Python列的推导式,遍历的技巧

#列的推导式, 如果需要推导出列表用中括号, 如果想推导出元组, 用小括号
#基础版
vec = [2, 4, 6]
var_list1 = [3*x for x in vec]
print(var_list1) #[6, 12, 18]

#不一样的数据类型
var_list2 = [[x, x**2] for x in vec]
print(var_list2) #[[2, 4], [4, 16], [6, 36]]

var_dic = {x: x**2 for x in (2, 4, 6)}
print(var_dic) #{2: 4, 4: 16, 6: 36}

#数据改变
freshfruit = ['  banana', '  loganberry ', 'passion fruit  ']
var_list3 = [weapon.strip() for weapon in freshfruit]
print(var_list3) #['banana', 'loganberry', 'passion fruit']

#条件判断
var_list4 = [3*x for x in vec if x > 3]
print(var_list4) #[12, 18]

#多个遍历
vec1 = [2, 4, 6]
vec2 = [4, 3, -9]
var_list5 = [x*y for x in vec1 for y in vec2] #笛卡尔积
print(var_list5) #[8, 6, -18, 16, 12, -36, 24, 18, -54]

var_list6 = [vec1[i]*vec2[i] for i in range(len(vec1))]
print(var_list6) #[8, 12, -54]

#嵌套列表的一些技巧
matrix = [
    [1, 2, 3, 4],
    [5, 6, 7, 8],
    [9, 10, 11, 12],
]
#   矩阵的转置
var_list7 = [[row[i] for row in matrix] for i in range(4)]
print(var_list7) #[[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]

#字典遍历的技巧
knights = {'gallahad': 'the pure', 'robin': 'the brave'}
for k, v in knights.items():
    print(k, v)

#带有下标的遍历序列
for i, v in enumerate(['tic', 'tac', 'toe']):
    print(i, v)

#同时遍历多个序列
questions = ['name', 'quest', 'favorite color']
answers = ['lancelot', 'the holy grail', 'blue']
for q, a in zip(questions, answers):
    print('What is your {0}?  It is {1}.'.format(q, a))

#反向遍历序列
for i in reversed(range(1, 20, 2)):
    print(i)

#按指定顺序遍历序列, 且不修改原始值
basket = ['apple', 'orange', 'apple', 'pear', 'orange', 'banana']
for f in sorted(set(basket)):
    print(f)