gistfile1.m

+ (double)popOperandOffProgramStack:(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 popOperandOffProgramStack:stack] +
            [self popOperandOffProgramStack:stack];
        } 
        else if ([@"*" isEqualToString:operation]) 
        {
            result = [self popOperandOffProgramStack:stack] *
            [self popOperandOffProgramStack:stack];
        } 
        else if ([operation isEqualToString:@"-"]) 
        {
            double subtrahend = [self popOperandOffProgramStack:stack];
            result = [self popOperandOffProgramStack:stack] - subtrahend;
        }
        else if ([operation isEqualToString:@"/"])
        {
            double divisor = [self popOperandOffProgramStack:stack];
            if (divisor) result = [self popOperandOffProgramStack:stack] / divisor;
        }
        else if ([operation isEqualToString:@"sin"])
        {
            result = sin([self popOperandOffProgramStack:stack]);
        }
        else if ([operation isEqualToString:@"cos"])
        {
            result = cos([self popOperandOffProgramStack:stack]);
        }
        else if ([operation isEqualToString:@"sqrt"])
        {
            result = sqrt([self popOperandOffProgramStack:stack]);
        }    
        else if ([operation isEqualToString:@"π"])
        {
            result = M_PI * [self popOperandOffProgramStack:stack];
        }
    }
    
    return result;
}