enum in C++Permalink

RecallPermalink

• In an early discussion, we were trying to give driving direction, and there was a finite set of actions:
  • turn right
  • turn left
  • continue straight
  • arrive
• switch provided a convenient way to choose among specific, finite values
  • if...else also works, but switch is often more convenient for finite values

SwitchPermalink

int action = ...; 	
switch (action) {			
    case 0:					//turn right
    	Serial.println("Turn right (you can turn on red)");
    break; 
    case 1:					//turn left
    	Serial.println("Turn left (wait for arrow)");
    break;
    case 2:	 				//continue
    	Serial.println("Keep going straight");
    break;
    case 3:	 				//arrived
    	Serial.println("You have arrived!");
    break;
    default:
	    Serial.println("Error!");
}

Another improvementPermalink

• This was cleaning and easier to read
• However, the actions (or states) are still a little confusing
  • Ex: We need to remember that case 2 means continue straight

case 2:	 				//continue
	Serial.println("Keep going straight");
 

• These is another improvement we can make

enumPermalink

• enum is a user-defined data type
  • This means we get to decide what its values will be
• enum that allows us to give labels to literal (constant) values
• It can make our code much more self-explanatory and logical

enum SyntaxPermalink

enum <<enum_name>> { <<value1>>, <<value2>>, ...}

• enum is the type keyword
• <<enum_Name>> is a name we define (it will become like int or String)
• <<value1>> … <<value2>> are the allowed value we will let our variable take
• By default, C++ treats each of these values as starting at 0 and increasing by 1

RecallPermalink

int action = ...; 	
switch (action) {			
    case 0:					//turn right
    	Serial.println("Turn right (you can turn on red)");
    break; 
    case 1:					//turn left
    	Serial.println("Turn left (wait for arrow)");
    break;
    case 2:	 				//continue
    	Serial.println("Keep going straight");
    break;
    case 3:	 				//arrived
    	Serial.println("You have arrived!");
    break;
    default:
	    Serial.println("Error!");
}

enum ExamplePermalink

• There are four actions or states we need to represent
  • Turn left, Turn right, Go straight, arrive
• Let’s define our enum and call it Operation

enum Operation {left, right, straight, arrive};

• Operation value of left is equivalent to 0
• Operation value of right is equivalent to 1
• Operation value of straight is equivalent to 2
• Operation value of arrive is equivalent to 3

enum ExamplePermalink

• Now we can create a Operation variable

Operation action;

• action would be given a value from some function we write
• You could also assign a literal value to action like this

Operation action = straight;	//equivalent to int value of 2

• We can now use our Operation value to control the switch

enum ExamplePermalink

Operation action = ...; 	
switch (action) {			
    case right:					
    	Serial.println("Turn right (you can turn on red)");
    break; 
    case left:
    	Serial.println("Turn left (wait for arrow)");
    break;
    case straight:
    	Serial.println("Keep going straight");
    break;
    case arrive:
    	Serial.println("You have arrived!");
    break;
    default:
	    Serial.println("Error!");
}

Tip: Converting String to enumPermalink

• If you have an int stored as a String but want to use enum for your a switch statement, first covert to an int with toInt() and then cast to enum

enum Operation {left, right, straight, arrive};
String opStr = "0";		//this often comes from an event handler
int opInt = opStr.toInt(); 	//int 0
Operation op = (Operation) opInt;	//Operation LEFT