Python 以指定的几率选取元素

Python 以指定的几率选取元素

Problem

You want to pick an item at random from a list, just about as random.choice does, but you need to pick the various items with different probabilities given in another list, rather than picking any item with equal probability as random.choice does.

Solution

Module random in the standard Python library offers a wealth of possibilities for generating and using pseudo-random numbers, but it does not offer this specific functionality, so we must code it as a function of our own:

import random 

def random_pick(some_list, probabilities): 
    x = random.uniform(0,1) 
    cumulative_probability = 0.0 
    for item, item_probability in zip(some_list, probabilities): 
         cumulative_probability += item_probability 
         if x < cumulative_probability:
               break 
    return item 

 

random.uniform(0,1)->生成0.0到1.0之间的伪随机数,以后循环元素及其几率,计算累积几率.

如：random_pick([1,2,3,4],[0.1,0.2,0.3,0.4])

当x处于0.0到0.1之间,则输出1

当x处于0.1到0.3之间,则输出2

……

 

Example

#以指定的几率获取元素 以一个列表为基准几率，从一个列表中随机获取元素 

import random 

def random_pick(some_list, probabilities): 
x = random.uniform(0,1) 
cumulative_probability = 0.0 
for item, item_probability in zip(some_list, probabilities): 
cumulative_probability += item_probability 
if x < cumulative_probability:break 
return item 

some_list = [1,2,3,4] 
probabilities = [0.2,0.1,0.6,0.1] 

print random_pick(some_list,probabilities) 

 

【注意】要求 some_list 的长度和 probabilities 的长度一致,以及全部元素的几率相加为1.0

 

【References】

[1] Randomly Picking Items with Given Probabilities

[2] Python 以指定几率获取元素