【20171001】python_语言设计（2）循环与分支

1.简单分支
e.g.PM2.5指数分级程序结构分级结构

#pm25.pydef main()    PM=eval(input("what is today's PM2.5?"))    if PM>75:        print("unhealthy!be careful!")    if PM<35:        print("good!go running!")main()

条件满足执行，条件不满足则跳过，无论如何都跳到下一句。
关系操作符：<,<=,==,>=,>,!=（数字，字符串）

#计算二次方程的实数根程序，如果没有实数根则报错import mathdef main():    print("this program finds the real solutions to a quadratic\n")    a,b,c=eval(input("please enter the coefficients(a,b,c):"))    discRoot=math.sqrt(b*b-4*a*c)    root1=(-b+discRoot)/(2*a)    root2=(-b-discRoot)/(2*a)    print("\nthe solutions are: ",root1,root2)main()

改进后程序

#计算二次方程的实数根程序import mathdef main():    print("this program finds the real solutions to a quadratic\n")    a,b,c=eval(input("please enter the coefficients(a,b,c):"))    delta=b*b-4*a*c    if delta>=0:        discRoot=math.sqrt(b*b-4*a*c)           root1=(-b+discRoot)/(2*a)        root2=(-b-discRoot)/(2*a)        print("\nthe solutions are: ",root1,root2)main()

2.二分支结构
if else结构

#计算二次方程的实数根程序import mathdef main():    print("this program finds the real solutions to a quadratic\n")    a,b,c=eval(input("please enter the coefficients(a,b,c):"))    delta=b*b-4*a*c    if delta<0:        print("the equation has no real roots!")    else:        discRoot=math.sqrt(b*b-4*a*c)           root1=(-b+discRoot)/(2*a)        root2=(-b-discRoot)/(2*a)        print("\nthe solutions are: ",root1,root2)main()

3.多分支结构
解决复杂问题，三分支结构可以由两个二分支机构。
if-elif-else结构

#计算二次方程的实数根程序import mathdef main():    print("this program finds the real solutions to a quadratic\n")    a,b,c=eval(input("please enter the coefficients(a,b,c):"))    delta=b*b-4*a*c    if delta<0:        print("\nthe equation has no real roots!")    elif delta==0:        discRoot=math.sqrt(b*b-4*a*c)           root1=(-b+discRoot)/(2*a)        print("\nsingleroot is：",root1)    else:        discRoot=math.sqrt(b*b-4*a*c)           root1=(-b+discRoot)/(2*a)        root2=(-b-discRoot)/(2*a)        print("\nthe solutions are: ",root1,root2)main()

改进：考虑a=0时退化为一次问题求解

#计算二次方程的实数根程序import mathdef main():    print("this program finds the real solutions to a quadratic\n")    a,b,c=eval(input("please enter the coefficients(a,b,c):"))    delta=b*b-4*a*c    if a==0:        x=-b/c        print("\nsolution is:",x)    elif delta<0:        print("\nthe equation has no real roots!")    elif delta==0:        discRoot=math.sqrt(b*b-4*a*c)           root1=(-b+discRoot)/(2*a)        print("\nsingleroot is：",root1)    else:        discRoot=math.sqrt(b*b-4*a*c)           root1=(-b+discRoot)/(2*a)        root2=(-b-discRoot)/(2*a)        print("\nthe solutions are: ",root1,root2)main()

#空气质量提醒程序#newpm25.pydef main():    PM=eval(input("what is today's pm2.5?"))    if PM<35:        print("good!")    elif PM<75:        print("moderate!")    elif PM<115:        print("unhealthy!")    elif PM<150:        print("limit!")    elif PM<250:        print("very unhealthy!")    else:        print("hazadous!")main()

4.异常处理
异常处理机制（程序），try…except…清理工作场合
else–必须在finally之前,finally–必须最后，关键字

def main():    try:        number1,number2=eval(input("enter two numbers,seperated by a comma:"))        result=number1/number2    except ZeroDivisionError:        print("division by zero!")    except SyntaxError:        print("A comma may be missing in the input")    except:        print("something wrong in the input")    else:        print("No exceptions,the result is",result)    finally:        print("executing the final clause")main()

#计算二次方程的实数根程序，如果没有实数根则报错import mathdef main():    print("this program finds the real solutions to a quadratic\n")    try:        a,b,c=eval(input("please enter the coefficients(a,b,c):"))        discRoot=math.sqrt(b*b-4*a*c)        root1=(-b+discRoot)/(2*a)        root2=(-b+discRoot)/(2*a)        print("\nthe solutions are: ",root1,root2)    except ValueError:        print("\nNo real roots")main()

