I have some code where I apply a "where clause" like this:
I put in "n" as an example of the database table.
List<KeyValuePair<int, int>> n = new List<KeyValuePair<int, int>>();
n.Add(new KeyValuePair<int, int>(1,2));
n.Add(new KeyValuePair<int, int>(1,3));
n.Add(new KeyValuePair<int, int>(4,6));
n.Add(new KeyValuePair<int, int>(4,3));
n.Add(new KeyValuePair<int, int>(5,3));
var zzz = n.Where(z => z.Key == 1); // this returns "1,2" and "1,3"
Then somewhere else in my code I did this:
zzz.Where(x => x.Value == 3); // this should return "1,3"... Instead it seems to return "4,3" and "5,3" and "1,3".
Isn't the second Where supposed to return only "1,3"??? The second Where clause, should be applied to the result of the "zzz" shouldn't it?
The second Where clause, should be applied to the result of the "zzz" shouldn't it?
Yes, and it in fact does. Take this sample code:
List<KeyValuePair<int, int>> n = new List<KeyValuePair<int, int>>();
n.Add(new KeyValuePair<int, int>(1,2));
n.Add(new KeyValuePair<int, int>(1,3));
n.Add(new KeyValuePair<int, int>(4,6));
n.Add(new KeyValuePair<int, int>(4,3));
n.Add(new KeyValuePair<int, int>(5,3));
var zzz = n.Where(z => z.Key == 1); // this returns "1,2" and "1,3"
zzz = zzz.Where(x => x.Value == 3); // this then filters to the 2nd option
foreach(var pair in zzz)
Console.WriteLine("{0}:{1}", pair.Key, pair.Value);
This prints 1:3, as expected.
I suspect the problem may be that you're not re-assigning the results of the second filter: zzz.Where(x => x.Value == 3); The result of that needs to be assigned to a variable (or enumerated) in order to see the actual, filtered results.
Related
How would I go about changing my if statement and foreach to something cleaner in linq using select and where.
I've tried to make the if statement into a where clause and then use the select query as a replacement for the Foreach loop but that seem to have type issues and wasn't working.
{
StripeConfiguration.ApiKey = _appSettings.StripeSecretKey;
var profile = await _userManager.FindByIdAsync(customerServiceID);
var stripeId = profile.StripeAccountId;
if (stripeId == null)
throw new ArgumentException("No associated Stripe account found.");
List<PaymentMethodDto> result = new List<PaymentMethodDto>();
var options = new PaymentMethodListOptions
{
Customer = stripeId,
Type = "card",
};
var service = new PaymentMethodService();
var payments = await service.ListAsync(options);
if (payments != null && payments.Data?.Count > 0)
{
payments.Data.ForEach((x) =>
{
result.Add(
new PaymentMethodDto
{
Brand = x.Card.Brand,
LastDigits = x.Card.Last4,
StripeToken = x.Id,
CustomerID = x.CustomerId
});
});
}
return result;
}
Just do a regular Select.
List<PaymentMethodDto> result = payments.Data.Select(x => new PaymentMethodDto
{
Brand = x.Card.Brand,
LastDigits = x.Card.Last4,
StripeToken = x.Id,
CustomerID = x.CustomerId
})
.ToList();
If payments.Data has nothing in it, this will give you an empty list, which is what you want.
If payments is null, you'll get an exception, which I think if you think about it really hard is probably what you really want in that case too. Why would .ListAsync() yield a null value?
I am busy with a small online voting web app, now I struggling to get the total number of votes for each party that I stored in a different table. Here is what I have tried, this method gets each party from the votes table named [dbo].[VoterCandidateMapping]
public List<int> GetAllPartIDs()
{
List<int> partieIDs = new List<int>();
var parties = (from votes in voteDB.VoterCandidateMappings
select votes.PartyID).Distinct().ToList();
partieIDs = parties;
return partieIDs;
}
Then I want to use this method to count each vote associated with a particular part, here is the code
public IQueryable<ResultsViewModel> GetResults()
{
int numberOfVotes = 0;
foreach (int IDs in GetAllPartIDs())
{
numberOfVotes = (from votes in voteDB.VoterCandidateMappings
where votes.PartyID == IDs ? true : false
select votes.VoterID).Count();
}
return (
from results in voteDB.VoterCandidateMappings
join parties in voteDB.Parties
on results.PartyID equals parties.Id
select new ResultsViewModel
{
PartyName = parties.Name,
TotalVotes = numberOfVotes
});
}
It runs and return almost every data but the total number of votes is the same
The reason why it does not work is that you are trying to store multiple values in a single numberOfVotes variable.
