C++ 如何在 switch 语句中选择一系列值？
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/9432226/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
How do I select a range of values in a switch statement?
提问by Robin Van den Broeck
When I try to compile I get this error:
当我尝试编译时，出现此错误：
1>------ Build started: Project: snake, Configuration: Debug Win32 ------ 1> exercise.cpp 1>c:\users\robin\documents\visual studio 2010\projects\snake\snake\exercise.cpp(13): error C2059: syntax error : '>=' 1>c:\users\robin\documents\visual studio 2010\projects\snake\snake\exercise.cpp(16): error C2059: syntax error : '>=' 1>c:\users\robin\documents\visual studio 2010\projects\snake\snake\exercise.cpp(19): error C2059: syntax error : '>=' 1>c:\users\robin\documents\visual studio 2010\projects\snake\snake\exercise.cpp(22): error C2059: syntax error : '>=' 1>c:\users\robin\documents\visual studio 2010\projects\snake\snake\exercise.cpp(25): error C2059: syntax error : '>' 1>c:\users\robin\documents\visual studio 2010\projects\snake\snake\exercise.cpp(28): error C2059: syntax error : '==' 1>c:\users\robin\documents\visual studio 2010\projects\snake\snake\exercise.cpp(34): warning C4065: switch statement contains 'default' but no 'case' labels ========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========
Code:
代码：
#include <iostream>
using namespace std;
int main(){
int score;
//Vraag de score
cout << "Score:";
cin >> score;
//Switch
switch(score){
case >= 100:
cout << "a";
break;
case >= 50:
cout << "b";
break;
case >= 25:
cout << "c";
break;
case >= 10:
cout << "d";
break;
case > 0:
cout << "e";
break;
case == 0:
cout << "f";
break;
default:
cout << "BAD VALUE";
break;
}
cout << endl;
return 0;
}
How can I fix this problem? It's a console application, Win32 and my IDE is Windows Enterprise C++ 2010.
我该如何解决这个问题？这是一个控制台应用程序，Win32，我的 IDE 是 Windows Enterprise C++ 2010。
I'm learning from Beginning C++ Through Game Programming.
我正在从游戏编程开始学习C++。
回答by Ankit Patel
Some compilers support case rangeslike case x ... yas an extensionto the C++ language.
一些编译器支持大小写范围，例如case x ... y作为C++ 语言的扩展。
Example:
例子：
#include <iostream>
using namespace std;
int main(){
int score;
//Vraag de score
cout << "Score:";
cin >> score;
//Switch
switch(score){
case 0:
cout << "a";
break;
case 0 ... 9:
cout << "b";
break;
case 11 ... 24:
cout << "c";
break;
case 25 ... 49:
cout << "d";
break;
case 50 ... 100:
cout << "e";
break;
default:
cout << "BAD VALUE";
break;
}
cout << endl;
return 0;
}
GCC 4.9, Clang 3.5.1 and Intel C/C++ Compiler 13.0.1 seem to support it (tried on http://gcc.godbolt.org/). On the other hand, Visual C++ 19 doesn't (tried on http://webcompiler.cloudapp.net/).
GCC 4.9、Clang 3.5.1 和 Intel C/C++ Compiler 13.0.1 似乎支持它（在http://gcc.godbolt.org/ 上尝试过）。另一方面，Visual C++ 19 没有（在http://webcompiler.cloudapp.net/ 上尝试过）。
回答by dasblinkenlight
In C++ case labels are constantexpressions, not expressions in general. You need a chain of if-then-else statements to do what you are trying to do.
在 C++ 情况下，标签是常量表达式，而不是一般的表达式。您需要一串 if-then-else 语句来完成您想要做的事情。
Alternatively, you can enumerate the values in the switch. This runs marginally faster (though it does not matter in cases like yours), but it is considerably less readable:
或者，您可以枚举开关中的值。这运行速度稍快（尽管在像您这样的情况下无关紧要），但可读性要低得多：
switch(score) {
case 0: cout << "f"; break;
case 1:
case 2:
case 3:
case 4:
case 5:
case 6:
case 7:
case 8:
case 9:
case 10: cout << "e"; break;
case 11:
case 12:
case 13:
case 14:
case 15:
case 16:
case 17:
case 18:
case 19:
case 20:
case 21:
case 22:
case 23:
case 24:
case 25: cout << "c"; break;
// ...and so on, you get the idea...
}
回答by mah
You can fix this problem by using a series of if/else ifstatements. Switch/case cannot be used like this in C++.
