Don't know how to access property from class method in Xcode - objective-c

Question

I want to access the property storedValue from the class method popOperandOffStack:, but I get an error. I read that you cannot access storedValue because it is a class method or something like that. Can anyone tell me how can I access storedValue or how can I make storedValue global so that I can use it in popOperandOffStack:?
Here is my header:
#import <Foundation/Foundation.h>
#interface CalculatorBrain : NSObject
- (void)pushOperand:(double)operand; // - = instance method
- (double)performOperation: (NSString *)operation;
#property (nonatomic,readonly) id program;
+ (double)runProgram:(id)program; // + = class method
+ (NSString *)descriptionOfProgram:(id)program;
#end
And this is my m file:
#import "CalculatorBrain.h"
#interface CalculatorBrain()
#property (nonatomic,strong) NSMutableArray *programStack;
#property (nonatomic) double storedValue;
#end
#implementation CalculatorBrain
#synthesize programStack = _programStack;
#synthesize storedValue = _storedValue;
- (NSMutableArray *)programStack
{
if (_programStack == nil)
_programStack = [[NSMutableArray alloc] init];
return _programStack;
}
- (void)setOperandStack:(NSMutableArray *)programStack
{
_programStack = programStack;
}
- (void)pushOperand:(double)operand
{
[self.programStack addObject:[NSNumber numberWithDouble:operand]];
}
- (double)performOperation: (NSString *)operation
{
[self.programStack addObject:operation];
return [[self class] runProgram:self.program];
}
- (id)program
{
return [self.programStack copy];
}
+ (NSString *)descriptionOfProgram:(id)program
{
return #"Implement this in Assignment 2";
}
+ (double) popOperandOffStack:(NSMutableArray *)stack
{
double result = 0;
id topOfStack = [stack lastObject];
if (topOfStack)
[stack removeLastObject];
if ([topOfStack isKindOfClass:[NSNumber class]])
{
result = [topOfStack doubleValue];
}
else if ([topOfStack isKindOfClass:[NSString class]])
{
NSString *operation = topOfStack;
if ([operation isEqualToString:#"+"]){
result = [self popOperandOffStack:stack] + [self popOperandOffStack:stack];
} else if ([operation isEqualToString:#"-"]){
result = -[self popOperandOffStack:stack] + [self popOperandOffStack:stack];
} else if ([operation isEqualToString:#"*"]){
result = [self popOperandOffStack:stack] * [self popOperandOffStack:stack];
} else if ([operation isEqualToString:#"/"]){
double divide = [self popOperandOffStack:stack];
if(divide){
result = [self popOperandOffStack:stack] / divide;
} else {
result = 0;
}
} else if ([operation isEqualToString:#"1/x"]){
result = 1 / [self popOperandOffStack:stack];
} else if ([operation isEqualToString:#"sqrt"]){
double sqrtNumber = [self popOperandOffStack:stack];
if (sqrtNumber > 0){
result = sqrt(sqrtNumber);
} else {
result = 0;
}
} else if ([operation isEqualToString:#"sin"]){
result = sin( [self popOperandOffStack:stack]);
} else if ([operation isEqualToString:#"cos"]){
result = cos( [self popOperandOffStack:stack]);
} else if ([operation isEqualToString:#"MS"]){
self.storedValue = [self popOperandOffStack:stack];
} else if ([operation isEqualToString:#"MR"]){
result = self.storedValue; //error here
} else if ([operation isEqualToString:#"MC"]){
result = [self popOperandOffStack:stack];
self.storedValue = 0; //error here
} else if ([operation isEqualToString:#"M+"]){
result = [self popOperandOffStack:stack];
self.storedValue = self.storedValue + result; //error here
} else if ([operation isEqualToString:#"M-"]){
result = [self popOperandOffStack:stack];
self.storedValue = self.storedValue - result; //error here
} else if ([operation isEqualToString:#"C"]){
result = 0;
while ( [self popOperandOffStack:stack] )
{
//clear operands until returns NO
}
} else if ([operation isEqualToString:#"CE"]){
result = 0;
self.storedValue = 0; //error here
while ( [self popOperandOffStack:stack] )
{
//clear operands until returns NO
}
}
if (![operation isEqualToString:#"MS"] && ![operation isEqualToString:#"CE"])
{
return result;
} else {
return self.storedValue;
}
}
return result;
}
+ (double)runProgram:(id)program
{
NSMutableArray *stack;
if ([program isKindOfClass:[NSArray class]]) {
stack = [program mutableCopy];
}
return [self popOperandOffStack:stack];
}
#end
I cannot use self.storedValue. What should I do besides changing the + (static method) to - (instance method) (I am not allowed to do that)?

Answer 1

You can't use instance getter/setter methods from inside a class method because you don't have an object that is an instance of CalculatorBrain. (I haven't used that course but I would hope that it explains the difference between classes and objects.)
Even though it's not a good idea from the point of view of creating an object-oriented program, you could use a static variable to hold your double. Look at a C language reference for the phrase "external static" to see examples.