Question

My LINQ query:

Dim groupedData = (From p In pData _
                   Group By p.TruncParam Into Group) _
                  .SelectMany(Function(g) g.Group) _
                  .Select(Function(d, idx) New With { _
                      .NewParameter = String.Concat(If(d.TruncParam.Length < 5, d.TruncParam, d.TruncParam.Substring(0, 5)), idx.ToString("0000")), _
                      .FullParameter = String.Format("{0}-{1} [{2}] <{3}>", d.LabName, d.TestName, d.Parameter, d.Unit)})

Produces these results:

ID: SOLUB0000  Name: 001-AMT SOLUBL  [SOLUBLES]       <%>
ID: SOLUB0001  Name: CHEM-C4:SOL     [SOLUBLES]       <%>
ID: SOLUB0002  Name: CHEM-EMCARB:SOL [SOLUBLES]       <%>
ID: INSOL0003  Name: 001-AMT:INSOL   [INSOLUBLES]     <%>
ID: INSOL0004  Name: CHEM-AMT:INSOL  [INSOLUBLES]     <%>
ID: INSOL0005  Name: CHEM-W:INSOL    [INSOLUBLES]     <%>
ID: INSOL0006  Name: CHEM-W:INSOL    [INSOLUBLES]     <mg/l>
ID: CLRES0007  Name: 001-CL RESIDUE  [CL RESIDUE]     <ppm>
ID: SUMCA0008  Name: 001-ELEMENTS    [SUM CA K NA SI] <%>
ID: SUMME0009  Name: 001-ELEMENTS    [SUM METALS + P] <%>

When the TruncParam changes, I would like the index (idx) in the Select clause to reset to 1. So, in the list above, the Index should be SOLUB0001, SOLUB0002, INSOL0001, INSOL0002…CLRES0001, SUMCA0001, SUME0001.

How should I alter the LINQ query?

Answer 1

You need to move the index selection inside the SelectMany statement (g.Group.Select(d, idx) => new {d, idx}) like this:

Dim groupedData = (From p In pData _
                   Group By p.TruncParam Into Group) _
    .SelectMany(Function(g) g.Group.Select(Function(d, idx) New With { _
        .Element = d, .Index = idx}) _
    .Select(Function(d) New With { _
        .NewParameter = String.Concat(If(d.Element.TruncParam.Length < 5, d.Element.TruncParam, d.Element.TruncParam.Substring(0, 5)), d.Index.ToString("0000")), _
        .FullParameter = String.Format("{0}-{1} [{2}] <{3}>", d.Element.LabName, d.Element.TestName, d.Element.Parameter, d.Element.Unit)})