python: 流程控制

Posted 2021-01-15 xinet

比较、相等和真值

• == 操作符测试值的相等性。
• is 表达式测试对象的一致性。

S1 = ‘spam‘
S2 = ‘spam‘

S1 == S2, S1 is S2

(True, True)

L1 = [1, (‘a‘, 3)]           # Same value, unique objects 
L2 = [1, (‘a‘, 3)]

L1 == L2, L1 is L2, L1 < L2, L1 > L2

(True, False, False, False)

bool(‘‘)

False

2 and 4

4

2 or 4

2

Python 语句就是告诉你的程序应该做什么的句子。

• 程序由模块构成。
• 模块包含语句。
• 语句包含表达式。
• 表达式建立并处理对象。

真值测试

• All objects have an inherent Boolean true or false value.
• Any nonzero number or nonempty object is true.
• Zero numbers, empty objects, and the special object None are considered false.
• Comparisons and equality tests are applied recursively to data structures.
• Comparisons and equality tests return True or False (custom versions of 1 and 0).
• Boolean and and or operators return a true or false operand object.
• Boolean operators stop evaluating (“short circuit”) as soon as a result is known

真值判定	结果
X and Y	Is true if both X and Y are true
X or Y	Is true if either X or Y is true
not X	Is true if X is false (the expression returns True or False)

短路计算

• or: 从左到右求算操作对象，然后返回第一个为真的操作对象。
• and: 从左到右求算操作对象，然后返回第一个为假的操作对象。

2 or 3, 3 or 2

(2, 3)

[] or 3

3

[] or {}

{}

2 and 3, 3 and 2

(3, 2)

[] and {}

[]

3 and []

[]

断言（assert）

num = -1
assert num > 0, ‘num should be positive!‘

---------------------------------------------------------------------------

AssertionError                            Traceback (most recent call last)

<ipython-input-13-68d5a766c1dc> in <module>()
      1 num = -1
----> 2 assert num > 0, ‘num should be positive!‘


AssertionError: num should be positive!

num = 5
assert num > 0, ‘num should be positive!‘

if 条件

year = int(input(‘请输入年份：‘))
if year % 4 == 0:
    if year % 400 == 0:
        print(‘闰年‘)
    elif year % 100 == 0:
        print(‘平年‘)
    else:
        print(‘闰年‘)
else:
    print(‘平年‘)

请输入年份：1990
平年

使用 and 与 or 的短路逻辑：

year = int(input(‘请输入年份：‘))
if (year % 4 == 0 and year % 100 != 0) or year % 400 == 0:
    print(‘闰年‘)
else:
    print(‘平年‘)

请输入年份：1990
平年

if 的短路（short-ciecuit）计算：A = Y if X else Z

year = int(input(‘请输入年份：‘))
print(‘闰年‘) if (year % 4 == 0 and year % 100 != 0) or year % 400 == 0 else print(‘平年‘)

请输入年份：1990
平年

‘t‘ if ‘spam‘ else ‘f‘

‘t‘

A = [Z, Y][bol(X)] 从列表中挑选真假值 (不推荐使用)：

[‘f‘, ‘t‘][bool(‘‘)]

‘f‘

[‘f‘, ‘t‘][bool(‘spam‘)]

‘t‘

a = ‘w‘ ‘d‘ ‘de‘
a

‘wdde‘

while 循环

通用循环结构：

while test:  # Loop test
    statements  # Loop body
else:  # Optional else 只有当循环正常离开时才会执行 (也就是没有碰到 `break` 语句)
    statements  # Run if didn‘t exit loop with break, 

• continue：跳到最近所在循环的开头处 (来到循环的首行)
• break：跳出最近所在的循环 (跳过整个循环语句)
• pass 或 ...：空占位语句

x = ‘spam‘
while x:
    print(x, end=‘ ‘)
    x = x[1:]

spam pam am m 

X = ...
X

Ellipsis

x = 1  # 初值条件
while x <= 100:  # 终止条件
    print(x)
    x += 27

1
28
55
82

# 拉兹猜想
num = int(eval(input(‘请输入初始值：‘)))
while num != 1:
    if num % 2 == 0:
        num /= 2
    else:
        num = num*3+1
    print(num)

