public static string GetBooksByCategory(BookShopContext context, string input)
        {
            //Return in a single string the titles of books by a given list of categories.
            //Order by title alphabetically.
            string[] categories = input.Split(' ');
            //------------------------------------------------------------------
            StringBuilder output = new StringBuilder();
            Category[] bCaterogies = new Category[categories.Length];
            for (int i = 0; i < categories.Length; i++) { bCaterogies[i].Name = categories[i]; }
            BookCategory[] addedCategories = new BookCategory[bCaterogies.Length];
            for (int i = 0; i < bCaterogies.Length; i++) { addedCategories[i].Category = bCaterogies[i]; } //add the string categories in BookCategory array to compare
             //------------------------------------------------------------------
            string[] bookTitles = context
                .Books

                 .Where(book => book.BookCategories.ToArray() == addedCategories )

                .Select(book => book.Title)
                .OrderBy(book => book)
                .ToArray();

                foreach(string title in bookTitles)
            {
                output.AppendLine(title);
            }

            return output.ToString().TrimEnd();
        }

 //problem 6
        public static string GetBooksByCategory(BookShopContext context, string input)
        {
            StringBuilder output = new StringBuilder();
            //Return in a single string the titles of books by a given list of categories.
            //Order by title alphabetically.
            string[] categories = input.Split(' ');

            string[] bookTitles = context
                .Books

                 .Where(book => categories.Contains(book.BookCategories.ToString()))

                .Select(book => book.Title)
                .OrderBy(book => book)
                .ToArray();

                foreach(string title in bookTitles)
            {
                output.AppendLine(title);
            }

            return output.ToString().TrimEnd();
        }