How to use while, if, and for statements in python-Python Tutorial-php.cn

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1, if conditional statement

Basic form:

if Conditional judgment:

Execution statement

elif conditional judgment:

Execution statement

else:

Execution statement

Example:

<span style='font-size: 14px; font-family: 微软雅黑, "Microsoft YaHei";'>number = int(input("输入一个数字："))<br>if number > 0:  <br>  print("正数")<br>elif number == 0:  <br>  print("零")<br>else:    <br>  print("负数")<br></span>

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<span style='font-size: 14px; font-family: 微软雅黑, "Microsoft YaHei";'>输入一个数字：2<br>正数<br></span>

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Analysis: As above, after running, the user enters a number 2 and starts to enter the if condition judgment, 2 > 0 is established , then enter the internal execution code block and output "positive number".

If the number entered by the user is -1, starts to enter the if condition judgment, -1 > 0 is not established, continue execution, "elif" is the condition judgment to continue execution , -1==0 is not established,

execute next, "else" is: if none of the above conditions are met, Then directly execute the else internal code block and print "negative number".

2. while loop statement

(1) while use

while Statements are used to execute programs in a loop, that is, under certain conditions, execute a certain program in a loop to handle the same tasks that need to be processed repeatedly. Its basic form is:

while Judgment condition:

循环语句...

执行语句可以是单个语句或语句块。判断条件可以是任何表达式，任何非零、或非空（null）的值均为true。

当判断条件假false时，循环结束。

例子：

<span style='font-size: 14px; font-family: 微软雅黑, "Microsoft YaHei";'>a = 1while a < 3:    print(a)<br>    a += 1<br></span>

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<span style='font-size: 14px; font-family: 微软雅黑, "Microsoft YaHei";'>结果为：1<br>    2<br></span>

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<span style='font-size: 14px; font-family: 微软雅黑, "Microsoft YaHei";'>解析：a 的值为1，进入while条件判断，a小于3，满足条件，则进入循环内部，<br></span>

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<span style='font-size: 14px; font-family: 微软雅黑, "Microsoft YaHei";'>执行代码块：打印a的值“1”，然后使a+1。则此时的a为2，进入下次循环，<br></span>

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<span style='font-size: 14px; font-family: 微软雅黑, "Microsoft YaHei";'>开始条件判断，2小于3，条件满足，进入循环内部执行代码块：打印a的值“2”，<br></span>

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<span style='font-size: 14px; font-family: 微软雅黑, "Microsoft YaHei";'>然后a+1。a的值为3，进入下次循环。开始条件判断，此时a的值为3，条件为a<3，<br></span>

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<span style='font-size: 14px; font-family: 微软雅黑, "Microsoft YaHei";'>可见不满足条件，所以结束循环。<br></span>

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（2）continue，break 跳出循环

continue 用于跳过该次循环，break 则是用于退出循环，此外"判断条件"还可以是个常值，表示循环必定成立，具体用法如下：

<span style='font-size: 14px; font-family: 微软雅黑, "Microsoft YaHei";'>continue<br></span>

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<span style='font-size: 14px; font-family: 微软雅黑, "Microsoft YaHei";'>a = 0<br>while a < 3:<br>    a += 1  <br>      if a == 2:     <br>         continue    <br>   print(a)<br></span>

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<span style='font-size: 14px; font-family: 微软雅黑, "Microsoft YaHei";'>结果为：1<br>    3<br></span>

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<span style='font-size: 14px; font-family: 微软雅黑, "Microsoft YaHei";'>循环过程：a的值为0，进行while条件判断 a<3 成立，执行代码块，a+1，此时a的<br></span>

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<span style='font-size: 14px; font-family: 微软雅黑, "Microsoft YaHei";'>值为1，进入if条件判断，不满足条件，所以不进入if内部，继续往下执行，打印<br></span>

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<span style='font-size: 14px; font-family: 微软雅黑, "Microsoft YaHei";'>a的值“1”。开始进入下次循环，a<3成立，进入循环，a+1，此时a的值为2，进入<br></span>

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<span style='font-size: 14px; font-family: 微软雅黑, "Microsoft YaHei";'>if条件判断，a等于2，条件成立，进入if内部执行代码块，continue 用于跳过该<br></span>

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<span style='font-size: 14px; font-family: 微软雅黑, "Microsoft YaHei";'>次循环，无论下面还有没有代码块都不会继续执行下去，直接将此次循环结束，<br></span>

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<span style='font-size: 14px; font-family: 微软雅黑, "Microsoft YaHei";'>进入下次循环。开始条件判断，a<3,成立，进入循环，a+1，此时a的值为3，进入<br></span>

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<span style='font-size: 14px; font-family: 微软雅黑, "Microsoft YaHei";'>if条件判断，不满足条件，所以不进入if内部，继续往下执行，打印a的值“3”。<br></span>

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<span style='font-size: 14px; font-family: 微软雅黑, "Microsoft YaHei";'>开始进入下次循环，a<3不成立，结束循环。<br></span>

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break
a = 0
while a < 4:
    a += 1   
     if a == 2:      
       break    
 print(a)

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结果为：1

将上面例子中的“continue”换为 “break”，循环是的流程还是一样的，只是在触发到“break”时不同，

break 为直接结束循环，所以运行的结果只有“1”被打印了。

三，for 循环语句

1，使用方式

for循环可以遍历任何序列的项目，如一个列表或者一个字符串。跟while都属于循环，for循环是在序列穷尽时停止，

while循环是在条件不成立时停止。

语法格式：

for iterating_var in sequence:
   statements(s)

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例子：遍历一个字符串的每个字符。

<span style="font-size: 14px; font-family: 微软雅黑, "Microsoft YaHei";">str1 = "Hello"<br/>for i in str1:   <br/>      print(i)<br/></span>

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<span style="font-size: 14px; font-family: 微软雅黑, "Microsoft YaHei";">结果为：H<br/>    e<br/>    l<br/>    l<br/>    o<br/></span>

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2，range（）函数，生成一个整数数列，前开后闭，即后边的数字不能被打印。

如：

   print

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结果为：1 2

3，xrange（）用法与 range 完全相同，所不同的是生成的不是一个数组，而是一个生成器。

>>> list(xrange(1,5))
[1, 2, 3, 4]

4，enumerate（）用于将一个可遍历的数据对象(如列表、元组或字符串)组合为一个索引序列，同时列出数据和数据下标。

如：

<span style="font-size: 14px; font-family: 微软雅黑, "Microsoft YaHei";">a = ["a","b","c"]<br/>    for i in enumerate(a):   <br/>         print(i)<br/></span>

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<span style="font-size: 14px; font-family: 微软雅黑, "Microsoft YaHei";">结果为：(0, &#39;a&#39;)<br/>    (1, &#39;b&#39;)<br/>    (2, &#39;c&#39;)<br/></span>

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