False ..., False False False]
 [False False False ..., False False False]
 [False False False ..., False False False]
 ..., 
 [False False False ..., False False False]
 [False False False ..., False False False]
 [False False False ..., False False False]]

1print forest.forest_fires

[[False False False ..., False False False]
 [False False False ..., False False False]
 [False False False ..., False False False]
 ..., 
 [False False False ..., False False False]
 [False False False ..., False False False]
 [False False False ..., False False False]]

使用 matshow 进行可视化：

1plt.matshow(forest.trees, cmap=plt.cm.Greens)
2
3plt.show()

png

模拟森林生长和火灾的过程

经过一段时间：

1forest.advance_one_step()
2plt.matshow(forest.trees, cmap=plt.cm.Greens)
3plt.show()

png

循环很长时间：

1for i in range(500):
2    forest.advance_one_step()
3plt.matshow(forest.trees, cmap=plt.cm.Greens)
4print forest.tree_fraction

0.253111111111

png

迭代更长时间：

 1forest = Forest()
 2tree_fractions = []
 3for i in range(5000):
 4    forest.advance_one_step()
 5    tree_fractions.append(forest.tree_fraction)
 6fig = plt.figure()
 7ax0 = fig.add_subplot(1,2,1)
 8ax0.matshow(forest.trees, cmap=plt.cm.Greens)
 9ax1 = fig.add_subplot(1,2,2)
10ax1.plot(tree_fractions)
11
12plt.show()

png