How to only add items to a list that haven't been added before? (C# - ASP.NET)

Question

I have a list that is being populated with the summed quantities of specific product ID's, for example:

My table:

My program checks for a user input and then compares that user input to the Quantity column in the table - once a match is found, the program checks for the ProductID of the matched quantity, and then adds ALL the quantities relating to that matched ProductID.

For example, if a user enters 2, the program will check for all the values in the Quantity column greater than or equal to 2 - so in this case, the first row to be found would be row 2 (which has a productID of 9 and a quantity of 2). The program should now search for ALL productID's equal to 9 and then sum their individual quantities (which will be 3 in this case, as the ProductID 9 appears only twice, with the quantities 2 and 1 respectively), and then save that final summed quantity (3) into the list. And then the program should continue to row 3 (which has a productID of 8 and a quantity of 3). The program should now search for ALL productID's equal to 8 and then sum their individual quantities (which will be 9 in this case, as the ProductID 8 appears three times, with the quantities 3 and 3 and 3 respectively), and then save that final summed quantity (9) into the list. And then the program should SKIP row 4, because the ProductID of 9 has already been dealt with - and so on and so forth.

So far i have tried this:

var dynamicReader = DBAccessor.InvoiceLines.Where(xx => xx.Quantity >= quantitySelected).Select(yy => yy.ProductID);

            foreach (var product in dynamicReader)
            {
                if (!quantityArrayList.Contains(product))
                {
                    quantityArrayList.Add(DBAccessor.InvoiceLines.Where(gg => gg.ProductID == product).Sum(g => g.Quantity));
                }
            }

If the user enters 2, the first 4 results using this method are correct, however, after that, the summed values fetched make little sense to me. Any help on this matter would be greatly appreciated.

Answer 1

It looks like you're not associating the ProductID with the Quantity stored in quantityArrayList . So, when you check quantityArrayList.Contains(product) you're not checking to see if that ProductID has already been processed, you're checking to see if there is a sum of quantities which match the ProductID .

I don't know what type quantityArrayList is, I'm assuming it's a List<int> . Instead, you should use a Dictionary where the key would be the ProductID and the value would be the sum of the Quantity .

Untested, but the updated code might look like...

Dictionary<int, int> productQuantities = new Dictionary<int, int>(); //I've changed the name, but this would replace quantityArrayList

var dynamicReader = DBAccessor.InvoiceLines.Where(xx => xx.Quantity >= quantitySelected).Select(yy => yy.ProductID);

foreach (var product in dynamicReader)
{
    if (!productQuantities.ContainsKey(product))
    {
        productQuantities.Add(product, DBAccessor.InvoiceLines.Where(gg => gg.ProductID == product).Sum(g => g.Quantity));
    }
}

Answer 2

I would use GroupBy with 2 values mapped to the grouping: Max and Sum . If the max quantity of one of the entries is greater than your desired amount, that grouping is 'in scope' and you want to get its sum. Like this:

    using System;
    using System.Collections.Generic;
    using System.Linq;
    
    public class Program
    {
        public static void Main()
        {
            var productCounts = new List<ProductCount>{
                new ProductCount{ProductId = 10, Quantity = 1},
                new ProductCount{ProductId = 9, Quantity = 2},
                new ProductCount{ProductId = 8, Quantity = 3},
                new ProductCount{ProductId = 9, Quantity = 1},
                new ProductCount{ProductId = 7, Quantity = 2},
                new ProductCount{ProductId = 5, Quantity = 3},
                new ProductCount{ProductId = 8, Quantity = 3},
                new ProductCount{ProductId = 6, Quantity = 2}
            };
            
            var desiredMinimum = 3;
            
            var output = productCounts.GroupBy(i => i.ProductId)
                .Select(i => new
                        {
                            ProductId = i.Key,
                            Max = i.Max(j => j.Quantity),
                            Total = i.Sum(j => j.Quantity)
                        })
                .Where(i => i.Max >= desiredMinimum)
                .ToList();
            
            foreach(var outputItem in output){
                Console.WriteLine("ProductId: " + outputItem.ProductId + "; Total: " + outputItem.Total);
            }
        }
        internal class ProductCount
        {
            public int ProductId{get;set;}
            public int Quantity{get;set;}
        }
    }

See:

https://dotnetfiddle.net/HmMc4h