c++ - C++ switch 语句不起作用

Question

我目前正在学习 C++ 课程并学习开关，我们的讲师希望我们编辑一些 openGL 3D 代码的主体。我们应该将它从 if then else 更改为 switch。我这样做了，但现在我的代码将无法运行。这是代码：

void key(unsigned char k, int x, int y)
{
    k = tolower(k);

    switch(k)
    {
             case 'a' : b_animate = !b_animate;
                        if (b_animate)
                        {
                           glutTimerFunc(33, myTimer, 1);
                        }
                        break;
             case 'h' : b_showHints = !b_showHints;
                        glutPostRedisplay();
                        break;
             case 'f' : toggleFullScreen();
                        break;
             case 'o' : gi_projection_type = ORTHO_3D;
                        myReshape(g_windowWidth, g_windowHeight);
                        glutPostRedisplay();
                        break;
             case 'p' : gi_projection_type = PERSPECTIVE;
                        myReshape(g_windowWidth, g_windowHeight);
                        glutPostRedisplay();
                        break;
             case '1' :
             case '2' :
             case '3' :
             case '4' :
             case '5' : 
             case '6' :
             case '7' : 
             case '8' :
             case '9' : double alpha = (k - '0') * 0.1;
                        if (alpha == 0)
                        alpha = 1.0;
                        setAlphaChannel(alpha);   
                        glutPostRedisplay();     
                        break; 
             case '+' : per_angle += 2;
                        myReshape(g_windowWidth, g_windowHeight);
                        glutPostRedisplay();
                        break;
             case '-' : per_angle -= 2;
                        myReshape(g_windowWidth, g_windowHeight);
                        glutPostRedisplay();
                        break;
             case 'm' : printModelViewMatrix();
                        break;
             case 'w' : b_wireFrame = !b_wireFrame;
                        glutPostRedisplay();
                        break;
             case 'q' : b_useOpenGLtransform = !b_useOpenGLtransform;
                        glutPostRedisplay();
                        break;
             case 'n' : g_model = (g_model + 1)%8;
                        glutPostRedisplay();
                        break; 
             case 't' : b_texture = !b_texture;
                        glutPostRedisplay();
                        break;

             case 'l' : b_lighting = !b_lighting;
                        glutPostRedisplay();
                        break;

    }

这是原始代码（抱歉有点搞砸了，我正在复制并粘贴到评论中）：

   if(k == 'a'){
   b_animate = !b_animate;
   if (b_animate){
       glutTimerFunc(33, myTimer, 1);

   }else if (k == 'h'){
    b_showHints = !b_showHints;
    glutPostRedisplay();

     }else if(k == 'f'){
     toggleFullScreen();

     }else if (k == 'o'){
         gi_projection_type = ORTHO_3D;
         myReshape(g_windowWidth, g_windowHeight);
         glutPostRedisplay();
   }else if (k == 'p'){
        gi_projection_type = PERSPECTIVE;
         myReshape(g_windowWidth, g_windowHeight);
         glutPostRedisplay();
    }else if (k >= '0' && k <= '9'){
    double alpha = (k - '0') * 0.1;
    if (alpha == 0)
       alpha = 1.0;
    setAlphaChannel(alpha);   
    glutPostRedisplay();     
             }else if (k == '+'){
      per_angle += 2;
      myReshape(g_windowWidth, g_windowHeight);
      glutPostRedisplay();

           }else if (k == '-'){
      per_angle -= 2;
      myReshape(g_windowWidth, g_windowHeight);
      glutPostRedisplay();

      }else if (k == 'm'){
       printModelViewMatrix();

       }else if (k == 'w'){
      b_wireFrame = !b_wireFrame;
      glutPostRedisplay();

      }else if (k == 'q'){
      b_useOpenGLtransform = !b_useOpenGLtransform;
      glutPostRedisplay();

       }else if (k == 'n'){
      g_model = (g_model + 1)%8;
      glutPostRedisplay();

      }else if (k == 't'){
      b_texture = !b_texture;
      glutPostRedisplay();

