从稀疏数据创建 python 列表理解

Question

我有两个列表（键和值），它们定义了一个稀疏定义的列表，每个键位置都有值。我想将其转换为密集列表，每个位置都有值。我可以使用 for 循环执行此操作，如下所示。

keys = [1,3,5]
values = [1.0, 3.0, 5.0]
column = [None]*10
for i, k in enumerate(keys):
    column[k] = values[i]

column
>>> [None, 1.0, None, 3.0, None, 5.0, None, None, None, None]

是否可以使用列表理解创建列？

Answer 1

简单地说，你可以使用这一行list comprehension:

column =[None if i not in dict(zip(keys,values)).keys() else dict(zip(keys,values))[i] for i in range(10)]

输出：

[None, 1.0, None, 3.0, None, 5.0, None, None, None, None]

Answer 2

keys = [1,3,5]
values = [1.0, 3.0, 5.0]

column = [values[keys.index(i)] if i in keys else None for i in range(10)]

print(column)

输出

[None, 1.0, None, 3.0, None, 5.0, None, None, None, None]

Answer 3

我接受了上面 zimdero 的回答，因为它肯定回答了这个问题。但是，存在关于大型列表的时间问题。我使用下面的代码为每个案例计时，列表大小为 10K，密度为 .4（60% 的值是 None）。

对于这种特殊情况，for 循环比列表理解快 2 个数量级，比使用 zip 的列表理解快 3 个数量级以上。查看下面的结果。

import time
import random

# setup test case
iterations = 10
sz = 10000
cutoff = sz*.60
a = random.sample(range(1,sz+1), sz)
dense = [x if x > cutoff else None for x in a]
keys = [i for i, n in enumerate(dense) if n]
values = [x for x in dense if x]

# case 1 for loop
start_time = time.time()
for i in range(iterations):
    column = [None]*sz
    for i, k in enumerate(keys):
        column[k] = values[i]

end_time = time.time()
print("For loop time            :", end_time - start_time)


# case 2 list comprehension
start_time = time.time()
for i in range(iterations):
    column = [values[keys.index(i)] if i in keys else None for i in range(sz)]
end_time = time.time()

print("List comprehension time 1:", end_time - start_time)


# case 2 list comprehension
start_time = time.time()
for i in range(iterations):
    column =[None if i not in dict(zip(keys,values)).keys() else 
dict(zip(keys,values))[i] for i in range(sz)]
end_time = time.time()

print("List comprehension time 2:", end_time - start_time)



For loop time            : 0.00599980354309082 
List comprehension time 1: 6.379000186920166 
List comprehension time 2: 36.09299993515015

从稀疏数据创建 python 列表理解

Create python list comprehension from sparse data

python

list-comprehension

sparse-matrix