switch and case - D Programming Language
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# switch and case

`switch` is a statement that allows comparing the value of an expression against multiple values. It is similar to but not the same as an "if, else if, else" chain. `case` is used for specifying the values that are to be compared with `switch`'s expression. `case` is a part of the `switch` statement, not a statement itself.

`switch` takes an expression within parentheses, compares the value of that expression to the `case` values, and executes the operations of the `case` that is equal to the value of the expression. Its syntax consists of a `switch` block that contains one or more `case` sections and a `default` section:

        switch (expression) {

        case value_1:
            // operations to execute if the expression is equal to value_1
            // ...
            break;

        case value_2:
            // operations to execute if the expression is equal to value_2
            // ...
            break;

        // ... other cases ...

        default:
            // operations to execute if the expression is not equal to any case
            // ...
            break;
        }

The expression that `switch` takes is not used directly as a logical expression. It is not evaluated as "if this condition is true", as it would be in an `if` statement. The value of the `switch` expression is used in equality comparisons with the `case` values. It is similar to an "if, else if, else" chain that has only equality comparisons:

        auto value = expression;

        if (value == value_1) {
            // operations for value_1
            // ...

        } else if (value == value_2) {
            // operations for value_2
            // ...
        }

        // ... other 'else if's ...

        } else {
            // operations for other values
            // ...
        }

However, the "if, else if, else" above is not an exact equivalent of the `switch` statement. The reasons why will be explained in the following sections.

If a `case` value matches the value of the `switch` expression, then the operations that are under the `case` are executed. If no value matches, then the operations that are under the `default` are executed.

##### The `goto` statement

The use of `goto` is generally advised against in most programming languages. However, `goto` is useful in `switch` statements in some situations, without being as problematic as in other uses. The `goto` statement will be covered in more detail in a later chapter.

`case` does not introduce a scope as the `if` statement does. Once the operations within an `if` or `else` scope are finished the evaluation of the entire `if` statement is also finished. That does not happen with the `case` sections; once a matching `case` is found, the execution of the program jumps to that `case` and executes the operations under it. When needed in rare situations, `goto case` makes the program execution jump to the next `case`:

        switch (value) {

        case 5:
            writeln("five");
            goto case;    // continues to the next case

        case 4:
            writeln("four");
            break;

        default:
            writeln("unknown");
            break;
        }

If `value` is 5, the execution continues under the `case 5` line and the program prints "five". Then the `goto case` statement causes the execution to continue to the next `case`, and as a result "four" is also printed:

    five
    four

`goto` can appear in three ways under `case` sections:

- `goto case` causes the execution to continue (fallthrough) to the next `case`.
- `goto default` causes the execution to continue to the `default` section.
- `goto case expression` causes the execution to continue to the `case` that matches that expression.

The following program demonstrates these three uses by taking advantage of a `foreach` loop:

    import std.stdio;

    void main() {
        foreach (value; [ 1, 2, 3, 10, 20 ]) {
            writefln("-- value: %s --", value);

            switch (value) {

            case 1:
                writeln("case 1");
                goto case;

            case 2:
                writeln("case 2");
                goto case 10;

            case 3:
                writeln("case 3");
                goto default;

            case 10:
                writeln("case 10");
                break;

            default:
                writeln("default");
                break;
            }
        }
    }

The output:

    -- value: 1 --
    case 1
    case 2
    case 10
    -- value: 2 --
    case 2
    case 10
    -- value: 3 --
    case 3
    default
    -- value: 10 --
    case 10
    -- value: 20 --
    default

##### The expression must be an integer, string, or `bool` type

Any type can be used in equality comparisons in `if` statements. On the other hand, the type of the `switch` expression is limited to all integer types, all string types, and `bool`.

        string op = /* ... */;
        // ...
        switch (op) {

        case "add":
            result = first + second;
            break;

        case "subtract":
            result = first - second;
            break;

        case "multiply":
            result = first * second;
            break;

        case "divide":
            result = first / second;
            break;

        default:
            throw new Exception(format("Unknown operation: %s", op));
        }

Note: The code above throws an exception when the operation is not recognized by the program. We will see exceptions in a later chapter.

Although it is possible to use `bool` expressions as well, because `bool` has only two values it may be more suitable to use an `if` statement or the ternary operator (`?:`) with that type.

##### Value ranges

Ranges of values can be specified by `..` between `case`s:

        switch (dieValue) {

        case 1:
            writeln("You won");
            break;

        case 2: .. case 5:
            writeln("It's a draw");
            break;

        case 6:
            writeln("I won");
            break;

        default:
            /* The program should never get here because the cases
             * above cover the entire range of valid die values.
             * (See 'final switch' below.) */
            break;
        }

The code above determines that the game ends in a draw when the die value is 2, 3, 4, or 5.

##### Distinct values

Let's assume that it is a draw for the values 2 and 4, rather than for the values that are in the range [2, 5]. Distinct values of a `case` are separated by commas:

        case 2, 4:
            writeln("It's a draw");
            break;

##### The `final switch` statement

The `final switch` statement works similarly to the regular `switch` statement, with the following differences:

- It cannot have a `default` section. Note that this section is meaningless when the `case` sections cover the entire range of values anyway, as has been with the six values of the die above.
- Value ranges cannot be used with `case`s (distinct values can be).
- If the expression is of an `enum` type, all of the values of the type must be covered by the `case`s (we will see `enum` types in the next chapter).

        int dieValue = 1;

        final switch (dieValue) {

        case 1:
            writeln("You won");
            break;

        case 2, 3, 4, 5:
            writeln("It's a draw");
            break;

        case 6:
            writeln("I won");
            break;
        }

##### When to use

`switch` is suitable for comparing the value of an expression against a set of values that are known at compile time.

When there are only two values to compare, an `if` statement may make more sense. For example, to check whether it is heads or tails:

        if (headsTailsResult == heads) {
            // ...

        } else {
            // ...
        }

As a general rule, `switch` is more suitable when there are three or more values to compare.

When all of the values need to be handled, then prefer `final switch`. This is especially the case for `enum` types.

##### Problemler

1. Write a calculator program that supports arithmetic operations. Have the program first read the operation as a `string`, then two values of type `double` from the input. The calculator should print the result of the operation. For example, when the operation and values are "add" and "5 7", respectively, the program should print 12.

   The input can be read as in the following code:

           string op;
           double first;
           double second;

           // ...

           op = strip(readln());
           readf(" %s %s", &first, &second);

2. Improve the calculator to support operators like "+" in addition to words like "add".
3. Have the program throw an exception for unknown operators. We will cover exceptions in a later chapter. For now, adapt the `throw` statement used above to your program.

... çözümler

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