[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
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
