Python基础（三） - 分支与循环

分支语法

当程序运行的结果可能有两种或多种时，我们就可以使用分支语法来选择输出哪种结果，只要满足相应的条件，程序就会进入某个分支继续下去。

Python中分支语法的格式为：

if 条件 :
相应的代码
elif 条件 :
相应的代码
else:
相应的代码

程序运行时，先从if语句开始，若满足条件，则进入运行相应的代码，并且不会再进入elif与else的语句；如果if条件不成立，则进入下一个elif语句的判断，满足条件则运行相应代码，并且不再进入其他分支；若if与elif语句条件都不满足，则直接进入else语句运行代码

循环结构

当一段程序需要运行多次或者不知道会运行多少次时，我们可以使用循环结构来满足，设定相应的次数或者设定满足条件，则可以让程序按照要求执行多次

循环结构语法：

for循环

# xxx：变量名，整型，会随着循环渐渐增大，第一次进入语句时xxx = a，若xxx命名为‘_‘则表示在循环中不需要使用此变量
# a：a表示起始数值，默认为0
# b：b表示最大数值，当xxx大于b时，循环结束
# c：c表示每一次循环后xxx的增量，默认为1
for xxx in range(a, b, c):
相应的代码

while循环

'''
当满足while后面的条件时，while里的代码会一直执行，直到条件不满足是循环结束
使用while语句需要注意不要变成死循环
'''
while 条件 :
相应的代码

分支与循环语法应用

1. 模拟登入，输入账号密码

import getpass

acount = input('请输入用户名：')
# 使用getpass库使密码输入不再是明文
password = getpass.getpass('请输入密码：')

# 分支需要注意缩进，如果不缩进则不受分支语句控制
if acount == 'admin'  and  password == '123456':
print('成功登入')
else:
print('用户名或密码错误')

2. 求方程式解

'''
5x+3 (x < -1)
f(x) =  2x-1 (-1 <= x <= 1)
3x-5 (x > 1)

'''
x = float(input('请输入x的值：'))

if x < -1:
y = 5*x+3
print('y:%.2f' % y)
elif x>=-1 and x<=1:
y = 2*x-1
print('y:%.2f' % y)
else:
y = 3*x-5
print('y:%.2f' % y)

3. 输入成绩，评价等级

'''
90分及以上：A
80-89:B
70-79:C
60-69:D
60分以下：E
'''
grade = float(input('请输入成绩：'))

if grade >= 90:
print('等级为A')
elif grade >= 80:
print('等级为B')
elif grade >= 70:
print('等级为C')
elif grade >= 60:
print('等级为D')
else:
print('不及格')

# 官方建议使用扁平化代码优于嵌套型
'''
if grade >= 90:
print('等级为A')
else:
if grade >= 80:
print('等级为B')
else:
if grade >= 70:
print('等级为C')
else:
if grade >= 60:
print('等级为D')
else:
print('不及格')

'''

4. 生成随机数

import random # 随机数模块
# from random import random
# from math import pi, sqrt
# 使用from语法可直接从模块导入需要使用的方法而不用再在方法前加模块名 如 print(random())

# random生成随机小数[0,1)
print(random.random())
# randint生成随机整数[a,b],a和b自行给，a<=b
print(random.randint(1,6))

5. 投骰子

import random

random_num = random.randint(1,6)

if random_num == 1:
print('骰出了1，你是非洲酋长')
elif random_num == 2:
print('骰出了2，你比非洲酋长好一点')
elif random_num == 3:
print('骰出了3，你的运气一般般')
elif random_num == 4:
print('骰出了4，你的运气还不错')
elif random_num == 5:
print('骰出了5，你是欧洲人呢')
else:
print('哇，骰出了6，欧皇诞生！')

