[Python知识库]python之选择、循环结构

前置疑问

Q1 双分支与三元运算符之间的转换？
Q2 python循环代码如果简洁优化？

学习内容

1、三种选择结构
2、循环结构
3、break、continue、else关键字
4、推导式生成序列

学习产出

1、选择结构

1.1 单分支结构

>>> if 3:
	print("ok")
ok

>>> if a = 3:
SyntaxError: invalid syntax

python中条件语句不能用赋值符"="

1.2 双分支结构

>>> a = 56
>>> 
>>> if a > 60:
	print("good")
else:
	print("bad")

bad

1.3 多分支结构

>>> grade = 80
>>> if grade >= 90:
	print("A")
elif grade >= 80 and grade < 90:
	print("B")
elif grade >= 70 and grade < 80:
	print("C")
elif grade >= 60 and grade < 70:
	print("D")
else:
	print("E")

B

#另外一种写法
score = int(input("请输入一个0~100区间的数字："))

degree =  "ABCDE"

num = 0

if score > 100 or score < 0:
    print("输入错误!")
else:
    num = score // 10
    if num < 6:num = 5
    print("分数为{0}， 对应等级为{1}".format(score, degree[9 - num]))

请输入一个0~100区间的数字：46
分数为46， 对应等级为E

A1

1.4 三元运算符

条件为真执行A if 条件表达式 else 条件为假执行B

s = input("please input a number: ")

if int(s) < 10:
    print("good")
else:
    print("false")

# 三元运算符的测试
print("good" if int(s) < 10 else "false")

2、循环

循环流程图

• while循环

>>> num = 0
>>> sum = 0
>>> while num <= 100:
	sum = sum + num
	num +=1

>>> print("所有的和为{0}".format(sum))
所有的和为5050

• for循环 用处很多
  for 变量 in 迭代对象
  对象：列表、元组、字典、集合、迭代器iterator 生成器函数 generator、 文件对象

>>> a1 = [6,5,4,3,2,1]
>>> for i in a1:
	print(i)

6
5
4
3
2
1
>>> 
>>> a2 = ("t","u","p","l","e")
>>> for i in a2:
	print(i)

t
u
p
l
e
>>> 
>>> a3 = {6,5,4,3,2,1} #观察集合遍历和元组遍历的区别
>>> for i in a3:
	print(i)

1
2
3
4
5
6
>>> for i in a4:
	print(i)
p
e
u
l
t

>>> a5 = {'name':'xiaoming','age':20,'job':'programmer'}
>>> for i in a5:
	print(i)

name
age
job
>>> for i in a5.values():
	print(i)

xiaoming
20
programmer
>>> for i in a5.items():
	print(i)

('name', 'xiaoming')
('age', 20)
('job', 'programmer')


>>> num = 0
>>> sum = 0
>>> for i in range(101):
	sum = sum + num
	num += 1

>>> print("所有100和为:{0} ".format(sum))
所有100和为:5050

• 嵌套循环

#打印九九乘法表
for i in range(1,10):
    for j in range(1,i+1):
        print("{0}*{1}={2}".format(i,j,(i*j)), end="\t")
    print()

>>> r1 = dict(name="xiaoming",age=20, salary=30000, city="A")
>>> r2 = dict(name="xiaohong",age=20, salary=20000, city="B")
>>> r3 = dict(name="xiaogang",age=20, salary=15000, city="C")
>>> 
>>> L1 = [r1,r2,r3]
>>> 
>>> for i in L1:
	if i.get("salary") > 15000:
		print(i)

{'name': 'xiaoming', 'age': 20, 'salary': 30000, 'city': 'A'}
{'name': 'xiaohong', 'age': 20, 'salary': 20000, 'city': 'B'}

break、continue语句
break用来结束整个循环
continue 结束当前这一次循环

while True:
    a = input("请输入一个字符(Q或者q退出):")
    if a == "Q" or a == "q":
        print("!!!exit")
        break;
    else:
        print(a)

#打印10以内的奇数
for i in range(11):
    if i % 2 == 0:
        continue
    else:
        print(i)

