python の zip 関数の動き

zipが何をしているか？

zipがどのように動くか気になったのでpythonチュートリアルで調べた。

pythonチュートリアル 5. データ構造より抜粋

matrix = [
    [1, 2, 3, 4],
    [5, 6, 7, 8],
    [9, 10, 11, 12],
]
print(list(zip(*matrix)))

[(1, 5, 9), (2, 6, 10), (3, 7, 11), (4, 8, 12)]

ネストしたfor

vec = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
list = [num for elem in vec for num in elem]
list = []
for elem in vec:
     print(elem)
     for num in elem:
         print(num)
         list.append(num)
print(list)

[1, 2, 3]
1
2
3
[4, 5, 6]
4
5
6
[7, 8, 9]
7
8
9
[1, 2, 3, 4, 5, 6, 7, 8, 9]

matrix = [
    [1, 2, 3, 4],
    [5, 6, 7, 8],
    [9, 10, 11, 12],
]

list_first = [[row[i] for row in matrix] for i in range(4)]

list_first = []
for i in range(4):
     print(f'first:{list_first}')
     list_second = []
     for row in matrix:
         list_second.append(row[i])
         print(f'matrix → second:{list_second}')
     list_first.append(list_second)
     print(f'first → second:{list_first}')
print(list_first)

first:[]
matrix → second:[1]
matrix → second:[1, 5]
matrix → second:[1, 5, 9]
first → second:[[1, 5, 9]]
first:[[1, 5, 9]]
matrix → second:[2]
matrix → second:[2, 6]
matrix → second:[2, 6, 10]
first → second:[[1, 5, 9], [2, 6, 10]]
first:[[1, 5, 9], [2, 6, 10]]
matrix → second:[3]
matrix → second:[3, 7]
matrix → second:[3, 7, 11]
first → second:[[1, 5, 9], [2, 6, 10], [3, 7, 11]]
first:[[1, 5, 9], [2, 6, 10], [3, 7, 11]]
matrix → second:[4]
matrix → second:[4, 8]
matrix → second:[4, 8, 12]
first → second:[[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]
[[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]

組み込み関数のzipと同じ動きをする

matrix = [
    [1, 2, 3, 4],
    [5, 6, 7, 8],
    [9, 10, 11, 12],
]
print(list(zip(*matrix)))

[(1, 5, 9), (2, 6, 10), (3, 7, 11), (4, 8, 12)]

unpackについて

アンパックも曖昧だったので記載した。

a = list(range(3, 6))
print(a)

[3, 4, 5]

args = [3, 6]
b = list(range(*args))
print(b)

[3, 4, 5]