6. 统计点数出现次数

from random import randint

f1 = 0
f2 = 0
f3 = 0
f4 = 0
f5 = 0
f6 = 0

# x为自定义变量名，在每次循环后数字+1
for x in range(6000):
random_num = randint(1,6)
if random_num == 1:
f1 += 1
elif random_num == 2:
f2 += 1
elif random_num == 3:
f3 += 1
elif random_num == 4:
f4 += 1
elif random_num == 5:
f5 += 1
else:
f6 += 1

print('1点次数：%d' % f1)
print('2点次数：%d' % f2)
print('3点次数：%d' % f3)
print('4点次数：%d' % f4)
print('5点次数：%d' % f5)
print('6点次数：%d' % f6)

7. 华氏度与摄氏度转换

# 华氏度 = 32 + 摄氏度*1.8
# 摄氏度 = （华氏度 - 32）/1.8

choose = int(input('华氏度转摄氏度请输入0，摄氏度转华氏度请输入1：'))
temper = float(input('请输入温度:'))

if choose == 0:
# print('华氏度')
temper2 = (temper -32)/1.8
print('%.2f华氏度 = %.2f摄氏度' %  (temper,temper2))
elif  choose == 1:
# print('摄氏度')
temper2 = 32 + temper*1.8
print('%.2f摄氏度 = %.2f华氏度' % (temper,temper2))
else:
print('输入错误')

8. 输入三边长，求三角形周长和面积

import math

a = float(input('请输入第一条边长：'))
b = float(input('请输入第二条边长：'))
c = float(input('请输入第三条边长：'))

if (a+b>c and a+c>b and b+c>a):
dd = (a+b+c)*0.5
ss = math.sqrt(dd*(dd-a)*(dd-b)*(dd-c))
print('三角形周长：%.2f  三角形面积：%.2f' % (2*dd, ss))
else:
print('输入边长不可组成三角形')

9. 求1到100的和

### 1到100求和

x = 0
for num in range (101):# range括号内若只写一个数值，则默认从零开始计数，每次数值+1
x += num
print(x)

10. 求1到100偶数的和

### 1到100的偶数的和

x = 0
for num in range (101):# range(x,y,z)x是起始值，y是终止值，z是步长
if num % 2 == 0:
x += num
print(x)

11. 求1到100 中3或5的倍数的和

### 1到100 中3或5的倍数的和

x = 0
for num in range (101):
if num % 3 ==0 or num % 5 == 0:
x += num
print(x)

----------

x = 0
num = 0
while num < 101:
if num % 3 == 0 or num % 5 == 0:
x += num
num += 1
print(x)

12. 猜1到100之间的一个数

# 人猜数字
import random

real_num = random.randint(1,100)
the_num = 0
count_num = 0
while the_num != real_num:
the_num = int(input('请输入你猜的数字：'))
if the_num > real_num:
print('很遗憾，猜的数字大了')
elif the_num < real_num:
print('很遗憾，猜的数字小了')
count_num += 1
if count_num > 7:
print('已经猜了%d次了哦，智商需要提高了～' % count_num)
print('恭喜猜对数字：%d，一共猜了%d次' % (real_num, count_num))

----------
# 机器猜数字
answer = 0
min_num = int(input('最小数是？'))
max_num = int(input('最大数是？'))
game_over = False
while not game_over:
answer = (min_num + max_num) // 2
my_answer = input('我猜是%d,对了吗？' % answer)
if my_answer == '大一点':
min_num = answer
elif my_answer == '小一点':
max_num = answer
elif my_answer == '猜对了':
game_over = True
else:
print('不太明白你的意思')

