Class methods, saving state, a better way to register functions - objective-c

Question

I'm working on an assignment which involves making an RPN calculator. I am currently using class methods to check whether a string is an operation as below:
+ (NSSet *) noOpOperations {
return [NSSet setWithObjects:#"π", nil];
}
+ (NSSet *) unaryOperations {
return [NSSet setWithObjects:#"sin",#"cos",#"log",#"+/-", nil];
}
+ (NSSet *) binaryOperations {
return [NSSet setWithObjects:#"+",#"-",#"*",#"/", nil];
}
+ (NSSet *) operations {
/* surely there is a better way - is it possible to save this call and reuse it? Or what about having the objects register themselves so you can add operations more easily? */
return [[self noOpOperations] setByAddingObjectsFromSet:
[[self unaryOperations] setByAddingObjectsFromSet:
[self binaryOperations]]];
}
+ (BOOL) isOperation:operand {
return [[self operations] containsObject:operand];
}
I believe it would be better to implement a kind of function registry system to allow dynamic adding of operations from another location in the project, but I think it would require a class variable. Is there a better way to do this than how I am doing it now?

Answer 1

My Personal solution for situations like this:
#define INT_OBJ(x) [NSNumber numberWithInt:x]
#implementation MyClass
static NSDictionary *operations;
enum {
kOperationNoOp = 1,
kOperationUnaryOp,
kOperationBinaryOp,
};
+(void) initialize
{
operations = [NSDictionary dictionaryWithObjectsAndKeys:#"π", INT_OBJ(kOperationNoOp),
// unary operations
#"sin", INT_OBJ(kOperationUnaryOp),
#"cos", INT_OBJ(kOperationUnaryOp),
#"log", INT_OBJ(kOperationUnaryOp),
#"+/-", INT_OBJ(kOperationUnaryOp),
// binary operations
#"+", INT_OBJ(kOperationBinaryOp),
#"-", INT_OBJ(kOperationBinaryOp),
#"*", INT_OBJ(kOperationBinaryOp),
#"/", INT_OBJ(kOperationBinaryOp), nil];
}
-(BOOL) isNoOpOperation:(NSString *) arg
{
return [[operations objectForKey:arg] intValue] == kOperationNoOp;
}
-(BOOL) isUnaryOperation:(NSString *) arg
{
return [[operations objectForKey:arg] intValue] == kOperationUnaryOp;
}
-(BOOL) isBinaryOperation:(NSString *) arg
{
return [[operations objectForKey:arg] intValue] == kOperationBinaryOp;
}
-(BOOL) isAnOperation:(NSString *) arg
{
// if objectForKey: returns nil, intValue will return 0, telling us that the input is not an operation
return [[operations objectForKey:arg] intValue] != 0;
}
#end
I find it to be very straightforward and easy to extend.