Cornerstone Qinnengbuzhuo labyrinth trip - the fourteenth day (Python ternary operator list ancestral derivation dictionaries, recursion, anonymous functions and built-in functions)

A ternary operator (ternary operator)

    That is if ... else ... syntactic sugar, using the premise else and if only one statement.

grammar

result = 结果一条件成立时执行 if 条件 else 结果二条件不成立时执行

Results ternary operator does not necessarily have a direct relationship with the conditions

a = 20
b = 30
res = a if a > b else b  # 求大值
print(res)

res = b if a > b else a  # 求小值
print(res)

Second, derivations

    List (tuples) syntactic sugar conversion dictionary
listing derivation formula

dic = {'a': 1, 'b': 2, 'c': 3}  # => [('a', 1), ('b', 2), ('c', 3)]
res = [(k, v) for k, v in dic.items()]
print(res)

Derivation of formula tuple

res = ((k, v) for k, v in dic.items())
print(tuple(res))

Dictionary derivations

ls = [('a', 1), ('b', 2), ('c', 3)]  # => {'a': 1, 'b': 2, 'c': 3}
res = {k: v for k, v in ls}
print(res)

Case

可以被推导为列表
range(10) 
res_ls = [arg for arg in range(10)]
print(res_ls)  # [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

推导字典只保留最后一个数据
res_dic = {'a': arg for arg in range(10)}
print(res_dic)  # {'a': 9}

迭代出可解压为的单列容器可以推导出字典
res_dic = {v: k for k, v in enumerate('abc')}
print(res_dic)  # {'a': 0, 'b': 1, 'c': 2}

Second, recursive

Backtracking and recursion

• Back: answer the inquiry process
• Recursive: the launch of the process of the answer

Recursive premise

• Back to a specific value of the result, start Recursive
• Conditions should be the law of backtracking and recursion

Two ways

Recursive nature is self-calling function
(1). Own their own tune

def a():
    global count
    count += 1
    if count > 50:
        return
    a()
a()

(2) function calls between each other form a circulation

def b():
    c()
def c():
    d()
def d():
    b()
b()

Case

阶乘 

def factorial(num):
    if num == 1:
        return 1
    temp = num * factorial(num - 1)
    return temp
res = factorial(5)
print(res)

Third, the anonymous function

    As the name suggests, there is no function name, no function body, only a return value

grammar

lambda parameter list omitted (): Returns the value of the expression a return keyword also omitted

Scenarios

1. anonymous function address can be a variable to accept, as a function of the variable names can be used, but it is contrary to the original intention of anonymity
2. The combination of built-in functions to use: built-in functions require a certain parameter function address
    - can be assigned a well-known function name, you can also directly assign an anonymous function

薪资最高
iterable = {
    'Bob': 12000,
    'Tom': 37000,
    'Jerry': 76000,
    'Zero': 120,
}
res = max(iterable, key=lambda x: iterable[x])  # x: 字典的k  返回值：做比较的值
print(res)


iterable = {
    'Bob': {'no': 100, 'salary': 12000},
    'Tom': {'no': 200, 'salary': 37000},
    'Jerry': {'no': 50, 'salary': 76000},
    'Zero': {'no': 150, 'salary': 120},
}
res = max(iterable, key=lambda k: iterable[k]['no'])
print(res)  # Tom   编号最高

res = max(iterable, key=lambda k: iterable[k]['salary'])
print(res)  # Jerry  薪资最高