第三章 C++的三种基本控制结构

0x C++提供的三种基本控制结构

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• 顺序结构：按照先后顺序依次执行程序中的语句
• 选择结构：按照给定条件有选择地执行程序中的语句
• 循环语句：按照给定规则重复地执行程序中的语句

1x 第一节 C++语句

---

语句以分号表示结束

• 六种C++语句
  • 声明语句：用于对程序中的各种实体进行声明、定义和初始化。
  • 表达式语句：用于对程序中的数据进行具体操作和处理。
  • 选择语句：用于实现程序的选择结构。
  • 循环语句：用于实现程序的循环结构。
  • 跳转语句：用于实现程序执行流程的变换。
  • 复合语句：用于表示程序中的语句块。

2x 第二节 顺序结构

---

顺序结构会按照程序的书写的先后顺序，从左到右，至上而下的执行。

2x.1 声明语句

• 变量声明
• 常量声明
• 函数声明
• 类型声明

2x.2 表达式语句

略

2x.3 基本输入输出

I/O操作通过标准库中的输入/输出流对象来完成。

• 标准输入输出流对象：cin cout
• 在使用标准输入/输出流之前必须用处理命令在引用iostream.h

      #include <iostream.h>
      using namespace std;
  

  • 输出操作 cout<<Exper<<endl;
    
    整个语句的含义是：将表达式Exper的值输出到屏幕上的当前光标位置。
    • Exper代表一个表达式,endl(end line的意思)是输入/输出操作符
    • 一个cout语句可以分写成为若干行，分写的行间有无分号亦可。
  • 输入操作 cin>>Var;
    • Var代表一个变量，并且支持连续性输入多项数据。
    • cin语句的一般格式为

        cin>>变量1>>变量2>>...>>变量N;
    

2x.4 符合语句和空语句

• 复合语句：用花括号{}将若干语句包围起来而组成的一条语句：
  • 函数的函数体
  • 循环语句的循环体
  • 选择语句的分支
• 空语句

      ;//无任何操作
  

3x 选择结构

---

3x.1 if语句——条件语句

• 基本的if语句
  • 格式：if(<条件表达式>) <语句>
  • 若条件表达式的结果为真则执行后面的语句。
  • 若想执行多条语句，必须用花括号{}把后面多条语句括起来。

      *C++中规定如果条件表达式中的值不为0就是“真”*,只有为0才为“假”。
  

3x.2 if...else语句

• 格式:

    if (<条件表达式>)
        <语句1>
    else
        <语句二>

3x.3 if语句的嵌套

• 格式：

    if(<条件表达式>)
        <语句1>；
    else
        if(<条件表达式>)
            <语句2>；
        else
            <语句>；

• else关键字总是于它前面最近的未配对且可见的那个if关键字匹配

• 复合语句内的if关键字对于外界是不可见的。
  
  例如：

      if(<条件表达式>)        //第一个if
          if(<条件表达式>)    //第二个if
              {
                  if(<条件表达式>)    //第三个if
              }
          else
              <语句>；  //与第二个else相匹配
  

友情提示:在真正的程序设计当中，必须要使用缩进、程序块等等手段来尽可能地避免程序可读性差，以后我会开辟博文写有关这个的方面

3x.2 switch语句——开关语句

• 根据给定表达式不同取值来决定从语句序列中的哪一个开始执行。
• 常用语法：

    switch (表达式)
        {
            case 常量表达式1: 语句1；break;
            case 常量表达式2：语句2; break;
            ...
            case 常量表达式n: 语句n; break;
            default:语句n+1;
        }

• 注意事项：
  • 表达式的值必须是整型、字符型或枚举型
  • 多个case标号可以共用一组语句序列，以实现对多个常量执行同一个操作
  • default标号是可选择的
  • switch语句是可以嵌套的
  • 每一个case表达式的值必须是不相同的
  • case和default的出现次序不影响程序的运行

4x 循环结构

---

• 三种循环语句：
  • for语句
  • while语句
  • do...while语句

4x.1 for语句

• 常用格式：

    for(表达式1;表达式2;表达式3)
        {
            <语句>;
        }
    //表达式1:循环变量初始化
    //表达式2:循环条件
    //表达式3:循环变量增值

• 注意事项:
  1. 先执行表达式1。
  2. 执行表达式2，若其值为真(值为非0)执行for中指定的内嵌语句，然后再执行表达式3，重新进行判断；若其值为假(值为0).
  3. 三个表达式从语法上可以省略

4x.2 While语句——"当型循环"

