[2021 Spring] CS61A 学习笔记 lecture 9 Function Examples

lecture 9 Function Examples
课本： http://composingprograms.com/pages/16-higher-order-functions.html

目录

• Exercise: Reversing Digits
• Exercise: Interleaving Digits
• Environment Detective
• Exercise: Tracing
• decorators 装饰器

Exercise: Reversing Digits

反转数字(不使用字符串、数组）

def reverse_digits(n):
    """Assuming N >= 0 is an integer.  Return the number whose
    base-10 representation is the reverse of that of N.
    >>> reverse_digits(0)
    0
    >>> reverse_digits(1)
    1
    >>> reverse_digits(123)
    321
    >>> reverse_digits(10)
    1
    >>> reverse_digits(12321)
    12321
    >>> reverse_digits(2222222)
    2222222
    >>> reverse_digits(102)
    201
    """
    assert type(n) is int and n >= 0
    if n < 10:
        return n
    return reverse_digits(n // 10) + (n % 10) * 10 ** (num_digits(n) - 1)

def num_digits(x):
    """Return the number of decimal digits in the positive integer X."""
    x_count = 1
    while x >= 10:
        x_count += 1
        x //= 10
    return x_count

Exercise: Interleaving Digits

交错数字

def interleave(a, b):
    """Assuming A and B are non-negative integers with the same 
    number of base-10 digits, return the number whose base-10 
    representation is the interleaving of A's and B's digits,
    starting with A.
    >>> interleave(1, 2)
    12
    >>> interleave(0, 1)
    1
    >>> interleave(1, 0)
    10
    >>> interleave(123,456)
    142536
    >>> interleave(111111, 222222)
    121212121212
    """
    if a <= 9:
        return a * 10 + b
    return interleave(a // 10, b // 10) * 100 + interleave(a % 10, b %10)

Environment Detective

从环境模型推导原始代码

Exercise: Tracing

（课件上有，要下课了，大概率推到lecture 10）
lecture10更新：

def trace1(f):
    """Return a function that takes a single argument, x, prints it,
    computes and prints F(x), and returns the computed value."""
    def traced(x):
        print("->", x)
        r = f(x)
        print("<-", r)
        return r
    return traced

decorators 装饰器

装饰器decorator下函数相当于作为参数传入装饰器后返回的函数。

@trace1
def square(x):
    return x * x

@trace1
def traced_reverse(n):
    assert type(n) is int and n >= 0
    if n < 10:
        return n
    return traced_reverse(n // 10) + (n % 10) * 10 ** (num_digits(n) - 1)

菜鸟教程的decorator：https://www.runoob.com/w3cnote/python-func-decorators.html