Python 以指定的機率選取元素

Python 以指定的機率選取元素

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之間的僞隨機數,以後循環元素及其機率,計算累積機率.

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

當x處於0.0到0.1之間,則輸出1

當x處於0.1到0.3之間,則輸出2

……

 

Example

#以指定的機率獲取元素 以一個列表爲基準機率，從一個列表中隨機獲取元素 

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 的長度和 probabilities 的長度一致,以及全部元素的機率相加爲1.0

 

【References】

[1] Randomly Picking Items with Given Probabilities

[2] Python 以指定機率獲取元素