Python中表达式和语句及for、while循环实例代码

1)表达式

       常用的表达式操作符：
        x + y, x - y
        x * y, x / y, x // y, x % y

        逻辑运算：
        x or y, x and y, not x

        成员关系运算：
        x in y, x not in y

        对象实例测试：
        x is y, x not is y

        比较运算：
        x < y, x > y, x <= y, x >= y, x == y, x != y

        位运算：
        x | y, x & y, x ^ y, x << y, x >> y

        一元运算：
        -x, +x, ~x:

        幂运算：
        x ** y

        索引和分片：
        x[i], x[i:j], x[i:j:stride]

        调用：
        x(...)

        取属性：
            x.attribute

        元组：(...)
        序列：[...]
        字典：{...}

        三元选择表达式：x if y else z

        匿名函数：lambda args: expression

        生成器函数发送协议：yield x
        
  运算优先级：
        (...), [...], {...}
        s[i], s[i:j]
        s.attribute
        s(...)
        +x, -x, ~x
        x ** y
        *, /, //, %
        +, -
        <<, >> 
        &
        ^
        |
        <, <=, >, >=, ==, !=
        is, not is
        in, not in
        not
        and
        or
        lambda

2)语句：

    赋值语句
    调用
    print： 打印对象
    if/elif/else: 条件判断
    for/else: 序列迭代
    while/else: 普通循环
    pass: 占位符
    break: 
    continue
    def
    return
    yield
    global: 命名空间
    raise: 触发异常
    import: 
    from: 模块属性访问
    class: 类
    try/except/finally: 捕捉异常
    del: 删除引用
    assert: 调试检查
    with/as: 环境管理器
    
        赋值语句：
        
            隐式赋值：import, from, def, class, for, 函数参数
        
            元组和列表分解赋值：当赋值符号(=)的左侧为元组或列表时，Python会按照位置把右边的对象和左边的目标自左而右逐一进行配对儿；个数不同时会触发异常，此时可以切片的方式进行；
        
            多重目标赋值
        
            增强赋值: +=, -=, *=, /=, //=, %=,

3)for循环练习

    练习1：逐一分开显示指定字典d1中的所有元素，类似如下
        k1 v1
        k2 v2
        ...
            
            >>> d1 = { 'x':1,'y':2,'z':3,'m':4 }
            >>> for (k,v) in d1.items():
                    print k,v  
            y 2
            x 1
            z 3
            m 4
            
    练习2：逐一显示列表中l1=["Sun","Mon","Tue","Wed","Thu","Fri","Sat"]中的索引为奇数的元素；
    
            >>> l1 = ["Sun","Mon","Tue","Wed","Thu","Fri","Sat"]
            >>> for i in range(1,len(l1),2):
                    print l1[i]
            
            Mon
            Wed
            Fri
            
    练习3：将属于列表l1=["Sun","Mon","Tue","Wed","Thu","Fri","Sat"]，但不属于列表l2=["Sun","Mon","Tue","Thu","Sat"]的所有元素定义为一个新列表l3; 
    
            >>> l1 = ["Sun","Mon","Tue","Wed","Thu","Fri","Sat"]
            >>> l2 = ["Sun","Mon","Tue","Thu","Sat"]
            >>> l3 = [ ]
            >>> for i in l1:
                    if i not in l2:
                        l3.append(i)
            >>> l3
            ['Wed', 'Fri']
            
     练习4：已知列表namelist=['stu1','stu2','stu3','stu4','stu5','stu6','stu7']，删除列表removelist=['stu3', 'stu7', 'stu9']；请将属于removelist列表中的每个元素从namelist中移除(属于removelist，但不属于namelist的忽略即可)；
     
            >>> namelist= ['stu1','stu2','stu3','stu4','stu5','stu6','stu7']
            >>> removelist = ['stu3', 'stu7', 'stu9']   
            >>> for i in namelist:
                    if i in removelist :
                        namelist.remove(i)
            >>> namelist
            ['stu1', 'stu2', 'stu4', 'stu5', 'stu6']

4)while循环练习

        练习1：逐一显示指定列表中的所有元素；
        
            >>> l1 = [1,2,3,4,5]
            >>> i = 0
            >>> while i < len(l1)
                    print l1[i]
                    i += 1
            
            1
            2
            3
            4
            5
        
            >>> l1 = [1,2,3,4,5]
            >>> while l1:                        
                    print l1.pop(0)
                    
            1
            2
            3
            4
            5
            
        练习2：求100以内所有偶数之和；
            
            >>> i = 0
            >>> sum = 0 
            >>> while i < 101:
                    sum += i
                    i += 2
                print sum
            
            2550
            
            >>>  for i in range(0,101,2):
                    sum+=i   
                 print sum
            
            2550
            
       练习3：逐一显示指定字典的所有键；并于显示结束后说明总键数；
       
            >>> d1 = {'x':1, 'y':23, 'z': 78}
            >>> i1 = d1.keys()
            >>> while i1:
                    print i1.pop(0)
                else:
                    print len(d1)
            x
            y
            z
            3        

       练习4：创建一个包含了100以内所有奇数的列表；
       
            >>> d1 = [ ]
            >>> i = 1
            >>> while i < 101:
                    d1.append(i)
                    i+=2
            >>> print d1
            [1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37, 39, 41, 43, 45, 47, 49, 51, 53, 55, 57, 59, 61, 63, 65, 67, 69, 71, 73, 75, 77, 79, 81, 83, 85, 87, 89, 91, 93, 95, 97, 99]    
           
           >>> d1 = [ ] 
            >>> for i in range(1,101,2)
                    d1.append(i)
            >>> print d1
            [1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37, 39, 41, 43, 45, 47, 49, 51, 53, 55, 57, 59, 61, 63, 65, 67, 69, 71, 73, 75, 77, 79, 81, 83, 85, 87, 89, 91, 93, 95, 97, 99]
            
        练习5：列表l1=[0,1,2,3,4,5,6], 列表l2=["Sun","Mon","Tue","Wed","Thu","Fri","Sat"]，以第一个列表中的元素为键，以第二个列表中的元素为值生成字典d1；
             
            >>> l1 = [0,1,2,3,4,5,6] 
            >>> l2 = ["Sun","Mon","Tue","Wed","Thu","Fri","Sat"]
            >>> d1 = {}
            >>> count = 0
            >>> if len(l1) == len(l2):
                    while count < len(l1):
                        d1[l1[count]] = l2[count] 
                        count += 1

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