13. 人机猜拳

import random

print('人机猜拳规则：0为石头，1为剪刀，2为布')
person_num = 1000
comput_num = 1000
game_over = False
while not game_over:
is_plus = 0
is_over = False
while not is_over:
is_plus = int(input('人类：%d分，电脑：%d分，请下注：' % (person_num, comput_num)))
if is_plus > comput_num  or is_plus > person_num:
print('电脑或你的点数不够，下注失败')
else:
is_over = True
person_ans = int(input('请选择：'))
comput_ans = random.randint(0,2)
if person_ans == comput_ans:
print('平手')
elif person_ans == 0 and comput_ans == 1:
print('人出石头，电脑出剪刀，人胜')
person_num += is_plus
comput_num -= is_plus
elif person_ans == 0 and comput_ans == 2:
print('人出石头，电脑出布，电脑胜')
person_num -= is_plus
comput_num += is_plus
elif person_ans == 1 and comput_ans == 0:
print('人出剪刀，电脑出石头，电脑胜')
person_num -= is_plus
comput_num += is_plus
elif person_ans == 1 and comput_ans == 2:
print('人出剪刀，电脑出布，人胜')
person_num += is_plus
comput_num -= is_plus
elif person_ans == 2 and comput_ans == 0:
print('人出布，电脑出石头，人胜')
person_num += is_plus
comput_num -= is_plus
elif person_ans == 2 and comput_ans == 1:
print('人出布，电脑出剪刀，电脑胜')
person_num -= is_plus
comput_num += is_plus
else:
print('输入有误')

if person_num == 0:
print('电脑获胜！')
game_over = True
elif comput_num == 0:
print('人类获胜！')
game_over = True

14. 计算个人所得税

# 不要在if else里面直接print 重复率太高且不好修改

money = float (input('请输入本月收入：'))
wu_xian_yi_jin = float(input('请输入五险一金所扣费用：'))
diff = money - wu_xian_yi_jin - 3500
tax = 0
deduction = 0
if diff > 80000:
tax = 0.45
deduction = 13505
elif diff > 55000:
tax = 0.35
deduction = 5505
elif diff > 35000:
tax = 0.3
deduction = 2755
elif diff > 9000:
tax = 0.25
deduction = 1005
elif diff > 4500:
tax = 0.2
deduction = 555
elif diff > 1500:
tax = 0.1
deduction = 0
else:
tax = 0
deduction = 0

personal = abs(diff * tax - deduction) # abs() 求绝对值
print('个人所得税为：%d元' % personal)

15. 买鸡问题

# 100元买鸡，公鸡5元，母鸡3元，小鸡一元三只，刚好100只鸡，问分别多少只
'''
for gong_ji in range(0, 21):
for mu_ji in range(0, (100-(5*gong_ji))//3):
for xiao_ji in range(0, 100,3):
if gong_ji+mu_ji+xiao_ji == 100 and 5*gong_ji+3*mu_ji+xiao_ji//3 == 100:
print(gong_ji,mu_ji,xiao_ji)

'''
# 提升效率
for gong_ji in range(0, 21):
for mu_ji in range(0, (100-(5*gong_ji))//3):
xiao_ji = 100 - gong_ji - mu_ji
if  xiao_ji % 3 == 0 and 5*gong_ji+3*mu_ji+xiao_ji//3 == 100:# 此处使用//原因请看 #警告
print(gong_ji,mu_ji,xiao_ji)

16. 捕鱼问题

# 作业3 有5人捕鱼，鱼很多，第二早，a把鱼分五份，多一条扔掉，拿一份，b分五份，多一条扔了，拿一份，cde同样操作，问至少多少条鱼
fish = 1
while True:
total = fish
is_over = True
for _ in range (5):
if (total - 1) % 5 == 0:
total = (total - 1) // 5 * 4
else:
is_over = False
break
if is_over:
print(fish)
break
fash += 1

17. 完美数问题

# 完美数：一个数等于除了自身以外所有因子的和
# 输出10000以内所有完美数
import time
import math

start = time.time()

for num in range(2,10001):
total_factor = 0
for factor in range(2, int(math.sqrt(num)) + 1):
if num % factor == 0:
total_factor += (factor + (num / factor == factor and 0 or num / factor))
if total_factor + 1 == num:
print(num)

end = time.time()

print(end - start)