Python 入门（五）条件判断和循环

if语句

计算机之所以能做很多自动化的任务，因为它可以自己做条件判断。

比如，输入用户年龄，根据年龄打印不同的内容，在Python程序中，可以用if语句实现：

age = 20
if age >= 18:
    print 'your age is', age
    print 'adult'
print 'END'

注意: Python代码的缩进规则。具有相同缩进的代码被视为代码块，上面的3，4行 print 语句就构成一个代码块（但不包括第5行的print）。如果 if 语句判断为 True，就会执行这个代码块。

缩进请严格按照Python的习惯写法：4个空格，不要使用Tab，更不要混合Tab和空格，否则很容易造成因为缩进引起的语法错误。

注意: if 语句后接表达式，然后用:表示代码块开始。

如果你在Python交互环境下敲代码，还要特别留意缩进，并且退出缩进需要多敲一行回车：

>>> age = 20
>>> if age >= 18:
...     print 'your age is', age
...     print 'adult'
...
your age is 20
adult

任务

如果成绩达到60分或以上，视为passed。

假设Bart同学的分数是75，请用if语句判断是否能打印出 passed:

aaarticlea/png;base64,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" alt="" />

if-else

当 if 语句判断表达式的结果为 True 时，就会执行 if 包含的代码块：

if age >= 18:
    print 'adult'

如果我们想判断年龄在18岁以下时，打印出 'teenager'，怎么办？

方法是再写一个 if:

if age < 18:
    print 'teenager'

或者用 not 运算：

if not age >= 18:
    print 'teenager'

细心的同学可以发现，这两种条件判断是“非此即彼”的，要么符合条件1，要么符合条件2，因此，完全可以用一个 if ... else ... 语句把它们统一起来：

if age >= 18:
    print 'adult'
else:
    print 'teenager'

利用 if ... else ... 语句，我们可以根据条件表达式的值为 True 或者 False ，分别执行 if 代码块或者 else 代码块。

注意: else 后面有个“:”。

任务

如果成绩达到60分或以上，视为passed，否则视为failed。

假设Bart同学的分数是55，请用if语句打印出 passed 或者 failed:

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" 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if-elif-else

有的时候，一个 if ... else ... 还不够用。比如，根据年龄的划分：

条件1：18岁或以上：adult
条件2：6岁或以上：teenager
条件3：6岁以下：kid

我们可以用一个 if age >= 18 判断是否符合条件1，如果不符合，再通过一个 if 判断 age >= 6 来判断是否符合条件2，否则，执行条件3：

if age >= 18:
    print 'adult'
else:
    if age >= 6:
        print 'teenager'
    else:
        print 'kid'

这样写出来，我们就得到了一个两层嵌套的 if ... else ... 语句。这个逻辑没有问题，但是，如果继续增加条件，比如3岁以下是 baby：

if age >= 18:
    print 'adult'
else:
    if age >= 6:
        print 'teenager'
    else:
        if age >= 3:
            print 'kid'
        else:
            print 'baby'

这种缩进只会越来越多，代码也会越来越难看。

要避免嵌套结构的 if ... else ...，我们可以用 if ... 多个elif ... else ... 的结构，一次写完所有的规则：

if age >= 18:
    print 'adult'
elif age >= 6:
    print 'teenager'
elif age >= 3:
    print 'kid'
else:
    print 'baby'

elif 意思就是 else if。这样一来，我们就写出了结构非常清晰的一系列条件判断。

特别注意: 这一系列条件判断会从上到下依次判断，如果某个判断为 True，执行完对应的代码块，后面的条件判断就直接忽略，不再执行了。

请思考下面的代码：

age = 8
if age >= 6:
    print 'teenager'
elif age >= 18:
    print 'adult'
else:
    print 'kid'

