基本語法：

{key_expr: value_expr for item in iterable if condition}

# 基本字典推導式
# 創建平方數字典
squares = {x: x**2 for x in range(5)}  # {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

# 使用條件過濾
even_squares = {x: x**2 for x in range(10) if x % 2 == 0}  # {0: 0, 2: 4, 4: 16, 6: 36, 8: 64}

# 從現有字典創建新字典
prices = {'apple': 5, 'banana': 3, 'orange': 4}
discount_prices = {k: v*0.8 for k, v in prices.items()}  # 所有商品打八折

基本語法：

{expr for item in iterable if condition}

# 基本集合推導式
# 創建平方數集合
square_set = {x**2 for x in range(5)}  # {0, 1, 4, 9, 16}

# 從字符串創建集合
vowels = {char for char in 'hello world' if char in 'aeiou'}  # {'e', 'o'}

# 過濾數字列表
numbers = [1, 2, 2, 3, 3, 4, 4, 5]
unique_evens = {x for x in numbers if x % 2 == 0}  # {2, 4}

數據轉換和清理

# 將列表轉換為字典（索引作為鍵）
words = ['apple', 'banana', 'orange']
word_dict = {i: word for i, word in enumerate(words)}

# 清理字典數據
data = {'a': 1, 'b': None, 'c': 3, 'd': None}
cleaned = {k: v for k, v in data.items() if v is not None}

# 字符串處理
text = "Hello World"
char_count = {char: text.count(char) for char in set(text)}

數據分析應用

# 學生成績分類
scores = {'小明': 85, '小華': 92, '小芳': 78, '小美': 95}
grades = {name: 'A' if score >= 90 else 'B' if score >= 80 else 'C' 
         for name, score in scores.items()}

# 商品分類
products = {'蘋果': 5, '香蕉': 3, '橘子': 4, '葡萄': 8}
price_levels = {name: '貴' if price > 5 else '普通' if price > 3 else '便宜'
               for name, price in products.items()}