# switch proc

Source: https://ref.dm-lang.org/proc/switch

### Format
- `if(A1,A2,...) Statement1`
- `if(B1,B2,...) Statement1`
- `else Statement3`


The "switch" instruction is a compact notation for a lengthy "else-if" chain. The expression E is compared to the values A1, A2, B1, B2, etc. When a match is found, the following statement (or code block) is executed. An optional "else" statement is run if no match is found. Once a matching switch condition is found, no further conditions will be tested.

The values A1, A2, etc. must be constants. As a convenience, a range of values may be specified in the form: A1 to An.

The switch instruction is MUCH more efficient than a lengthy "else-if" chain, because the expression E is evaluated only once. The conditional values may be any constant expression, such as a number or text string.

```dm /switch/
switch (2)
  if(1)       world << "ONE"
  if(4)       world << "FOUR"
  if(2,3)     world << "TWO or THREE"
  if(5 to 10) world << "FIVE to TEN"
  else        world << "not ONE to TEN"
```

This outputs:

```dm /switch/
TWO or THREE
```

> [!note]
> 
> Currently the compiler does not throw a warning or error if there is a conflict between two different `if` blocks in a `switch`, e.g. when you define `if(1 to 10)` and `if(5 to 20)` which overlap from 5 to 10. If two different blocks could handle a given value, the choice of which block takes over is not defined.

## C-like syntax

Using the [`#pragma syntax` directive](https://ref.dm-lang.org/DM/preprocessor/pragma/syntax.md) you can change `switch()` to be more like other languages such as C. This replaces the if/else instructions with `case *value*` and `default` with a trailing colon. If you don't use the [`break` statement](https://ref.dm-lang.org/proc/break.md) at the end of a block, it will fall through to the next block.

```dm /switch/
// make this syntax change temporary
#pragma push
#pragma syntax C switch

mob/proc/Greeting(friend)
    switch(roll(6))
        case 4:
            friend << "I'm [name]. \..."
            // fall through to cases 1 and 2
        case 1,2:
            friend << "Hi!"
            break
        case 3:
            friend << "Hello there!"
        default:
            friend << "Yo."
            break

#pragma pop
```

Multiple cases can be put together with either commas between the values, or separate `case` statements. The `*A* to *B*` syntax is still allowed also.

### See also
- [if proc](https://ref.dm-lang.org/proc/if.md)
- [#pragma syntax directive](https://ref.dm-lang.org/DM/preprocessor/pragma/syntax.md)