How do use a Switch Case Statement in Dart

Question

I am trying to understand how the switch is working in the dart. I have very simple code:

methodname(num radians) {   switch (radians) {     case 0:       // do something       break;     case PI:       // do something else       break;   } } 

This unfortunately does not work. If left like this the error is: case expressions must have the same type (I think the type is num, but not the editor). If I change 0 to 0.0 it says: The switch type expression double cannot override == operator - I have no idea what this means!

So what is the way to do this switch case? I can turn it onto if/else probably but I wanted to know how to make the switch work and why is it not working in the first place.

I am running the latest stable version of DartEditor.

Answer 1

The comparsion of double values using '==' is not very reliable and should be avoided (not only in Dart but in most languages).

You could do something like

methodname(num radians) {   // you can adjust this values according to your accuracy requirements   const myPI = 3142;    int r = (radians * 1000).round();    switch (r) {     case 0:       // do something       break;     case myPI:        // do something else       break;   } } 

This question contains some additional information that might interest you

• comparing float/double values using == operator
• How should I do floating point comparison?

some more information:

• https://www.dartlang.org/docs/spec/latest/dart-language-specification.html#h.50ae78s6gbw2
• http://floating-point-gui.de/errors/comparison/

Answer 2

Switch statements in Dart compare integer, string, or compile-time constants using ==. The compared objects must all be instances of the same class (and not of any of its subtypes), and the class must not override ==. Enumerated types work well in switch statements.

Each non-empty case clause ends with a break statement, as a rule. Other valid ways to end a non-empty case clause are a continue, throw, or return statement.

Use a default clause to execute code when no case clause matches:

var command = 'OPEN'; switch (command) {   case 'CLOSED':     executeClosed();     break;   case 'PENDING':     executePending();     break;   case 'APPROVED':     executeApproved();     break;   case 'DENIED':     executeDenied();     break;   case 'OPEN':     executeOpen();     break;   default:     executeUnknown(); } 

The following example omits the break statement in a case clause, thus generating an error:

var command = 'OPEN'; switch (command) {   case 'OPEN':     executeOpen();     // ERROR: Missing break    case 'CLOSED':     executeClosed();     break; } 

However, Dart does support empty case clauses, allowing a form of fall-through:

var command = 'CLOSED'; switch (command) {   case 'CLOSED': // Empty case falls through.   case 'NOW_CLOSED':     // Runs for both CLOSED and NOW_CLOSED.     executeNowClosed();     break; } 

If you really want fall-through, you can use a continue statement and a label:

var command = 'CLOSED'; switch (command) {   case 'CLOSED':     executeClosed();     continue nowClosed;   // Continues executing at the nowClosed label.    nowClosed:   case 'NOW_CLOSED':     // Runs for both CLOSED and NOW_CLOSED.     executeNowClosed();     break; } 

A case clause can have local variables, which are visible only inside the scope of that clause.