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186 lines
58 KiB
Markdown
186 lines
58 KiB
Markdown
# 使用 OOP 对森林火灾建模
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In [1]:
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```py
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%matplotlib inline
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import matplotlib.pyplot as plt
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import numpy as np
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```
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## 对森林建模
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In [2]:
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```py
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class Forest(object):
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def __init__(self, size=(150, 150), p_sapling=0.0025, p_lightning=5.e-6, name=None):
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self.size = size
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self.trees = np.zeros(self.size, dtype=bool)
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self.forest_fires = np.zeros(self.size, dtype=bool)
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self.p_sapling = p_sapling
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self.p_lightning = p_lightning
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if name is not None:
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self.name = name
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else:
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self.name = self.__class__.__name__
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@property
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def num_cells(self):
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return self.size[0] * self.size[1]
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@property
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def tree_fraction(self):
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return self.trees.sum() / float(self.num_cells)
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@property
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def fire_fraction(self):
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return self.forest_fires.sum() / float(self.num_cells)
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def advance_one_step(self):
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self.grow_trees()
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self.start_fires()
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self.burn_trees()
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def grow_trees(self):
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growth_sites = self._rand_bool(self.p_sapling)
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self.trees[growth_sites] = True
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def start_fires(self):
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lightning_strikes = (self._rand_bool(self.p_lightning) &
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self.trees)
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self.forest_fires[lightning_strikes] = True
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def burn_trees(self):
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fires = np.zeros((self.size[0] + 2, self.size[1] + 2), dtype=bool)
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fires[1:-1, 1:-1] = self.forest_fires
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north = fires[:-2, 1:-1]
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south = fires[2:, 1:-1]
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east = fires[1:-1, :-2]
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west = fires[1:-1, 2:]
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new_fires = (north | south | east | west) & self.trees
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self.trees[self.forest_fires] = False
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self.forest_fires = new_fires
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def _rand_bool(self, p):
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return np.random.uniform(size=self.trees.shape) < p
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```
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定义一个森林类之后，我们创建一个新的森林类对象：
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In [3]:
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```py
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forest = Forest()
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```
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显示当前的状态：
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In [4]:
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```py
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print forest.trees
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```
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```py
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[[False False False ..., False False False]
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[False False False ..., False False False]
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[False False False ..., False False False]
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...,
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[False False False ..., False False False]
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[False False False ..., False False False]
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[False False False ..., False False False]]
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```
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In [5]:
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```py
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print forest.forest_fires
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```
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```py
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[[False False False ..., False False False]
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[False False False ..., False False False]
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[False False False ..., False False False]
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...,
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[False False False ..., False False False]
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[False False False ..., False False False]
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[False False False ..., False False False]]
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```
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使用 `matshow` 进行可视化：
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In [6]:
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```py
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plt.matshow(forest.trees, cmap=plt.cm.Greens)
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plt.show()
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```
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## 模拟森林生长和火灾的过程
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经过一段时间：
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In [7]:
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```py
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forest.advance_one_step()
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plt.matshow(forest.trees, cmap=plt.cm.Greens)
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plt.show()
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```
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循环很长时间：
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In [8]:
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```py
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for i in range(500):
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forest.advance_one_step()
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plt.matshow(forest.trees, cmap=plt.cm.Greens)
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print forest.tree_fraction
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```
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```py
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0.253111111111
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```
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迭代更长时间：
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In [9]:
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```py
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forest = Forest()
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tree_fractions = []
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for i in range(5000):
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forest.advance_one_step()
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tree_fractions.append(forest.tree_fraction)
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fig = plt.figure()
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ax0 = fig.add_subplot(1,2,1)
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ax0.matshow(forest.trees, cmap=plt.cm.Greens)
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ax1 = fig.add_subplot(1,2,2)
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ax1.plot(tree_fractions)
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plt.show()
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```
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