[Python]List Comprehension リストの作り方いろいろ

List Comprehension

List Comprehensionを使うとListの作成が簡単にかける。
Comprehensionは「理解」ではなく「含む」という意味で、日本語ではリストの内包表現などと言うらしい。

###例）０～２９までの値のリストを作る
####List Comprehensionを使用しない場合

In [1]: L1 = []
   ...: for x in range(30):
   ...:     L1.append(x**2)    
   ...: 
   ...: print(L1)
   ...: 
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196, 225, 256, 289, 324, 361, 400, 441, 484, 529, 576, 625, 676, 729, 784, 841]

####List Comprehensionを使用した場合
いちいち空のListを作ってappendしなくても下のように１行でかける。

In [2]: L2 =[x**2 for x in range(30)]
In [3]: print(L2)
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196, 225, 256, 289, 324, 361, 400, 441, 484, 529, 576, 625, 676, 729, 784, 841]

for文のあとにif文を続けることができる。

In [4]: L3 = [x for x in range(30) if x % 2 == 0]
In [5]: print(L3)
[0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28]

for文のあとにfor文を続けることもできる。

In [6]: L4 = [(x,y) for x in range(10) for y in range(x,10)]
In [7]: print(L4)
[(0, 0), (0, 1), (0, 2), (0, 3), (0, 4), (0, 5), (0, 6), (0, 7), (0, 8), (0, 9), (1, 1), (1, 2), (1, 3), (1, 4), (1, 5), (1, 6), (1, 7), (1, 8), (1, 9), (2, 2), (2, 3), (2, 4), (2, 5), (2, 6), (2, 7), (2, 8), (2, 9), (3, 3), (3, 4), (3, 5), (3, 6), (3, 7), (3, 8), (3, 9), (4, 4), (4, 5), (4, 6), (4, 7), (4, 8), (4, 9), (5, 5), (5, 6), (5, 7), (5, 8), (5, 9), (6, 6), (6, 7), (6, 8), (6, 9), (7, 7), (7, 8), (7, 9), (8, 8), (8, 9), (9, 9)]

Generator Comprehension

[]を()にするとGeneratorが作成できる

In [8]: G4 = ((x,y) for x in range(10) for y in range(x,10))

In [9]: G4
Out[9]: <generator object <genexpr> at 0x000000000A395D80>

In [10]: L5 = list(G4)

In [11]: print(L5)
[(0, 0), (0, 1), (0, 2), (0, 3), (0, 4), (0, 5), (0, 6), (0, 7), (0, 8), (0, 9), (1, 1), (1, 2), (1, 3), (1, 4), (1, 5), (1, 6), (1, 7), (1, 8), (1, 9), (2, 2), (2, 3), (2, 4), (2, 5), (2, 6), (2, 7), (2, 8), (2, 9), (3, 3), (3, 4), (3, 5), (3, 6), (3, 7), (3, 8), (3, 9), (4, 4), (4, 5), (4, 6), (4, 7), (4, 8), (4, 9), (5, 5), (5, 6), (5, 7), (5, 8), (5, 9), (6, 6), (6, 7), (6, 8), (6, 9), (7, 7), (7, 8), (7, 9), (8, 8), (8, 9), (9, 9)]

詳細はここを参照