使用 reduce 计算节点的 gini 指数

Question

我正在尝试应用公式：

我不清楚为什么这不起作用：

def gini_node(node):
    count = sum(node)
    gini = functools.reduce(lambda p,c: p + (1 - (c/count)**2), node)
    print(count, gini)
    print(1 - (node[0]/count)**2, 1 - (node[1]/count)**2)
    return gini

正在评估 gini([[175, 330], [220, 120]]) 打印：

505 175.57298304087834
0.8799137339476522 0.5729830408783452
340 220.87543252595157
0.5813148788927336 0.8754325259515571

请注意，在给定示例输入的情况下，第二个打印语句打印了我想要求和的数字。 return 值（第一个 print 语句的第二个值）应该是 0 到 1 之间的数字。

我的 reduce 有什么问题？

我想写的完整功能是：

import functools

def gini_node(node):
    count = sum(node)
    gini = functools.reduce(lambda p,c: p + (1 - (c/count)**2), node)
    print(count, gini)
    print(1 - (node[0]/count)**2, 1 - (node[1]/count)**2)
    return gini

def gini (groups):
    counts = [ sum(node) for node in groups ]
    count = sum(counts)
    proportions = [ n/count for n in counts ]

    return sum([ gini_node(node) * proportion for node, proportion in zip(groups, proportions)])

# test
print(gini([[175, 330], [220, 120]]))

Answer 1

reduce 的工作方式是它从它的容器中获取 2 个参数（只有 2 个）
https://docs.python.org/3/library/functools.html#functools.reduce
并执行给它的操作，然后继续迭代相同的操作使用 2 个参数对列表进行操作。

gini = functools.reduce(lambda p,c: p + (1 - (c/count)**2), node)

对于第一个节点 (175, 330)，此 lambda 将在 p 中采用 175，在 c 中采用 330，在 return 中采用 [=19] =] 相反，我们想要

gini = functools.reduce(lambda p,c: (1 - (p/count)**2) + (1 - (c/count)**2), node)

我添加了一些打印语句，让我们看看它们的输出。

import functools

def gini_node(node):
    count = sum(node)
    gini = functools.reduce(lambda p,c: (1 - (p/count)**2) + (1 - (c/count)**2), node)
    print(count, gini)
    print(1 - (node[0]/count)**2, 1 - (node[1]/count)**2)
    return gini

def gini (groups):
    counts = [ sum(node) for node in groups ]
    count = sum(counts)
    proportions = [ n/count for n in counts ]
    print(count, counts, proportions) #This
    gini_indexes = [ gini_node(node) * proportion for node, proportion in zip(groups, proportions)]
    print(gini_indexes) #And this
    return sum(gini_indexes)

# test
print(gini([[175, 330], [220, 120]]))

rahul@RNA-HP:~$ python3 so.py
845 [505, 340] [0.5976331360946746, 0.40236686390532544]
505 1.4528967748259973 #Second number here is addition of 2 numbers below
0.8799137339476522 0.5729830408783452
340 1.4567474048442905 #Same for this
0.5813148788927336 0.8754325259515571
#The first number of this list is first 1.45289677.... * 0.597633...
#Basically the addition and then multiplication by it's proportion.
[0.868299255961099, 0.5861468847894187]
#What you are returning to final print statement is the addition of gini co-effs of each node i.e the sum of the list above
1.4544461407505178

如果参数超过 2 个，更简单的解决方法(*)

 gini = sum([(1 - (p/count)**2) for p in node])

与上面定义的 reduce() 函数相同。

使用 reduce 计算节点的 gini 指数

using reduce to calculate gini index for a node

python

functools