Python3 basics (five)-loop control

Python3 basics (five)-loop control

Foreword
1 What can I learn from reading this article?
  This article will introduce you to the usage of loop control in python3. In python3 where there are cycle control forand whiletwo kinds.

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1 while loop control

1.1 while structure

Grammatical structures:

while <condition>:
    <codeblock>

  Before the start of the loop, determine whether the <condition>condition is satisfied , that is, the result of the conditional expression is Boolean true. If it is satisfied, it will be executed <codeblock>. After the execution is completed, continue to judge whether it is satisfied <condition>. Continue to meet and continue to execute <codeblock>until the condition is not satisfied. <codeblock>The code block must have whileat least one space or Table indentation.
flow chart:

Code example:

a = 5
while(a > 0):               #循环边界条件
    a -= 1                  #循环执行内容
    print(a)                #循环执行内容

operation result:

4
3
2
1
0

  When the <condition>constant is true, the loop will continue to execute forever, for example while(True).

1.2 while——else结构

Grammatical structures:

while <condition>:
    <codeblock1>
else:
    <codeblock2>

  Python3 whilecan be elsecombined with the implementation of the elseinner code block if the loop body is not executed when the condition is not met .
Code example:

a = 5
while(a > 0):
    a -= 1
    print(a)
else:                                   #当a=0时不满足a > 0所以执行else中代码块
    print("else code block")

a = -1
while(a > 0):                           #初始a=-1不满足a > 0所以执行else中代码块
    a -= 1
    print(a)
else:
    print("else code block")

operation result:

4
3
2
1
0
else code block
else code block

  When the loop condition is false, the elsecurrent code block will be executed .

2 for loop control

2.1 for structure

Grammatical structures:

for <variable> in <sequence>:
    <codeblock1>
else
    <codeblock2>

  Python3's for loop is used to traverse the elements within a range of sequences. This sequence can be a range of values, strings, tuples, lists, sets, dictionaries. Using it flexibly will allow you to manipulate these data types more freely.
flow chart:

Code example

number = 3
for val in range(number):               #遍历整数元素，range函数限制了范围为整数从0~3，步长为1
    print(val)
print("----------------------")

string = "123"
for val in string:                      #遍历字符串中的字符
    print(val)
print("----------------------")

Tuple = (1, 2, 3)
for val in Tuple:                       #遍历元组元素
    print(val)
print("----------------------")

List = (1, 2, 3)
for val in List:                        #遍历列表元素
    print(val)
print("----------------------")

Set = {1, 2, 3}
for val in Set:                         #遍历集合元素
    print(val)
print("----------------------")

Dictionary = {"1":1, "2":2, "3":3}
for val in Set:                         #遍历字典元素
    print(val)
print("----------------------")

  val是循环体内的局部变量，变量名可以自己定义。它的会依次遍历序列里的每一个元素。
输出结果为：

0
1
2
----------------------
1
2
3
----------------------
1
2
3
----------------------
1
2
3
----------------------
1
2
3
----------------------
1
2
3
----------------------

3 循环内的break和continue关键字

  循环结构内可以使用关键字break和continue，break的作用的立即结束循环的执行，跳到循环外继续执行。coutinue则是立即结束循环的本次执行，跳到循环条件表达式处。
语法结构：

while <condition>:
    <codeblock1>
    break
    <codeblock2>
<codeblock3>

  当执行到break时表示该循环体的处理已经结束，不会执行<codeblock2>而是跳到<codeblock3>继续执行。
语法结构：

while <condition>:
    <codeblock1>
    continue
    <codeblock2>
<codeblock3>

  当执行到continue时表示循环本轮的处理已经结束，不会执行<codeblock2>而是跳到<condition>继续执行下一轮循环。
代码示例：

a = 5
while a > 0:
    a -= 1
    if a == 2:                      #轮循到a为2时即结束循环
        break
    print(a)
print("End while")

a = 5
while a > 0:
    a -= 1
    if a == 2:                      #轮询到a为2时结束本轮循环
        continue
    print(a)
print("End for")

运行结果：

4
3
End while
4
3
1
0
End for