Python推导式

"""
列表推导式
[表达式 for item in 可迭代对象]
或 [表达式 for item in 可迭代对象 if 条件判断]
"""
a = [x*2 for x in range(5)]
print(a)  # [0, 2, 4, 6, 8]

a = []
for x in range(5):
    a.append(x*2)
print(a)

a = [x*2 for x in range(5) if x % 2 == 0]
print(a)  # [0, 4, 8]

a = []
for x in range(5):
    if x % 2 == 0:
        a.append(x*2)
print(a)

cells = [(row, col) for row in range(1, 3) for col in range(1, 3)]  # 两层循环
print(cells)  # [(1, 1), (1, 2), (2, 1), (2, 2)]

cells = []
for row in range(1, 3):
    for col in range(1, 3):
        cells.append((row, col))
print(cells)

"""
字典推导式格式：  哈希表（用数组实现的）
{key_expression: value_expression for 表达式 in 可迭代对象}
还可以添加if判断语句
"""

text = "I love you, Can you marry me ?"
dic = {c:text.count(c) for c in text}
print(dic)  # {'I': 1, ' ': 7, 'l': 1, 'o': 3, 'v': 1, 'e': 2, 'y': 3, 'u': 2, ',': 1, 'C': 1, 'a': 2, 'n': 1, 'm': 2, 'r': 2, '?': 1}

dic = {}
for c in text:
    dic[c] = text.count(c)
print(dic)

"""
集合推导式
{key_expression for 表达式 in 可迭代对象}  集合的本质是字典，没有值只有键的字典
"""
a = [2,3,2,3,5,67,5]
b = {x for x in a}
print(b)  # {67, 2, 3, 5}

"""
生成器推导式 （生成元组）  只能使用一次
(表达式 for 对象 in 可迭代对象)
"""
a = (x**2 for x in range(10) if x % 2 == 0)
print(list(a))  # [0, 4, 16, 36, 64]


a = (x**2 for x in range(10) if x % 2 == 0)
for i in a:
    print(i, end=" ")