how to get index of list item in C#

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5.00/5 (1 vote)

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I want to find the index of list item whose age is 20 ..how can i get using the indexOf() method of list...now in this case its index is 3...but how to find it and display...

public class Person
   {
       public string Name { get; set; }
       public int Age { get; set; }
       public string Country { get; set; }
       public string EmailId { get; set; }
       public DateTime JoinedOn { get; set; }
   }

ObservableCollection<person> myList = new ObservableCollection<Person>()
           {
               new Person{ Name="Person 1", Age=21, Country="India",  EmailId="some@some.com", JoinedOn=DateTime.Now},
               new Person{ Name="Person 2", Age=29, Country="India",  EmailId="some@some.com", JoinedOn=DateTime.Now},
               new Person{ Name="Person 3", Age=20, Country="India",  EmailId="some@some.com", JoinedOn=DateTime.Now},
               new Person{ Name="Person 4", Age=22, Country="India",  EmailId="some@some.com", JoinedOn=DateTime.Now},
               new Person{ Name="Person 5", Age=23, Country="India",  EmailId="some@some.com", JoinedOn=DateTime.Now},

           };
</person>

Posted 5-Aug-11 0:37am

vishal_h

Updated 5-Aug-11 1:08am

v2

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Manfred Rudolf Bihy · Accepted Answer · 2011-08-05T01:27:00

Solution 2

In case you always want only one result do this:

C#

Person agedTwenty = myList.Where<Person>( x => return x.Age == 20; ).Single<Person>();
int index = myList.IndexOf(agedTwenty);

or alternatively

C#

int index = myList.Where<Person>( x => return x.Age == 20; ).Select<Person,int>( x => myList.IndexOf(x)).Single<int>();

In case there can be more than one result you'd do this:

C#

IEnumerable<Person> allAgedTwenty = myList.Where<Person>( x => return x.Age == 20; );
IEnumerable<int> indices = allAgedTwenty.Select<Person,int>( x => myList.IndexOf(x) );

The first case will get you only one int and the second case will leave you with a list of ints.

Best Regards,

—MRB

Posted 5-Aug-11 1:27am

Manfred Rudolf Bihy

Updated 5-Aug-11 2:40am

v3

Comments

fjdiewornncalwe 5-Aug-11 8:16am

You get my five for encouraging the OP to use LINQ for this purpose.

Manfred Rudolf Bihy 5-Aug-11 8:34am

Thanks Marcus,
I've only recently started using LinQ since we stepped from .NET 2.0 to 4.0 just a short while ago. I must say that I love it. It's so awesome in expressing combinations, transformations and filtering. Code that used to take up 10 to 15 lines can easily be expressed in 2 - 3 lines. And once you've gotten used to the syntax it's not so cryptic after all. :)

BobJanova 5-Aug-11 10:38am

5 for good and appropriate use of LINQ.

[no name] 5-Aug-11 11:31am

Nice and clear, my 5!

UJimbo · Accepted Answer · 2011-08-05T01:22:00

Solution 1

Make a foreach loop through your List items:

private void LookForPerson() 
{
   ObservableCollection<Person> myList = new ObservableCollection<Person>()
   {
      new Person{ Name="Person 1", Age=21, Country="India",  EmailId="some@some.com", JoinedOn=DateTime.Now},
      new Person{ Name="Person 2", Age=29, Country="India",  EmailId="some@some.com", JoinedOn=DateTime.Now},
      new Person{ Name="Person 3", Age=20, Country="India",  EmailId="some@some.com", JoinedOn=DateTime.Now},
      new Person{ Name="Person 4", Age=22, Country="India",  EmailId="some@some.com", JoinedOn=DateTime.Now},
      new Person{ Name="Person 5", Age=23, Country="India",  EmailId="some@some.com", JoinedOn=DateTime.Now},
 
   };

   int n = -1;
   foreach (Person item in myList)
   {
      if (item.Age == 20)
      {
          n = myList.IndexOf(item);
          break;
      }
   }        
}

Index will be 2, since items are stored like 0,1,2,3 etc in the List.

Posted 5-Aug-11 1:22am

UJimbo

Updated 5-Aug-11 4:38am

v5

Comments

Manfred Rudolf Bihy 5-Aug-11 7:31am

My 4!
Close, but not quite there. What happens in your code if there is more than one person aged twenty?

UJimbo 5-Aug-11 7:48am

I'm using this case only, like the OP requested :)

Manfred Rudolf Bihy 5-Aug-11 8:37am

Still doesn't do what OP wanted. You set n to zero which would be a valid index. So when no Person aged twenty is found your code would suggest the first entry on the list would fit.
At least set n to an invalid number for an index, like -1 for instance.

Manfred Rudolf Bihy 5-Aug-11 12:17pm

Looks better now! Have my 5!

vishal_h 5-Aug-11 8:10am

I try it but it gives me nullReferenceException and warn me to used new keyword..
if i used new like Person item=new Person()...then it gives error in foreach(item in MyList) line....what should i do sir

UJimbo 5-Aug-11 8:37am

Using your Person class, try using the above function, n will be 2 at the end

UJimbo 5-Aug-11 8:40am

person should be Person while defining the ObservableCollection, for some reason the page code marks it as an html tag and then tries closing it at the end. Probaly a bug in the web page

BobJanova 5-Aug-11 10:38am

I edited your solution to make the generic type parameters not fight with HTML tags.

UJimbo 5-Aug-11 11:43am

Thanks bob, I've got to rtfm at some point