      }else if (k == 'l'){
      b_lighting = !b_lighting;
      glutPostRedisplay();
         }
         }    
}

if then else 语句仍将运行。我在使用开关时遇到的错误是：“double”之前的语法错误

'alpha' 未声明（在此函数中首次使用）

score 2 · Accepted Answer

你不能在 switch 语句中声明变量，除非你把它们放在一个块中（用包围{}）：

switch(k)
{
         case 'a' : b_animate = !b_animate;
                    if (b_animate)
                    {
                       glutTimerFunc(33, myTimer, 1);
                    }
                    break;
         case 'h' : b_showHints = !b_showHints;
                    glutPostRedisplay();
                    break;
         case 'f' : toggleFullScreen();
                    break;
         case 'o' : gi_projection_type = ORTHO_3D;
                    myReshape(g_windowWidth, g_windowHeight);
                    glutPostRedisplay();
                    break;
         case 'p' : gi_projection_type = PERSPECTIVE;
                    myReshape(g_windowWidth, g_windowHeight);
                    glutPostRedisplay();
                    break;
         case '1' :
         case '2' :
         case '3' :
         case '4' :
         case '5' : 
         case '6' :
         case '7' : 
         case '8' :
         case '9' : {                               // Note: begin of block
                    double alpha = (k - '0') * 0.1;
                    if (alpha == 0)
                    alpha = 1.0;
                    setAlphaChannel(alpha);   
                    glutPostRedisplay();     
                    break;
         }                                          // Note: end of block
         case '+' : per_angle += 2;
                    myReshape(g_windowWidth, g_windowHeight);
                    glutPostRedisplay();
                    break;
         case '-' : per_angle -= 2;
                    myReshape(g_windowWidth, g_windowHeight);
                    glutPostRedisplay();
                    break;
         case 'm' : printModelViewMatrix();
                    break;
         case 'w' : b_wireFrame = !b_wireFrame;
                    glutPostRedisplay();
                    break;
         case 'q' : b_useOpenGLtransform = !b_useOpenGLtransform;
                    glutPostRedisplay();
                    break;
         case 'n' : g_model = (g_model + 1)%8;
                    glutPostRedisplay();
                    break; 
         case 't' : b_texture = !b_texture;
                    glutPostRedisplay();
                    break;

         case 'l' : b_lighting = !b_lighting;
                    glutPostRedisplay();
                    break;

}

或者，alpha在 switch 语句之前声明。

score 2 · Accepted Answer

您已成为“一体式”功能设计的受害者。你函数的主体是不可读的混乱。

当然，你可以尽可能多地挖掘这个烂摊子，直到你修复它。但最好重新考虑整体设计。将其拆分为较小的功能，每个键一个：

case 'a' : OnKeyA(); break;
case 'h' : OnKeyH(x, y, z); break
case 'f' : OnKeyF(x); break;

....

void OnKeyA()
{
   b_animate = !b_animate;
   if (b_animate)
   {
       glutTimerFunc(33, myTimer, 1);
   }
}

不是更干净吗？案例语句成为单行语句，所有逻辑都转到函数中。你永远不会迷失在花括号中。

告诉你的导师“关注点分离”和“单一责任原则”，他会印象深刻的=）

快乐编码！

score 1 · Accepted Answer

每当您需要在案例中声明变量时，请将其放在大括号之间。-

case '9' : { 
    double alpha = (k - '0') * 0.1;
    if (alpha == 0)
        alpha = 1.0;
    setAlphaChannel(alpha);   
    glutPostRedisplay();     
    break; 
}

score 0 · Accepted Answer

您应该将案例处理程序包装在 {} 中，例如

         case '9' :
                 {
                    double alpha = (k - '0') * 0.1;
                    if (alpha == 0)
                    alpha = 1.0;
                    setAlphaChannel(alpha);   
                    glutPostRedisplay();     
                    break; 
                 }

因为只能在 switch-case 构造内的范围内声明新变量

score 0 · Accepted Answer

您需要为以下内容添加范围double：

     case '9' : {
                   double alpha = (k - '0') * 0.1;
                   if (alpha == 0)
                      alpha = 1.0;
                   setAlphaChannel(alpha);   
                   glutPostRedisplay();     
                }
                break;

你需要添加

     case '0':

到名单前啦！（你不小心只'1'需要'9'）

c++ - C++ switch 语句不起作用

5 回答 5

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