Recursive query using LINQ

Question

Is it possible to write this in LINQ? I tried using LINQ. However, I figured it will have to be looped twice; first to verify whether a is present and next to iterate through qs.

So I came up with this code.

public a Traverse(List<q> qs,string id)
    {

        foreach (var q in qs)
        {
            if (q.as.Any(a => a.Id == id))
            {
                return q.as.First(a => a.Id == id);

            }

            foreach (var a in q.as)
            {
                var result =Traverse(a.qs, id);
                if(result != null)
                  return result;
            }
        }
        return null;
    }

I am reading it from XML which is like "q" have "a" and "a" have "q" in recursive manner.

I need to find unique Id belonging to a.

I know there have been discussions on other threads but they were not helpful.

EDIT: Here's snippet of XML

  <qs xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <qs>
    <q>q 1</q>
    <Id>1</Id>
    <as>
      <a>
        <aprop>a</aprop>
        <Id>a 1.1</Id>
        <qs>
          <q>
            <q>q 1.1.1</q>
            <Id>1.1.1</Id>
            <as>
              <a>
                <a>a</a>
                <Id>a 1.1.1.1</Id>
                <qs />
                <type>1</type>
              </a>
              <a>
                <a>a</a>
                <Id>a 1.1.1.2</Id>
                <qs />
                <type>1</type>
              </a>
            </as>
          </q>
          <q>
            <q>q 1.1.2</q>
            <Id>1.1.2</Id>
            <as>
              <a>
                <a>a</a>
                <Id>a 1.1.2.1</Id>
                <qs />
                <type>1</type>
              </a>
              <a>
                <a>a</a>
                <Id>a 1.1.2.2</Id>
                <qs />
                <type>1</type>
              </a>
            </as>
          </q>

FYI, you can use FirstOrDefault (and compare to null) rather than checking Any and then First, as long as it's a reference type. — Dan Bryant
– Dan Bryant, Commented Feb 20, 2013 at 20:52

Judah Gabriel Himango · Accepted Answer · 2013-02-20 21:14:12Z

7

I'm not sure what you're really going to achieve. That's how I understand the problem:

We are looking for <a> tag, that contains another tag named <Id> with value equals to id given as a method parameter. If element is not found method should return null.

I think it can be done on the XML using Descendants() method:

Let say, we have your XML loaded into a variable called doc, which is an instance of XDocument class.

var query= from a in doc.Descendants("a")
           let i = (string)a.Element("Id")
           where i == id
           select a;

return query.FirstOrDefault();

edited Feb 20, 2013 at 21:14

Judah Gabriel Himango

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answered Feb 20, 2013 at 21:10

MarcinJuraszek

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2 Comments

Larry Over a year ago

I was totally focused to LINQ to object... Thanks a lot to remind how important it is sometimes to think out the box.

Amit Over a year ago

Yeah this works with XDocument. Any solution using Linq <-> object ?

Ray Saltrelli · Accepted Answer · 2013-02-21 14:07:52Z

3

If LINQ to XML will work for you then MarcinJuraszek's answer is perfect. To address your LINQ to Objects question, what about something like this?

    public a Traverse(IQueryable<q> qList, string id)
    {
        return this.GatherAs(qList).FirstOrDefault(a => a.Id == id);
    }

    public IQueryable<a> GatherAs(IQueryable<q> qList)
    {
        IQueryable<a> aList = qList.SelectMany(q => q.aList);

        if (aList.Count != 0)
            aList = aList.Union(this.GatherAs(aList.SelectMany(a => a.qList)));

        return aList;       
    }

edited Feb 21, 2013 at 14:07

answered Feb 20, 2013 at 21:20

Ray Saltrelli

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7 Comments

Larry Over a year ago

It probably works. However, 1) both XML and this solution requires the whole list to be traversed and 2) The LINQ to object solution is a pain to read. Anyway, this answer deserves to be accepted because it is the requested solution. But IMHO, I would rather stick to the recursive code, honestly.

MarcinJuraszek Over a year ago

@Laurent: Using FirstOrDefault means the value will be returned as soon as it is found, even if not whole collection was traversed. Why do you think whole list have to be checked in my solution?

Ray Saltrelli Over a year ago

I think what @Laurent is saying is that in both solutions, Descendants() and GatherAs() scan the entire document/graph looking for A's then perform FirstOrDefault() on the results. I'm a little fuzzy on how LINQ works under the hood. This might be true.

MarcinJuraszek Over a year ago

@RaySaltrelli: no! Descendants has deferred execution - it returns only one element at the time, which is compared using where condition and returned when it's true. Next element is taken only when condition check returns false.

Larry Over a year ago

No Ray, Marcin is right and I was wrong: the IQueryable<T> usage defers the execution until a one by one enumeration is performed by a ToList(), ToArray() or a fetching extension method like FirstOrDefault().

|

Larry · Accepted Answer · 2013-02-20 21:07:36Z

0

Using LINQ to traverse things that references each other looks very hard to achieve to me. It would be easier if the q and a classes were merged together in a single one.

However, as a beginning, your function can be simplified this way :

public a Traverse(List<q> qs, string id)
{
    foreach (var q in qs)
    {
        foreach (var a in q._as)
        {
            if (a.Id == id)
                return a;
            else
                return Traverse(a.qs, id);
        }
    }
    return null;
}

answered Feb 20, 2013 at 21:07

community wiki

Larry

Comments

sa_ddam213 · Accepted Answer · 2013-02-20 21:25:01Z

-1

You should be able to use XDocument to query your xml a bit easier

Example:

  XDocument xdoc = XDocument.Load("c:\\MyXmlFile.xml");
  var results = xdoc.Descendants("a").Descendants("Id").Select(id => id.Value);

Returns all the Id values from a elements.

answered Feb 20, 2013 at 21:25

sa_ddam213

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