您可以使用一系列if/else if语句来解决此问题。在 C++ 中不能像这样使用 switch/case。
回答by Slava
It can be done using a std::mapwith switch:
可以使用std::mapwith来完成switch：
enum Interval {
One,
Two,
Three,
NotFound };
// [0,10[ is One, [10,30[ is Two, [30,55[ is Three
std::map<int,Interval> imap {
{ { 0, One },
{ 10, Two },
{ 30, Three },
{ 55, NotFound } };
Interval ivalue = NotFound;
auto f = imap.lower_bound( value );
if( f != imap.end() ) ivalue = f->second;
switch( ivalue ) {
case One : ...
case Two : ...
case Three : ...
default: ...
}
回答by Fifi
Switch-caseis not a great option for testing ranges. The best option is to use several if:
Switch-case不是测试范围的好选择。最好的选择是使用几个if：
if (score<0) cout << "BAD VALUE";
if (score == 0) cout << "f";
if (score>0 && score<10) cout << "e";
if (score>=10 && score <25) cout << "d";
if (score>=25 && score <50) cout << "c";
if (score>=50 && score <100) cout << "b";
If running time is an issue, the following solution is faster :
如果运行时间是一个问题，以下解决方案会更快：
if (score == 0) cout << "f";
else if (score<10) cout << "e";
else if (score <25) cout << "d";
else if (score <50) cout << "c";
else if (score <100) cout << "b";
else if (score>=100) cout << "a";
else cout << "BAD VALUE";
回答by BoBTFish
In C++ a switch statement can only match constant integer values:
在 C++ 中，switch 语句只能匹配常量整数值：
switch (i)
{
case 1:
//... stuff
break;
case 2:
//... stuff
break;
default:
//... stuff
}
回答by Janus Troelsen
There's a GCC extensionthat does exactly what you want.
有一个GCC 扩展可以完全满足您的需求。
回答by Sebastian Mach
The standard does not allow for this:
该标准不允许这样做：
6.4.2 The switch statement [stmt.switch]
[...] Any statement within the switch statement can be labeled with one or more case labels as follows:
case constant-expression :where the constant-expression shall be an integral constant expression (5.19).
6.4.2 switch 语句[stmt.switch]
[...] switch 语句中的任何语句都可以用一个或多个 case 标签进行标记，如下所示：
case constant-expression :其中，constant-expression 应为整数常量表达式 (5.19)。
In other words, you can only use case-values that expand into a single, integral, "hard" compile time constant (e.g. case 5+6:, enum {X = 3}; ... case X*X:).
换句话说，您只能使用扩展为单个、整数、“硬”编译时间常量（例如case 5+6:, enum {X = 3}; ... case X*X:）的大小写值。
The way around this is to use if-statements. E.g., to replace
解决这个问题的方法是使用if-statements。例如，替换
switch (x)
case 0..100:
you'd instead
你反而
if (x>=0 && x<=100)
.
.
回答by Bubu
I had the same problem with a score based problem and while the " if/elseif "statements were good to use, for intervals i found that the best option (for me at least because i like how it looks and it's easier for me as a beginner to see my mistakes) is " 1 ... 10 ". but don't forget to use a space between the number and the dots or the program will think that your interval is a number and u will get an error "2 many decimal dots...". Hope it helps.
我在基于分数的问题上遇到了同样的问题，虽然“if/elseif”语句很好用，但对于间隔我发现这是最好的选择（至少对我来说是因为我喜欢它的外观，而且对我来说更容易初学者看到我的错误）是“ 1 ... 10 ”。但不要忘记在数字和点之间使用一个空格，否则程序会认为您的间隔是一个数字，你会得到一个错误“2 个小数点...”。希望能帮助到你。
int score;
int main()
{
cout<<"Enter score"<<endl;
cin>>score;
switch(score){
case 100:
cout<<"Your score is Perfect"<<endl;
break;
case 90 ... 99:
cout<<"You got A"<<endl;
break;
case 80 ... 89:
cout<<"You got B"<<endl;
break;
case 70 ... 79:
cout<<"You got C"<<endl;
break;
case 60 ... 69:
cout<<"You got D"<<endl;
break;
case 50 ... 59:
cout<<"You got E"<<endl;
break;
case 0 ... 49:
cout<<"You got F"<<endl;}
}
回答by KitsuneYMG
That's simply not how switch works. It only takes single values. You'll have to use if-elseif blocks
这根本不是 switch 的工作方式。它只需要单个值。你必须使用 if-elseif 块
