Python Foundation II

While loop

结构：while 条件:    循环语句

Dead loop

while True:    没有break语句情况下

Terminating loops

First, change the condition (the concept of the sign bit): The flag is usually used as the mark bit by default.

flag = Truenum = 0while flag:    print(num)    num = num + 1    if num < 10:        flag = False

Second, break terminates the loop: jump out of the loop body where it is located.

num = 0while True:    print(num)    num = num + 1    if num < 10:        break

Calculate 1 + 2 + 3 .... 100 results (three ways) 1

result = 0count = 1while True:    result += count    count += 1    if count == 101:        breakprint(result)

2

result = 0count = 1while count < 101:    result += count    count += 1print(result)

3

result = 0count = 1flag = Truewhile flag:    result += count    count += 1    if count == 101:        flag = Falseprint(result)

Continue: Ends the loop and continues the next cycle.

while True:    print(111)    print(222)    continue    print(333)

The result is

111222111222...

While-else structure: If the while loop is interrupted by a break, the else code is not executed.

count = 1while count < 5:    print(count)    count = count + 1    if count == 3: breakelse:    print(666)print(222)

The result is

12222

Application Scenarios:

• Verify user name password, re-enter this function requires a while loop.
• Infinite number of display pages, unlimited input ...

Formatted output

• To make a template, the parameters in some locations are dynamic, and like this, you need to format the output.
• Dynamic substitution of strings

name = input('请输入姓名：')age = int(input('请输入年龄：'))sex = input('请输入性别：')

% placeholder s data type is string D-digit

The first way:

msg = '你的名字是%s，你的年龄%d,你的性别%s' % (name,age,sex)print(msg)

The second way

msg = '你的名字是%(name1)s，你的年龄%(age1)d,你的性别%(sex1)s' % {'name1':name,'age1':age,'sex1':sex}print(msg)

Bug points in formatted output, only want to simply represent a%, should be expressed in percent

msg = '我叫%s，今年%d，我的学习进度1%%' % ('alex',28)print(msg)

Operator

==  比较的两边的值是否相等= 赋值运算！= 不等于+= 举例： count = count + 1  简写 count += 1-=*=     : count = count * 5   简写 count *= 5/=**=//=...

Logical operators: And OR not precedence: () > Not > and > or first case, before and after conditions are comparison operations

print(1 < 2 or 3 > 1)print(1 < 2 and 3 > 4)print(1 < 2 and 3 > 4 or 8 < 6 and 9 > 5 or 7 > 2)print(1 > 2 and 3 < 4 or 4 > 5 and 2 > 1 or 9 < 8)print(1 > 1 and 3 < 4 or 4 > 5 and 2 > 1 and 9 > 8 or 7 < 6)

In the second case, the conditions on both sides are numeric

'''x or y if x is True,return x'''print(1 or 2)print(1 and 2)print(5 or 2)print(5 and 2)print(0 or 2)print(-1 or 2)

conversion between INT and bool

• 0 The corresponding bool value is false, and not 0 is true.
• True 1, False 0

print(bool(100))print(bool(-1))print(bool(0))print(int(True))print(int(False))

Perverted interview questions: Thinking

print(1 > 2 or 3 and 4 < 6)print(2 or 3 and 4 < 6)

The answer is:

True2如果 x or y 语句中x为真，即为x,否则为y    真 or 真 --> x    真 or 假 --> x    假 or 真 --> y    假 or 假 --> y>>> 3 or 43>>> 3 or 03>>> 0 or 33>>> '' or 00如果 x and y 语句中x为假，即为x,否则为y    真 and 真 --> y    真 and 假 --> y    假 and 假 --> x    假 and 真 --> x

深层次理解：由于or的特性是只要一个为真就判定为真，即判断x为真后，不用再来判断y。若x为假后才会判定y,所以是结果为y。   这样的话程序就能以最小的步数来执行并得到结果。由于and的特性是只要一个为假就判定为假，即判断x为假后，不用再来判断y。若x为真后才会判定y,所以是结果为y。  这样的话程序就能以最小的步数来执行并得到结果。

Python Foundation II