捕捉任何类型的错误，NameError,SyntaxError…
改进二次求根错误类型

#计算二次方程的实数根程序，如果没有实数根则报错import mathdef main():    print("this program finds the real solutions to a quadratic\n")    try:        a,b,c=eval(input("please enter the coefficients(a,b,c):"))        discRoot=math.sqrt(b*b-4*a*c)        root1=(-b+discRoot)/(2*a)        root2=(-b+discRoot)/(2*a)        print("\nthe solutions are: ",root1,root2)    except ValueError as excObj:        if str(excObj)=="math domain error":            print("No Real Roots.")        else:            print("You didn't give me the right number of coefficients.")    except NameError:        print("\nYou did't enter three numbers.")       except TypeError:        print("\nYour inputs were not all numbers.")    except SyntaxError:        print("\nYour input was not in the correct form.Missing comma?")    except:        print("\nSomething went wrong,sorry!")    #print("\nNo real roots")main()

5.三者最大实例

#三者最大值#1.通盘比较，分支结构def main():    x1,x2,x3=eval(input("请输入三个值："))    if x1>=x2 and x1>=x3:        max=x1    elif x2>=x1 and x2>=x3:        max=x2    else:        max=x3    print("max为：",max)main()

#三者最大值#2.决策树方法避免冗余比较def main():    x1,x2,x3=eval(input("请输入三个值："))    if x1>=x2:        if x1>=x3:            max=x1        else max=x3    else:        if x2>=x3:            max=x2        else:            max=x3    print("max为：",max)main()

#三者最大值#3.顺序处理，逐步扫描每个值，保留最大者def main():    x1,x2,x3=eval(input("请输入三个值："))    max=x1    if x2>max:        max=x2    if x3>max:        max=x3    print("max为：",max)main()

#maxn.py#寻找一组数中的最大值def main():    n=eval(input("总共几个数？："))    max=eval(input("输入一个数："))    for i in range(n-1):        x=eval(input("输入一个数："))        if x>max:            max=x        print("最大数为：",max)    #print("max为：",max)main()

#maxn.py#终极解决方案def main():    x1,x2,x3=eval(input())    print("最大数为：",max(x1,x2,x3))    #print("max为：",max)main()

6.基本循环结构
for循环语句，循环遍历整个序列，for i in …：

#average1.pydef main():    n=eval(input("sumnumber:"))    sum=0.0    for i in range(n):        x=eval(input("input:"))        sum=sum+x    print("\naverage is ",sum/n)main()

for循环缺点：必须提供循环次数
无限循环：while语句可以进行前测循环
continue，break语句
7.通用循环结构
交互式循环

#average2.pydef main():    sum=0.0    count=0    moredata="yes"    while moredata[0]=="y":        x=eval(input("输入一个数："))        sum=sum+x        count=count+1        moredata=input("want another number?(y/n)")    print("\nthe average is ",sum/count)main()

#average3.py 哨兵循环def main():    sum=0.0    count=0    #moredata="yes"    #while moredata[0]=="y":    x=eval(input("输入一个数："))    while x>=0:        sum=sum+x        count=count+1        #moredata=input("want another number?(y/n)")        x=eval(input("输入一个数："))    print("\nthe average is ",sum/count)main()

#average4.py 哨兵循环def main():    sum=0.0    count=0    xStr=input("输入一个数:")    while xStr!=" ":        x=eval(xStr)        sum=sum+x        count=count+1        xStr=input("输入一个数：")    print("\nthe average is ",sum/count)main()

#文件循环def main():    fileName=input("what file are the numbers in?")    infile=openfile(fileName,'r')    sum=0.0    count=0    for line in infile:        sum=sum+eval(line)        count=count+1    print("\nthe average is ",sum/count)main()

#文件循环def main():    fileName=input("what file are the numbers in?")    infile=openfile(fileName,'r')    sum=0.0    count=0    line=infile.readline()    while line!=" ":    #for line in infile:        sum=sum+eval(line)        count=count+1        line=infile.readline()    print("\nthe average is ",sum/count)main()

嵌套循环，数字以逗号分隔出现在文件的同一行上。

#嵌套文件循环def main():    fileName=input("what file are the numbers in?")    infile=openfile(fileName,'r')    sum=0.0    count=0    line=infile.readline()    while line!=" ":        #为line中的值更新其count和sum        for xStr in line.split(","):            sum=sum+eval(line)            count=count+1        line=infile.readline()    print("\nthe average is ",sum/count)main()

8.死循环和循环嵌套
死循环、死语句
while true，while 1 完成特定功能
后测循环，直到型循环，通过while语句间接实现，或break语句实现。
半路循环，出口在中部
9.布尔表达式
true/false
运算符：and，or，not
优先级：not>and>or