In a switch case statement, it says “duplicate case value” comes up as an error. Anyone know why?

问题

I am working on a rock paper scissors program, but this time the computer chooses rock half the time, scissors a third of the time, and paper only one sixth of the time. The way I did this was I enumerated six possible computer choice values:

enum choicec {rock1, rock2, rock3, scissors1, scissors2, paper};
choicec computer;

But then, after the computer makes its choice, I have to convert these enumerated values to either rock, paper, or scissors. I did this using a switch-case statement:

switch(computer) {
        case rock1 || rock2 || rock3:
            c = 1;
            break;
        case scissors1 || scissors2: //ERROR!
            c = 3;
            break;
        case paper:
            c = 2;
            break;
    }

one is rock, two is paper, and three is scissors. However, on the line where I have error written in as a comment, it gives me this error: [Error] duplicate case value.

I'm not sure why. Any ideas?

回答1:

I am not sure what you doing, but switch statement should look like this

switch(computer) 
{
    case rock1:
    case rock2:
    case rock3:
        c = 1;
        break;
    case scissors1:
    case scissors2:
        c = 3;
        break;
    case paper:
        c = 2;
        break;
}

回答2:

You can't use || in case branches. Sorry :(
When you use || it does a logical or on them, that says "is rock1 or rock2 or rock3 not a zero?". And the answer is yes, at least one of those is not zero. So rock1 || rock2 || rock3 is true, which is 1. And scissors1 || scissors is also true, which is 1. So you have two case branches for the 1 case.

You should simply use case fallthrough to select multiple conditions:

switch(computer) {
    case rock1: case rock2: case rock3:
        c = 1;
        break;
    case scissors1: case scissors2:
        c = 3;
        break;
    case paper:
        c = 2;
        break;
    default:
        std::cerr << "INVALID COMPUTER MOVE";
}

Also, I always have a default in my case switches. Sometimes mistakes happen, and we definitely want to know if it doesn't hit any of the case branches. I'm also pretty paranoid about missing else statements, but about half the time it's ok if there's no else.

回答3:

That switch statement does not do what you think.

Each case defines one value that the value of computer is matched against. Combining several values with logical disjunction to give the value associated with a single case label does not make the corresponding block be entered when the value of computer is equal to any of those values, but rather when it is equal to the result of their logical OR combination. Not very meaningful, indeed.

This is how you could rewrite your switch statement in order to make more sense:

switch(computer) {
    case rock1: // Is it rock1?
    case rock2: // Or perhaps rock2?
    case rock3: // Or maybe rock3?
        c = 1;  // Well, if it's one of the above, do this...
        break;
    case scissors1: // OK, it wasn't. So is it scissors1?
    case scissors2: // Or scissors2?
        c = 3;      // If it's one of the above, do this...
        break;
    case paper: // So is it paper?
        c = 2;
        break;
    default: // Always better to be explicit about this
        break;
}

回答4:

Change it to:

switch(computer) {
    case rock1:
    case rock2:
    case rock3:
        c = 1;
        break;
    case scissors1:
    case scissors2:
        c = 3;
        break;
    case paper:
        c = 2;
        break;
}

rock1 || rock2 || rock3 and scissors1 || scissors2 are both expressions which evaluate to "true", hence the conflict.

回答5:

The expression used in the switch statement must be integral type ( int, char and enum). In the Switch statement, all the matching case execute until a break statement is reached and Two case labels cannot have the same value.

But in the above case with logical or condition. At first case: rock1 || rock2 || rock3: This will evaluate to 1 and second case scissors1 || scissors2: will also evaluate to 1. This is cause error as said Two case labels cannot have the same value.

This is the reason compiler complains and giving an error:

Compiler Error: duplicate case value

To solve this convert to

switch(computer) {
        case rock1: 
        case rock2:  
        case rock3:
            c = 1;
            break;
        case scissors1:
        case scissors2: //Now will not give any error here...
            c = 3;
            break;
        case paper:
            c = 2;
            break;
    }

来源：https://stackoverflow.com/questions/17225023/in-a-switch-case-statement-it-says-duplicate-case-value-comes-up-as-an-error