• 常用格式：

    while(条件表达式)
    {
        <语句>;
    }

• 条件是为真（表达式为非0）时，执行while语句中的内嵌语句。
  
  先判断，后循环
• 有多条语句要执行时，用花括号{}把多个语句括起来。
• 循环体中应该有使循环体催向结束的语句。

4x.3 do...while语句——直到型循环

• 常用格式

    do
    {
        <语句>;
    }
    while(条件表达式)；

• 与while语句的区别,do...while语句先执行后判断，至少执行一次；while语句先判断后执行，可能一次也不执行。

4x.4 循环的嵌套

• 一个循环体内又包含另外一个完整的循环结构，称为循环的嵌套。
• 三种循环体都可以互相循环。

5x 跳转语句

---

• 使用跳转语句可以实现程序执行流程的无条件转换。
• C++提供了四种跳转语句，分别是：
  • break语句
  • continue语句
  • return语句
  • goto语句

5x.1 break语句

• 常用格式为

    break;

• break语句可以用于多分支选择语句——switch或循环语句的跳出操作，不可单独用于其他语句

5x.2 continue语句

• 常用格式为

    continue;

• 其作用为结束本次循环，即跳过该次循环，继续进行下一轮的循环。
• continue与break的区别是：
  • continue只结束本次循环而不是终止整个循环的进行。
  • break是终止整个循环。

5x.3 return语句

• 常用格式为：

    return ;
    //或者
    return 表达式;

• 值得注意：
  • 只能用在函数体中
  • 非void类型返回值函数，其函数体必须包含一条return语句并带有一个对应类型的返回值。
  • 返回值为void的函数，可以使用第一种形式的return语句，即不带返回值。

5x.4 goto语句

• 常用格式：

    goto <标志>;

标志常用格式

    <标志>:<语句>

• 作用：使程序执行流程跳转到标记的语句处

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.markdown-preview.markdown-preview input[type="text"],.markdown-preview.markdown-preview input[type="password"],.markdown-preview.markdown-preview textarea,.markdown-preview.markdown-preview select { display: inline-block; width: 210px; padding: 4px; font-size: 13px; font-weight: normal; line-height: 18px; height: 18px; color: #808080; border: 1px solid #ccc }
.markdown-preview.markdown-preview select,.markdown-preview.markdown-preview input[type="file"] { height: 27px; line-height: 27px }
.markdown-preview.markdown-preview textarea { height: auto }
.markdown-preview.markdown-preview :-moz-placeholder { color: #bfbfbf }
.markdown-preview.markdown-preview ::-webkit-input-placeholder { color: #bfbfbf }
.markdown-preview.markdown-preview input[type="text"],.markdown-preview.markdown-preview input[type="password"],.markdown-preview.markdown-preview select,.markdown-preview.markdown-preview textarea { }
.markdown-preview.markdown-preview input[type="text"]:focus,.markdown-preview.markdown-preview input[type="password"]:focus,.markdown-preview.markdown-preview textarea:focus { outline: none; border-color: rgba(82, 168, 236, 0.8) }
.markdown-preview.markdown-preview button { display: inline-block; padding: 4px 14px; font-family: "Helvetica Neue", Helvetica, Arial, sans-serif; font-size: 13px; line-height: 18px; background-color: #0064cd; background-repeat: repeat-x; background-image: linear-gradient(top, #049cdb, #0064cd); color: #fff; border: 1px solid #004b9a; border-bottom-color: #003f81; border-color: rgba(0, 0, 0, 0.1) rgba(0, 0, 0, 0.1) rgba(0, 0, 0, 0.25) }
.markdown-preview.markdown-preview button:hover { color: #fff; background-position: 0 -15px; text-decoration: none }
.markdown-preview.markdown-preview button:active { }
.markdown-preview.markdown-preview button::-moz-focus-inner { padding: 0; border: 0 }
.markdown-preview.markdown-preview table { border-spacing: 0; width: 100% }
.markdown-preview.markdown-preview table { border: solid #ccc 1px }
.markdown-preview.markdown-preview table tr:hover { background: #fbf8e9 }
.markdown-preview.markdown-preview table td,.markdown-preview.markdown-preview .table th { border-left: 1px solid #ccc; border-top: 1px solid #ccc; padding: 10px; text-align: left }
.markdown-preview.markdown-preview table th { background-color: #dce9f9; background-image: linear-gradient(top, #ebf3fc, #dce9f9); border-top: none; padding: 5px }
.markdown-preview.markdown-preview table td:first-child,.markdown-preview.markdown-preview table th:first-child { border-left: none }
.markdown-preview.markdown-preview table th:first-child { }
.markdown-preview.markdown-preview table th:last-child { }
.markdown-preview.markdown-preview table th:only-child { }
.markdown-preview.markdown-preview table tr:last-child td:first-child { }
.markdown-preview.markdown-preview table tr:last-child td:last-child { }