当 age = 8 时，结果正确，但 age = 20 时，为什么没有打印出 adult？

如果要修复，应该如何修复？--互换条件

任务

如果按照分数划定结果：

90分或以上：excellent

80分或以上：good

60分或以上：passed

60分以下：failed

请编写程序根据分数打印结果。

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" alt="" />

for循环

list或tuple可以表示一个有序集合。如果我们想依次访问一个list中的每一个元素呢？比如 list：

L = ['Adam', 'Lisa', 'Bart']
print L[0]
print L[1]
print L[2]

如果list只包含几个元素，这样写还行，如果list包含1万个元素，我们就不可能写1万行print。

这时，循环就派上用场了。

Python的 for 循环就可以依次把list或tuple的每个元素迭代出来：

L = ['Adam', 'Lisa', 'Bart']
for name in L:
    print name

注意:  name 这个变量是在 for 循环中定义的，意思是，依次取出list中的每一个元素，并把元素赋值给 name，然后执行for循环体（就是缩进的代码块）。

这样一来，遍历一个list或tuple就非常容易了。

任务

班里考试后，老师要统计平均成绩，已知4位同学的成绩用list表示如下：

L = [75, 92, 59, 68]

请利用for循环计算出平均成绩。

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" 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while循环

和 for 循环不同的另一种循环是 while 循环，while 循环不会迭代 list 或 tuple 的元素，而是根据表达式判断循环是否结束。

比如要从 0 开始打印不大于 N 的整数：

N = 10
x = 0
while x < N:
    print x
    x = x + 1

while循环每次先判断 x < N，如果为True，则执行循环体的代码块，否则，退出循环。

在循环体内，x = x + 1 会让 x 不断增加，最终因为 x < N 不成立而退出循环。

如果没有这一个语句，while循环在判断 x < N 时总是为True，就会无限循环下去，变成死循环，所以要特别留意while循环的退出条件。

任务

利用while循环计算100以内奇数的和。

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" 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break退出循环

用 for 循环或者 while 循环时，如果要在循环体内直接退出循环，可以使用 break 语句。

比如计算1至100的整数和，我们用while来实现：

sum = 0
x = 1
while True:
    sum = sum + x
    x = x + 1
    if x > 100:
        break
print sum

咋一看， while True 就是一个死循环，但是在循环体内，我们还判断了 x > 100 条件成立时，用break语句退出循环，这样也可以实现循环的结束。

任务

利用 while True 无限循环配合 break 语句，计算 1 + 2 + 4 + 8 + 16 + ... 的前20项的和。

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" alt="" />

continue继续循环

在循环过程中，可以用break退出当前循环，还可以用continue跳过后续循环代码，继续下一次循环。

假设我们已经写好了利用for循环计算平均分的代码：

L = [75, 98, 59, 81, 66, 43, 69, 85]
sum = 0.0
n = 0
for x in L:
    sum = sum + x
    n = n + 1
print sum / n

现在老师只想统计及格分数的平均分，就要把 x < 60 的分数剔除掉，这时，利用 continue，可以做到当 x < 60的时候，不继续执行循环体的后续代码，直接进入下一次循环：

for x in L:
    if x < 60:
        continue
    sum = sum + x
    n = n + 1

任务

对已有的计算 0 - 100 的while循环进行改造，通过增加 continue 语句，使得只计算奇数的和：

sum = 0
x = 1
while True:
    sum = sum + x
    x = x + 1
    if x > 100:
        break
print sum

aaarticlea/png;base64,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" alt="" />

多重循环

在循环内部，还可以嵌套循环，我们来看一个例子：

for x in ['A', 'B', 'C']:
    for y in ['1', '2', '3']:
        print x + y

x 每循环一次，y 就会循环 3 次，这样，我们可以打印出一个全排列：

A1
A2
A3
B1
B2
B3
C1
C2
C3

任务

对100以内的两位数，请使用一个两重循环打印出所有十位数数字比个位数数字小的数，例如，23（2 < 3）。

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" 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