How to do groupby with list<list<int>> and return list<list<int>> in C#?

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My data starts in a JSON object:

JSON
    {"page":"1",
     "per_page":10,
     "total":100,
     "total_pages":10,
     "data":[{"id":1,
              "userId":1,
              "userName":"Jim Silver", 
              "timestamp":16425382171,
              "txnType":"debit",
              "amount":"$1,000.07",
              "location":{"id":2,
                          "address":"654, Somewhere, Some Street", 
                          "city":"Some City",
                          "zipCode":12345},
                          "ip":"202.210.105.105"},
             {"id":2," ...}
             {"id":3," ...}
              ...
             {"id":n," ...}]

I need to get the user ID and amount into a List<list<int>> which I've done here:

C#

var model = JsonConvert.DeserializeObject<TxnResponse>(rawJSON);
var elements = model.data;

List<List<int>> userTxns = new List<List<int>>(elements.Count);
List<int> record = null;

int userID = 0;
int userAmount = 0;

for (int row = 0; row < elements.Count; row++)
{
    userID = elements[row].userId;
    userAmount = Convert.ToInt32(Math.Floor(decimal.Parse(model.data[row].amount, System.Globalization.NumberStyles.Currency)));

    record = new List<int>();
    record.Add(userID);
    record.Add(userAmount);

    userTxns.Add(record);
}

return userTxns.GroupBy();

The result should look like this when printed to screen (Console.WriteLine(String.Join("\n", list.Select(x => String.Join(" ", x))));):
1 1000
2 2000
3 3000
4 4000

Without the Group By, if the JSON object has 10 data items, I have 10 (userId, amount) records.

Model:

C#

public class TxnResponse
{
    public string page { get; set; }
    public int per_page { get; set; }
    public int total { get; set; }
    public int total_pages { get; set; }
    public List<data> data { get; set; }
}


public class data
{
    public int id { get; set; }
    public int userId { get; set; }
    public string userName { get; set; }
    public object timestamp { get; set; }
    public string txnType { get; set; }
    public string amount { get; set; }
    public Location location { get; set; }
    public string ip { get; set; }
}

public class Location
{
    public int LocationID { get; set; }
    public string Address { get; set; }
    public string City { get; set; }
    public int ZipCode { get; set; }
}

How do I do GroupBy on user ID? Thank you.

What I have tried:

C#

return userTxns.GroupBy(userID);
return userTxns.GroupBy(u => userID);

but get "the type arguments for method cannot be inferred from the usage..." or "Cannot implicitly convert type...".

Posted 1-Apr-20 5:19am

Member 12338931

Updated 1-Apr-20 9:27am

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Comments

Richard Deeming 1-Apr-20 12:09pm

Based on the sample data, your List<List<int>> will already provide the output you've said you want.

What are you trying to group by, and what output are you expecting?

Member 12338931 1-Apr-20 13:19pm

I'm trying to group by userId in the userTxns object which contains lists of userIDs and amounts.

The output should group userIds and sum each user's amounts like this:

1 1000
2 2000
3 3000
4 4000

where the first column is the userId and the second column is each respective user's summed amounts.

2 solutions

Solution 1

GroupBy doesn't return the records - it returns groups of records, which are (basically) in (key, value) pairs.

Have a look at this: Using Linq to create a Dictionary of sub-Lists by grouping from a collection.[^] It was written to group a days worth of logs together, but it'll work the same for IDs and return a Dictionary of Lists - one list for each ID value - with a minor change to the grouping clause.

Posted 1-Apr-20 6:02am

OriginalGriff

Comments

Member 12338931 1-Apr-20 13:31pm

Thanks ... I'll see if I can figure out how to implement this...

Member 12338931 1-Apr-20 13:40pm

So in your example, are all the example log files in the rawFiles object as in

Dictionary<string, list<string="">> groups = rawFiles.GroupBy(r => GetDateFromPath(r)).ToDictionary(g => g.Key, g => g.ToList());

Member 12338931 1-Apr-20 13:50pm

Also, the method must return a List<list<int>>.

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Richard Deeming · Accepted Answer · 2020-04-01T09:29:00

If you want to stick with the List<List<int>> approach, grouping by the first value and summing the second, something like this should work:

C#

return userTxns.GroupBy(u => u[0], (key, values) => new List<int> 
{ 
    key, 
    values.Select(l => l[1]).Sum() 
}).ToList();

Personally, I'd be inclined to use a dictionary instead:

C#

var model = JsonConvert.DeserializeObject<TxnResponse>(rawJSON);
var result = new Dictionary<int, int>(elements.Count);
foreach (data item in model.data)
{
    int userId = item.userId;
    int userAmount = Convert.ToInt32(Math.Floor(decimal.Parse(item.amount, System.Globalization.NumberStyles.Currency)));
    
    if (result.TryGetValue(userId, out int existingAmount))
    {
        userAmount += existingAmount;
    }
    
    result[userId] = userAmount;
}

return result;