Let's go through code what you have now.
First foreach loop calculate votes for each party and assigns to numberOfVotes variable. Each time value is assigned, existing value in numberOfVotes is overwritten. In the end of loop numberOfVotes contains number of votes for the last party. This is value you are seeing in your results as you use the same variable to return results.
Here is one way to do it correctly:
public IQueryable<ResultsViewModel> GetResults()
{
var groupedVotes = voteDB.VoterCandidateMappings
.GroupBy(x => x.PartyID)
.Select(x => new { PartyId = x.Key, NumberOfVotes = x.Count());
return voteDB.Parties
.Select(x => new ResultsViewModel
{
PartyName = x.Name,
TotalVotes = groupedVotes
.Where(y => y.PartyId == x.Id)
.Select(y => y.NumberOfVotes)
.FirstOrDefault()
});
}
by default I have this:
Expression<Func<ItemGroup, ItemGroupView>> Exp =
m => new ItemGroupView{
ID = m.id,
Name = m.name,
TotalCount = m.groupDetail.Sum(n => n.item.itemDetail.Count())
};
but in the runtime, I might want to add multiple filter. So for example, if I specify the status to 1 and category to mineral then it becomes
Expression<Func<ItemGroup, ItemGroupView>> Exp =
m => new ItemGroupView{
ID = m.id,
Name = m.name,
TotalCount = m.groupDetail.Sum(
n => n.item.itemDetail
.Where(o => o.status == 1 && o.category == "mineral")
.Count())
};
// ItemGroup.groupDetail is collection of ItemGroupDetail (n)
// ItemGroupDetail.item is Item
// Item.itemDetail is collection of ItemDetail (o)
// ItemDetail.item is Item
how do I modify the expression tree to insert multiple Where dynamically?
So far I do the default like this
private int _status;
private string _category;
internal Expression<Func<ItemDetail, bool>> whereStatus()
{
return o => o.status == _status;
}
internal Expression<Func<ItemDetail, bool>> whereCategory()
{
return o => o.category == _category ;
}
internal Expression<Func<ItemGroup, ItemGroupView>> GetEx()
{
return m => new ItemGroupView{
ID = m.id,
Name = m.name,
TotalCount = m.groupDetail.Sum(n => n.item.itemDetail.Count())
};
}
internal IQueryable<ItemGroupView> GetSelectQuery(IQueryabe<ItemGroup> ie)
{
ParameterExpression m = Expression.Parameter(typeof(ItemGroup), "m");
ParameterExpression n = Expression.Parameter(typeof(ItemGroupDetail), "n");
MemberInitExpression ex = (MemberInitExpression)GetEx().Body;
// ParameterReplacer is inherited from ExpressionVisitor
ex = (MemberInitExpression)new ParameterReplacer(
new ParameterExpression[] { m, n }).Visit(ex);
// ? ? ? ?
// how to modify the Expression if _status or _category is supplied?
Expression<Func<ItemGroup, ItemGroupView>> el =
Expression.Lambda<Func<ItemGroup, ItemGroupView>>
(ex, new ParameterExpression { m });
return ie.Select(el);
}
EDIT:
ItemGroup.itemDetail changed to ItemGroup.groupDetail, to avoid confusion between groups and items..
If you can create an expression that represents the Where() operation, you can then paste it into your main expression using LINQKit:
Expression<Func<IQueryable<ItemDetail>, IQueryable<ItemDetail>>> whereExpression=
id => id.Where(o => o.status == 1 && o.category == "mineral");
Expression<Func<ItemGroup, ItemGroupView>> Exp =
m => new ItemGroupView
{
ID = m.id,
Name = m.name,
TotalCount =
whereExpression.Invoke(m.itemDetail)
.Sum(n => n.item.itemDetail.Count())
};
Exp = Exp.Expand();
(Don't forget that last line, it's important.)
I am trying to use following code to create the PIVOT but its not working.
It's giving me compile time error. I don't know linq so unable to use it.