• else语句
  在python中else 还能与for 或者while一起搭配使用，循环正常结束执行else语句，若循环被break中断则不执行else语句,下面是输入员工薪水的代码：

employee = 0
salaryNum = 0
salary = []

for i in range(4):
   s = input("please input salary(Q or q is exit):")
   if s == "Q" or s == "q":
       print("ok-ok-ok")
       break
   if float(s) < 0:
       continue
   employee += 1
   salary.append(float(s))
   salaryNum = salaryNum + float(s)
else:
   print("已完成4名员工的薪资输入！！！")


print("员工人数{0}".format(employee))
print("录入薪资:",salary)
print("平均薪资{0}".format(salaryNum / employee))

A2

• 循环代码优化三个基本准则
  减少循环内部的不必要运算
  嵌套循环时，处于内部循环的运算，能外面写，就写在外面
  局部变量使用较快
  相关拼接字符串或者插入列表和删除，都要考虑效率。比如字符拼接join() 效率比 + 好

import time

start = time.time()

for i in range(1000):
    result = []
    for j in range(100000):
        result.append(i*1000 + j * 100)

end = time.time()

print("耗时：{0}".format((end - start)))


#优化
start = time.time()

for i in range(1000):
    result = []
    m = i * 1000
    for j in range(100000):
        result.append(m + j * 100)

end = time.time()
print("耗时：{0}".format((end - start)))

• zip()进行迭代
  zip()可以一次性对多个序列同时处理，迭代长度取最小的序列长度

>>> names = ("xiaoming","xiaohong","xiaogang","xiaoyang")
>>> ages = (18,19,20,21)
>>> jobs = ("teacher","programmer","student")
>>> for name, age, job in zip(names,ages,jobs):
	print("{0}-{1}-{2}".format(name,age,job))

xiaoming-18-teacher
xiaohong-19-programmer
xiaogang-20-student

• 各种序列推导式
  列表推导式 [表达式 for item in 可迭代对象]

>>> [i for i in range(1,5)]
[1, 2, 3, 4]
>>> [i for i in range(2,10) if(i % 2 != 0)]
[3, 5, 7, 9]

• 字典推导式
  {key:value for item in 可迭代对象}

>>> my_text = 'i love python, i love java, i love machine learning'
>>> char_count = {c:my_text.count(c) for c in my_text}
>>> char_count
{'i': 5, ' ': 9, 'l': 4, 'o': 4, 'v': 4, 'e': 5, 'p': 1, 'y': 1, 't': 1, 'h': 2, 'n': 4, ',': 2, 'j': 1, 'a': 4, 'm': 1, 'c': 1, 'r': 1, 'g': 1}

• 集合推导式

>>> {i for i in range(100) if i % 2 == 0 and i % 3 == 0}
{0, 96, 66, 36, 6, 72, 42, 12, 78, 48, 18, 84, 54, 24, 90, 60, 30}

• 生成推导式(元组无推导式)
  生成的对象是一次性的，用完之后就没有了

>>> (i for i in range(100) if i % 2 == 0 and i % 3 == 0)
<generator object <genexpr> at 0x0000017115EE3C10>

>>> generator1 = (i for i in range(100) if i % 2 == 0 and i % 3 == 0)
>>> for i in generator1:
	print(i,end=" ")

0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 96

>>> for i in generator1:
	print(i,end=" ")

>>>
> 

今日代码

import turtle


my_colors = ("red","green","yellow","black")

turtle.width(5)
turtle.speed(1)

for i in range(10):
    turtle.penup()
    turtle.goto(0, -i*10)
    turtle.pendown()
    turtle.color(my_colors[i%len(my_colors)])
    turtle.circle(i*10)

turtle.done()

import turtle

t = turtle.Pen()

t.width = 5
width = 30
num = 18
t.speed(0)

x1 = [(-300,300),(-300+width*num,300)]
y1 = [(-300,300),(-300,300-width*num)]


#划横线时 |x.end - x.start|长度不变， y坐标变
for i in range(19):
    t.penup()
    t.goto(-300,300 - 30 * i)
    t.pendown()
    t.goto(240,300 - 30 * i)

#划竖线时，x坐标变 |y.end - y.start长度不变
for i in range(19):
    t.penup()
    t.goto(-300 + 30 * i,300)
    t.pendown()
    t.goto(-300 + 30 * i,-240)

t.hideturtle()
turtle.done()