Query on table with nulls

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I have a query on a table where nulls are allowed in the columns. The method then returns the query results as an array to plot a chart. The chart command is in a windows form and the database query is in a class.
I thought the query line

where P!=null

excluded nulls, but I still get an error

"can't convert double[]? to double"

There has to be a simple fix but I can't find anything on the net or MSDN .[^]

I'm going in circles...thanks for the help.

What I have tried:

C#

//a command in a windows form to plot a chart
private void cmdCHART_Click(object sender, EventArgs e)
{

    nmCHART.clsCHART oCHART = new nmCHART.clsCHART();
            
    double[] Y = new double[21];
    Y = oCHART.GET_Y_SERIES("CONDOR");
}


//code in a class to retrieve data from a table
namespace nmCHART
{
    public class clsCHART
    {
        public double?[] GET_Y_SERIES(string GREEK)        
        {

            var qryY = (from P in Globals.DATA.PAYOFF_EAVs
                       where P.GREEK == GREEK
                       where P != null
                       orderby P.DP_NO
                       select P.DATA).ToArray();

            return qryY;
        }
    }
}

Posted 16-Mar-16 13:14pm

Trader999

Updated 17-Mar-16 18:16pm

Matt T Heffron

v7

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Comments

Garth J Lancaster 16-Mar-16 20:37pm

how do you call GET_Y_SERIES("its-all-greek-to-me") ?

- for instance, if you have

double myGreekValue = GET_Y_SERIES("kali spera");

it will fail as Matt indicates, because myGreekValue is a single double, yet you return 'qryY.ToArray()' which is an array of values

so you'd need

double [] myGreekValues = GET_Y_SERIES("ti kanis");

for example as you have GET_Y_SERIES currently coded

Trader999 16-Mar-16 21:22pm

Garth, I've added some more code - what you said made me think the problem maybe elsewhere. Basically I'm tying to plot a chart from a windows form when a command is pressed. Another class retrieves and returns the data as an array. Sorry I'm not sure how to format the code in the question.

Trader999 16-Mar-16 21:56pm

...still getting the error msg!

3 solutions

Solution 2

You select P.DATA

So you should check P.DATA for not being null, not P:

C#

var qryY = (from P in Globals.DATA.PAYOFF_EAVs
            where P.GREEK == GREEK
            && P.DATA != null
            orderby P.DP_NO
            select P.DATA
           ).ToArray();

Posted 16-Mar-16 22:24pm

Sascha Lefèvre

Comments

Trader999 18-Mar-16 0:13am

thanks Sascha, I tried this but kept getting the same error

Solution 3

here is another solution with the help of others

C#

double[] Y = new double[21];
            Y = oCHART.GET_Y_SERIES("CONDOR").Where(d => d.HasValue).Cast<double>().ToArray();

Posted 17-Mar-16 18:16pm

Trader999

Updated 18-Mar-16 8:36am

v2

Comments

Matt T Heffron 18-Mar-16 14:38pm

This moves the handling of the nulls out of the GET_Y_SERIES method and makes it the responsibility of all of the callers.
(You don't need to allocate the "new double[21]" since you are about to replace it on the next line.)

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Matt T Heffron · Accepted Answer · 2016-03-16T13:41:00

You've declared the method to return a double but are trying to return double?[].

Edit [MTH]:

C#

//a command in a windows form to plot a chart
private void cmdCHART_Click(object sender, EventArgs e)
{
    // Does this really need to be created new each time it is used?
    // If nmCHART carries some important state, then it should be a singleton
    // Otherwise it should be a static class
    nmCHART.clsCHART oCHART = new nmCHART.clsCHART();
            
    double[] Y = oCHART.GET_Y_SERIES("CONDOR");
}
 

//code in a class to retrieve data from a table
namespace nmCHART
{
    public class clsCHART
    {
        public double[] GET_Y_SERIES(string GREEK)        
        {
            var qryY = (from P in Globals.DATA.PAYOFF_EAVs
                       where P.GREEK == GREEK
                             && P.DATA != null // as Sascha Lefèvre noted in Solution 2
                       orderby P.DP_NO
                       select P.DATA).OfType<double>().ToArray();
 
            return qryY;
        }
    }
}

The .OfType<double>() casts all of the values to double from double? so the caller can use it without having to be concerned about nulls.
Also, the .OfType<double>() will filter out the null values, so the null check in the query is not necessary, but, if this is actually a database query, the null check would be executed on the server side, so it could be more efficient.

Query on table with nulls

3 solutions

Solution 1

Solution 2

Solution 3

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