Please help :
DataTable Pivot(DataTable dt, DataColumn pivotColumn, DataColumn pivotValue) {
// find primary key columns
//(i.e. everything but pivot column and pivot value)
DataTable temp = dt.Copy();
temp.Columns.Remove( pivotColumn.ColumnName );
temp.Columns.Remove( pivotValue.ColumnName );
string[] pkColumnNames = temp.Columns.Cast(<DataColumn>)
.Select( c => c.ColumnName )
.ToArray();
// prep results table
DataTable result = temp.DefaultView.ToTable(true, pkColumnNames).Copy();
result.PrimaryKey = result.Columns.Cast(<DataColumn>).ToArray();
dt.AsEnumerable()
.Select(r =>; r[pivotColumn.ColumnName].ToString())
.Distinct().ToList()
.ForEach (c => result.Columns.Add(c, pivotColumn.DataType));
// load it
foreach( DataRow row in dt.Rows ) {
// find row to update
DataRow aggRow = result.Rows.Find(
pkColumnNames
.Select( c => row[c] )
.ToArray() );
// the aggregate used here is LATEST
// adjust the next line if you want (SUM, MAX, etc...)
aggRow[row[pivotColumn.ColumnName].ToString()] = row[pivotValue.ColumnName];
}
return result;
}
Code from : http://michaeljswart.com/2011/06/forget-about-pivot/
Moreover it tried to use following code, it works well except for it is not giving total sum for Value Column
public DataTable GetInversedDataTable(DataTable table, string columnX, string columnY, string columnZ, string nullValue, bool sumValues)
{
//Create a DataTable to Return
DataTable returnTable = new DataTable();
DataTable tempTable = table.Clone();
if (string.IsNullOrEmpty(columnX))
{
columnX = table.Columns[0].ColumnName;
}
tempTable.Columns.Remove(columnX);
//Add a Column at the beginning of the table
//returnTable.Columns.Add(columnY);
returnTable = tempTable.Clone();
//Read all DISTINCT values from columnX Column in the provided DataTale
List<string> columnXValues = new List<string>();
foreach (DataRow dr in table.Rows)
{
string columnXTemp = dr[columnX].ToString();
if (!columnXValues.Contains(columnXTemp))
{
//Read each row value, if it's different from others provided, add to the list of values and creates a new Column with its value.
columnXValues.Add(columnXTemp);
returnTable.Columns.Add(columnXTemp);
}
}
//Verify if Y and Z Axis columns re provided
if (!string.IsNullOrEmpty(columnY) && !string.IsNullOrEmpty(columnZ))
{
//Read DISTINCT Values for Y Axis Column
List<string> columnYValues = new List<string>();
foreach (DataRow dr in table.Rows)
{
if (!columnYValues.Contains(dr[columnY].ToString()))
{
columnYValues.Add(dr[columnY].ToString());
}
}
//Loop all Column Y Distinct Value
foreach (string columnYValue in columnYValues)
{
//Creates a new Row
DataRow drReturn = returnTable.NewRow();
drReturn[0] = columnYValue;
//foreach column Y value, The rows are selected distincted
DataRow[] rows = table.Select((columnY + "='") + columnYValue + "'");
//Read each row to fill the DataTable
foreach (DataRow dr in rows)
{
string rowColumnTitle = dr[columnX].ToString();
//Read each column to fill the DataTable
foreach (DataColumn dc in returnTable.Columns)
{
if (dc.ColumnName == rowColumnTitle)
{
//If Sum of Values is True it try to perform a Sum
//If sum is not possible due to value types, the value displayed is the last one read
if (sumValues)
{
try
{
drReturn[rowColumnTitle] = Convert.ToDecimal(drReturn[rowColumnTitle]) + Convert.ToDecimal(dr[columnZ]);
}
catch
{
drReturn[rowColumnTitle] = dr[columnZ];
}
}
else
{
drReturn[rowColumnTitle] = dr[columnZ];
}
}
}
}
returnTable.Rows.Add(drReturn);
}
}
else
{
throw new Exception("The columns to perform inversion are not provided");
}
//if a nullValue is provided, fill the datable with it
if (!string.IsNullOrEmpty(nullValue))
{
foreach (DataRow dr in returnTable.Rows)
{
foreach (DataColumn dc in returnTable.Columns)
{
if (string.IsNullOrEmpty(dr[dc.ColumnName].ToString()))
{
dr[dc.ColumnName] = nullValue;
}
}
}
}
return returnTable;
}
GetInversedDataTable(dtNormal, "Dated", "OrderStatus", "Qty", " ", true);
Please help :)
Here is the code with the compilation errors corrected:
DataTable Pivot(DataTable dt, DataColumn pivotColumn, DataColumn pivotValue) {
// find primary key columns
//(i.e. everything but pivot column and pivot value)
DataTable temp = dt.Copy();
temp.Columns.Remove( pivotColumn.ColumnName );
temp.Columns.Remove( pivotValue.ColumnName );
string[] pkColumnNames = temp.Columns.Cast<DataColumn>()
.Select( c => c.ColumnName )
.ToArray();
// prep results table
DataTable result = temp.DefaultView.ToTable(true, pkColumnNames).Copy();
result.PrimaryKey = result.Columns.Cast<DataColumn>().ToArray();
dt.AsEnumerable()
.Select(r => r[pivotColumn.ColumnName].ToString())
.Distinct().ToList()
.ForEach (c => result.Columns.Add(c, pivotColumn.DataType));
// load it
foreach( DataRow row in dt.Rows ) {
// find row to update
DataRow aggRow = result.Rows.Find(
pkColumnNames
.Select( c => row[c] )
.ToArray() );
// the aggregate used here is LATEST
// adjust the next line if you want (SUM, MAX, etc...)
