C++ 在 if-else 语句中递增全局计数器变量

Question

所以我的总体目标是制作一个程序来显示与用户输入的波长数相关的颜色（400 到 445 是紫色等）。输入需要在 400 到 700 之间，并让用户知道输入是高还是低。我还需要实现一个全局计数器变量，它将计数并显示在用户输入和颜色范围之间执行的比较量（对于它通过的每个 else 语句，递增 1）。这是我到目前为止所拥有的。 代码：

#include <iostream>
using namespace std;
int g_counter;

int main()
{
g_counter = 0;
int userinput;

cout<< "please enter wavelength between 400 and 700: "<< endl;
cin>>userinput;
++g_counter;
if (userinput < 400){
    cout << "Wavelength to small, try again"<< endl;
    
    cout << "You evauluated this many conditions: "<< g_counter << endl; 
}
++g_counter;
else (userinput >= 400 && userinput <=445){
    cout<<  "Your wavelength corresponds to the color violet"<< endl;

    cout << "You evauluated this many conditions: "<< g_counter << endl; 
}
++g_counter;
else (userinput > 445 && userinput <= 475){
    cout<< "Your wavelength corresponds to the color blue"<< endl;
    
    cout << "You evauluated this many conditions: "<< g_counter << endl; 
}
++g_counter;
else (userinput > 475 && userinput <= 510){
    cout<< "Your wavelength corresponds to the color green"<< endl;
    
    cout << "You evauluated this many conditions: "<< g_counter << endl; 
}
++g_counter;
else (userinput > 510 && userinput <= 570){
    cout<< "Your wavelength corresponds to the color Yellow"<< endl;

    cout << "You evauluated this many conditions: "<< g_counter << endl; 
}
++g_counter;
else (userinput > 570 && userinput <= 590){
    cout<< "Your wavelength corresponds to the color Orange"<< endl;
    
    cout << "You evauluated this many conditions: "<< g_counter << endl; 
}
++g_counter;
else ( userinput > 590 && userinput <= 700){
    cout<< "Your wavelength corresponds to the color red"<< endl;
    
    cout << "You evauluated this many conditions: "<< g_counter << endl; 
}
++g_counter;
else ( userinput > 700 ){
    cout<< "wavelength to large, try again"<< endl;

    cout << "You evauluated this many conditions: "<< g_counter << endl; 
}

return 0;
}

我的问题是我无法理解编译时遇到的错误。 错误：

Homework2.cpp: In function ‘int main()’:
Homework2.cpp:19:1: error: ‘else’ without a previous ‘if’
 else (num >= 400 && num <=445){
 ^
Homework2.cpp:19:31: error: expected ‘;’ before ‘{’ token
 else (num >= 400 && num <=445){
                               ^
Homework2.cpp:26:1: error: ‘else’ without a previous ‘if’
 else (num > 445 && num <= 475){
 ^
Homework2.cpp:26:31: error: expected ‘;’ before ‘{’ token
 else (num > 445 && num <= 475){
                               ^
Homework2.cpp:32:1: error: ‘else’ without a previous ‘if’
 else (num > 475 && num <= 510){
 ^
Homework2.cpp:32:31: error: expected ‘;’ before ‘{’ token
 else (num > 475 && num <= 510){
                               ^
Homework2.cpp:38:1: error: ‘else’ without a previous ‘if’
 else (num > 510 && num <= 570){
 ^
Homework2.cpp:38:31: error: expected ‘;’ before ‘{’ token
 else (num > 510 && num <= 570){
                               ^
Homework2.cpp:44:1: error: ‘else’ without a previous ‘if’
 else (num > 570 && num <= 590){
 ^
Homework2.cpp:44:31: error: expected ‘;’ before ‘{’ token
 else (num > 570 && num <= 590){
                               ^
Homework2.cpp:50:1: error: ‘else’ without a previous ‘if’
 else ( num > 590 && num <= 700){
 ^
Homework2.cpp:50:32: error: expected ‘;’ before ‘{’ token
 else ( num > 590 && num <= 700){
                                ^
Homework2.cpp:56:1: error: ‘else’ without a previous ‘if’
 else ( num > 700 ){
 ^
Homework2.cpp:56:19: error: expected ‘;’ before ‘{’ token
 else ( num > 700 ){
                   ^

例如，错误“Homework2.cpp:19:1: error: ‘else’ without a previous ‘if’”对我来说没有意义，因为之前存在 if。此外，即使它实际上检查了更多比较，全局计数器（当程序在我搞砸之前工作时）仍然给出 1 的答案。任何关于这些问题的指示都会很棒。

Answer 1

您的问题是 if/else 构造应该如下所示：

if (...) 
{ ... /* then case code */ }
else 
{ ... /* else case code */ }

“then case 代码”末尾的“}”和开始 else 分支的“else”之间没有任何内容。

在上述情况下，这两个组件之间有一个“g_counter++”，它不会形成有效的 if/else 构造。

Answer 2

else 命令必须立即 以if(){} 块开头。在 betwwen 中设置 g_counter 会使 next else 出错。

要计算比较，您可以使用comma operator，它返回(E1,E2) 中E2 的评估结果

所以你可以这样编码：

if (++g_counter, userinput < 400){
    cout << "Wavelength to small, try again"<< endl;
    
    cout << "You evauluated this many conditions: "<< g_counter << endl; 
}

else if ((++g_counter, userinput >= 400) && (++g_counter, userinput <=445)){
    cout<<  "Your wavelength corresponds to the color violet"<< endl;

    cout << "You evauluated this many conditions: "<< g_counter << endl; 
}

else....

注意使用&&时的括号（两个比较）