Python｜リスト内包表記でコードを簡潔に書く方法

# 一般的な構文
[式 for 変数 in イテラブル]

# 通常のfor文
result = []
for i in range(5):
    result.append(i)
print(result)  # 実行結果：[0, 1, 2, 3, 4]

# リスト内包表記
result = [i for i in range(5)]
print(result)  # 実行結果：[0, 1, 2, 3, 4]

# 偶数だけを抽出
evens = [i for i in range(10) if i % 2 == 0]
print(evens)  # 実行結果：[0, 2, 4, 6, 8]

words = ["apple", "banana", "cherry"]
lengths = [len(word) for word in words]
print(lengths)  # 実行結果：[5, 6, 6]

matrix = [[1, 2], [3, 4], [5, 6]]
flat = [num for row in matrix for num in row]
print(flat)  # 実行結果：[1, 2, 3, 4, 5, 6]

# 辞書内包表記
squares = {i: i**2 for i in range(5)}
print(squares)  # 実行結果：{0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

# 集合内包表記
unique_lengths = {len(word) for word in ["apple", "banana", "fig", "kiwi"]}
print(unique_lengths)  # 実行結果：{3, 4, 5, 6}

# OK
result = ["even" if i % 2 == 0 else "odd" for i in range(5)]
print(result)  # 実行結果：['even', 'odd', 'even', 'odd', 'even']

# NG（構文エラー）
# result = [i for i in range(5) if i % 2 == 0 else "odd"]