aggRow[row[pivotColumn.ColumnName].ToString()] = row[pivotValue.ColumnName];
}
return result;
}
I changed Cast(<DataColumn>) to Cast<DataColumn>() in two locations and got rid of the semicolon in the middle of a lambda expression. The second part of your question is a little trickier. You may want to ask it as its own question.
Good one., but you might want to replace the below line
.ForEach (c => result.Columns.Add(c, pivotColumn.DataType));
with this (change pivotColumn to pivotValue)
.ForEach (c => result.Columns.Add(c, pivotValue.DataType));
Works perfectly for my requirement.
I have a list of prices ordered by date. I need to select all monotonously decreasing values. The following code works:
public static List<DataPoint> SelectDecreasingValues(List<DataPoint> dataPoints)
{
var ret = new List<DataPoint>(dataPoints.Count);
var previousPrice = dataPoints[0].Price;
for (int i = 0; i < dataPoints.Count; i++)
{
if (dataPoints[i].Price <= previousPrice)
{
ret.Add(dataPoints[i]);
previousPrice = dataPoints[i].Price;
}
}
return ret;
}
However, is there a shorter/cleaner way to accomplish it with Linq?
This code is equivalent:
previousPrice = dataPoints[0].Price;
var ret = dataPoints.Where(x => {
if(x.Price <= previousPrice)
{ previousPrice = x.Price; return true;}
return false;
}).ToList();
However, if you don't need to have a list, go with plain enumerables and drop the ToList at the end. That way you can make use of the deferred execution feature built into LINQ.
The following code is also equivalent:
DataPoint previous = dataPoints.FirstOrDefault();
var ret = dataPoints.Where(x => x.Price <= previous.Price)
.Select(x => previous = x).ToList();
This works because of the deferred execution in LINQ. For each item in dataPoints it will first execute the Where part and then the Select part and only then will it move to the second item in dataPoints.
You need to decide which version you want to use. The second one is not as intention revealing as the first one, because you need to know about the internal workings of LINQ.
public IEnumerable<T> WhereMonotonicDecreasing<T>(
IEnumerable<T> source,
Func<T, IComparable> keySelector)
{
IComparable key;
bool first = true;
foreach(T t in source)
{
if (first)
{
key = keySelector(t);
yield return t;
first = false;
}
else
{
IComparable newKey = keySelector(t);
if (newKey.CompareTo(key) < 0)
{
key = newKey;
yield return t;
}
}
}
}
Called by:
dataPoints.WhereMonotonicDecreasing(x => x.Price);
previousPrice = dataPoints[0];
dataPoints.Where(p => p.Price <= previousPrice.Price)
.Select(p => previousPrice = p);
You can then use .ToList() if you really need one.
How about (untested):
return dataPoints.Take(1)
.Concat(dataPoints.Skip(1)
.Zip(dataPoints,
(next, previous) =>
new { Next = next, Previous = previous })
.Where(a => a.Next.Price <= a.Previous.Price)
.Select(a => a.Next))
.ToList();
Essentially, this overlays a "one-deferred" version of the sequence over the sequence to produce "next, previous" tuples and then applies the relevant filters on those tuples. The Take(1) is to pick the first item of the sequence, which it appears you always want.
If you don't care for the readability of the variable names, you could shorten it to:
return dataPoints.Take(1)
.Concat(dataPoints.Skip(1)
.Zip(dataPoints, Tuple.Create)
.Where(a => a.Item1.Price <= a.Item2.Price)
.Select(a => a.Item1))
.ToList();