在 python 中的列表中使用 for 循环

Question

I am confused between these 2 implementations:

Implementation 1:

arr1 = []
arr2 = []
for j in range(2):
    arr1.append({"vals":[random() for j in range(2)]} )
arr2.append(arr1)

for i in arr2:
   print(i)

Implementation 2:

arr3 = [{'vals': [random() for i in range(2)]} for i in range(2)]

for i in arr3:
   print(i)

Outputs:

Implementation1:

[{'vals': [0.36439704360819525, 0.8234398731777764]}, {'vals': [0.8452416981328936, 0.42974230274939684]}]

Implementation2:

{'vals': [0.2974249134210081, 0.515705525022607]} {'vals': [0.8142081726400429, 0.6701375952682302]}

Is there a difference between the 2 implementation of for loop and can I do the second implementation with a external for loop?

Answer 1

In your implementation1, you create arr1 as an array with 2 items like {vals:[]}, and append to arr2 which is an array with one item that is arr1

in implementation2, arr3 is created like arr1 in implementation1. But there isn't any arr2 - I guess you have a typo here, you tend to print i in arr3, which are like {vals:[]}.