请输入初始值：5
16
8.0
4.0
2.0
1.0

x = 10
while x:
    x -= 1
    if x % 2 != 0:
        continue  # 跳过打印
    print(x, end=‘ ‘)

8 6 4 2 0 

while True:
    name = input(‘Enter name: ‘)
    if name == ‘stop‘: break
    age = input(‘Enter age: ‘)
    print(‘Hello ‘, name, ‘=>‘, int(age)**2)

Enter name: H
Enter age: 25
Hello  H => 625
Enter name: stop

和循环 else 子句结合，break 语句通常可以忽略其他语言中所需要的搜索状态标志位。

y = int(input(‘输入数字：‘))
x = y // 2
while x > 1:
    if y % x == 0:
        print(y, ‘有因子‘, x)
        break
    x -= 1
else:  # 没有碰到 break 才会执行
    print(y, ‘是质数！‘)

输入数字：6
6 有因子 3

循环 else 分句是 Python 特有的，它提供了常见的编写代码的明确语法：这是编写代码的结构，让你捕捉循环的“另一条”出路，而不通过设定和检查标志位或条件。

例如，假设你要写一个循环搜索列表的值，而且需要知道在离开循环后该值是否已经找到，可能会用下面的方式编写该任务：

found = False
while x and not found:
    if match(x[0]):
        print(‘Ni‘)
        found = True
    else:
        x = x[1:]
if not found:
    print(‘not found‘)

我们亦可使用循环 else 分句来简化上述代码：

while x:
    if match(x[0]):
        print(‘Ni‘)
        break
    x = x[1:]
else:
    print(‘not found‘)

for 循环

遍历序列对象：

for target in object:                 # Assign object items to target    
    statements                        # Repeated loop body: use target 
else:                                 # Optional else part    
    statements                        # If we didn‘t hit a ‘break‘

for i in range(5):
    ...  # 等价于 pass

nums=[1,2,3,4,5]
for i in nums:
    print(i)

1
2
3
4
5

list(range(1,10,6))

[1, 7]

# 阶乘
x=1
for i in range(1,11):
    x*=i
print(‘10!=‘,x)

10!= 3628800

b = [[9, 7, 3, 6, 5], [10, 2, 4, 6, 7], [0, 5, 3, 2, 9], [7, 3, 5, 6, 1]]
s = 0
for i in range(len(b)):
    for j in range(len(b[i])):
        s += b[i][j]
print(s)

100

for x in range(1, 10):
    for y in range(1, x + 1):
        print(end=‘|‘)
        print(‘%d*%d=%2d‘ % (x, y, x * y), end=‘|‘)
    print()

|1*1= 1|
|2*1= 2||2*2= 4|
|3*1= 3||3*2= 6||3*3= 9|
|4*1= 4||4*2= 8||4*3=12||4*4=16|
|5*1= 5||5*2=10||5*3=15||5*4=20||5*5=25|
|6*1= 6||6*2=12||6*3=18||6*4=24||6*5=30||6*6=36|
|7*1= 7||7*2=14||7*3=21||7*4=28||7*5=35||7*6=42||7*7=49|
|8*1= 8||8*2=16||8*3=24||8*4=32||8*5=40||8*6=48||8*7=56||8*8=64|
|9*1= 9||9*2=18||9*3=27||9*4=36||9*5=45||9*6=54||9*7=63||9*8=72||9*9=81|

d = {‘k1‘: 1, ‘k2‘: 2, ‘k3‘: 3}
for i in d:
    print(i, ‘:‘, d[i])

k1 : 1
k2 : 2
k3 : 3

d = {‘k1‘: 1, ‘k2‘: 2, ‘k3‘: 3}
for key, value in d.items():
    print(key, ‘:‘, value)

k1 : 1
k2 : 2
k3 : 3

a, *b, c = 1, 2, 3, 4, 5
a, b, c

(1, [2, 3, 4], 5)