gistfile1.m

// Assignment 2, Problem 2
+ (NSString *)descriptionOfProgram:(id)program
{
    NSMutableArray *stack;
    if ([program isMemberOfClass:[NSArray class]])
    {
        stack = [program mutableCopy];
    }
    
    return [self descriptionOfTopOfStack:stack];
}

+ (NSString *)descriptionOfTopOfStack:(NSMutableArray *)stack
{
    // recursive code to describe each object on the program stack goes here
    NSString *descriptionOfTopOfStack;
    NSString *operation;
    
    id topOfStack = [stack lastObject];
    if (topOfStack) [stack removeLastObject];
    
    if ([topOfStack isKindOfClass:[NSNumber class]])
    {
        descriptionOfTopOfStack = [topOfStack stringValue];
    }
    else if ([topOfStack isKindOfClass:[NSString class]])
    {
        if ([self isOperation:topOfStack])
        {
            operation = [topOfStack stringValue];
            
            if ([[self isKindOfOperation:topOfStack] isEqualToString:@"two-operand operation"])
            {   
                // should display all multi-operand operations using “infix” notation if appropriate, else function notation. For example, 3 Enter 5 + should display as 3 + 5.
                descriptionOfTopOfStack = [[[self descriptionOfTopOfStack:stack] stringByAppendingString:operation] stringByAppendingString:[self descriptionOfTopOfStack:stack]];
            }
            else if ([[self isKindOfOperation:topOfStack] isEqualToString:@"single-operand operation"])
            {
                // should display all single-operand operations using “function” notation. For example, 10 sqrt should display as sqrt(10).
                descriptionOfTopOfStack = [[[operation stringByAppendingString:@"("] stringByAppendingString:[self descriptionOfTopOfStack:stack]] stringByAppendingString:@")"];
            }
            else if ([[self isKindOfOperation:topOfStack] isEqualToString:@"no-operand operation"])
            {
                // no-operand operations, like π, should appear unadorned. For example, π.
                descriptionOfTopOfStack = [topOfStack stringValue];
            }
        }
        else // topOfStack must be a variable
        {
            // Variables (Required Task #1) should also appear unadorned. For example, x.
            descriptionOfTopOfStack = [topOfStack stringValue];
        }
    }

    
    return descriptionOfTopOfStack;
}

+ (BOOL)isOperation:(NSString *)operation
{
    if ([operation isEqualToString:@"+"] ||
        [operation isEqualToString:@"-"] ||
        [operation isEqualToString:@"*"] ||
        [operation isEqualToString:@"/"] ||
        [operation isEqualToString:@"sin"] ||
        [operation isEqualToString:@"cos"] ||
        [operation isEqualToString:@"sqrt"] ||
        [operation isEqualToString:@"π"])
    {
        return YES;
    }
    else return NO;
}

+ (NSString *)isKindOfOperation:(NSString *)operation
{
    NSString *kindOfOperation = @"no operation";
    // code to determine whether a given string on the stack is...
    if ([self isOperation:operation])
         {
             kindOfOperation = @"no-operand operation";
             
             if ([operation isEqualToString:@"+"] ||
                 [operation isEqualToString:@"-"] ||
                 [operation isEqualToString:@"*"] ||
                 [operation isEqualToString:@"/"])
             {
                 kindOfOperation = @"two-operand operation";
             }
             else if ([operation isEqualToString:@"sin"] ||
                      [operation isEqualToString:@"cos"] ||
                      [operation isEqualToString:@"sqrt"])
             {
                 kindOfOperation = @"single-operand operation";
             }
         }    
    return kindOfOperation;
}