Python中的条件判断语句与循环语句用法小结
if语句
>>通用格式
if语句一般形式如下:
if <test1>: <statements1> elif <test2>: <statements2> else: <statements3>
另外需要注意的是，Python中是没有switch/case语句的
while循环
while语句是Python语言中最通用的迭代结构，简而言之，只要顶端测试一直计算到真值，就会重复执行一个语句块。
>>一般格式
while <test>: <statements1> else: <statements2> >>break,continue,pass和循环else
break
跳出最近所在的循环(跳过整个循环语句)。
continue
跳到最近所在循环的开头处(来到循环的首行)。
pass
什么事也不做，只是空占位符语句。
循环else块
只有当前循环正常离开时才会执行(也就是没有碰到break语句)
>>一般循环格式
加入break和continue语句后，while的一般格式变为:
while <test1>: <statements1> if <test2>:break if <test3>:continue else: <statements2> >>pass
pass语句是无运算的占位符，当语法需要语句并且还没有任何实用的语句可写时，就可以使用它。
>>循环else
在while语句中加入else和C/C++中的语法不太一样，这里详细说明一下。else后面的代码只有当循环正常结束时才会执行，如果是用break跳出循环的，这部分代码就不会运行，具体看一个求质数的例子:
x = y // 2 while x > 1: if y % x == 0: print(y,'has factor',x) break x -= 1 else: print(y,'is prime')
再看一个对比的例子，没有使用else的情况:
found=False
while x and not found:
if (matchx[0]):
print('Ni')
found=True
else:
x=x[1:]
if not found:
print('not found')
使用else后的情况:
while x:
if (match(x[0])):
print('Ni')
break
else:
print('not found')
for循环
for循环在Python中是一个通用的序列迭代器:可以遍历任何有序的序列对象内元素。for语句可以用于字符串、列表、元组、其他内置可迭代对象。
>>一般格式
for <target> in <object>: <statements> else: <statements>
此处的else的作用和while语句中的一样。另外需要注意的是，当Python运行for循环时，会逐个将序列对象中的元素赋值给目标，然后为每个元素执行循环体。
编写循环的技巧
内置range函数:返回一系列连续增加的整数，可作为for中的索引
内置zip函数:返回并行元素的元组的列表，可用于在for中遍历数个数列
>>循环计数器:while和range
range
当range函数只有一个参数时，会返回从零算起的整数列表，但其中不包括该参数的值。如果传进两个参数，那第一个参数是上边界，第二个参数是下边界。如果传进三个参数时，第三个参数表示步进值。
range提供了一种简单的方法，重复特定次数的动作:
for i in range(5): print(i,'Pythons')
相应的C++代码则是:
int i;
for(i = 0;i < 5;++i)
{
std::cout<<i<<"Python";
}
>>并行遍历:zip和map
zip会取得一个或多个序列为参数，然后返回元组的列表，将这些序列中的并排的元素配成对。
L1=[1,2,3,4] L2=[5,6,7,8] list(zip(L1,L2))
上述代码的执行结果是:
[(1,5),(2,6),(3,7),(4,8)]
当参数的长度不同时，zip会以最短序列的长度为准来截断所得到的元组。
使用zip构造字典:
keys=['spam','eggs','totast'] values=[1,2,5] D = dict(zip(keys,values))
>>产生偏移和元素:enumerate
enumerate函数一个比较新的内置函数，它能同时返回元素值和偏移值:
s='spam' for (offset,item) in enumerate(s): print(item,'appears at offset',offset)
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