How to get the index of an item in a list in a single step

Question

How can I find the index of an item in a list without looping through it   Currently this doesn t look very nice - searching through the list for the same item twice  just to get the index   var oProp   something   int theThingIActuallyAmInterestedIn   myList IndexOf myList Single i   gt  i Prop    oProp

User · Answer

For simple types you can use  IndexOf     List lt string gt  arr   new List lt string gt     arr Add  aaa    arr Add  bbb    arr Add  ccc    int i   arr IndexOf  bbb       RETURNS 1

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Simple solution to find index for any string value in the List    Here is code for List Of String   int indexOfValue   myList FindIndex a   gt  a Contains  insert value from list        Simple solution to find index for any Integer value in the List    Here is Code for List Of Integer       int indexOfNumber   myList IndexOf   insert number from list

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If you don t want to use LINQ  then   int index  for  int i   0  i  lt  myList Count  i          if  myList i  Prop    oProp               index   i         break            this way you are iterating list only once

User · Answer

How about the List FindIndex Method   int index   myList FindIndex a   gt  a Prop    oProp        This method performs a linear search  therefore  this method is an   O n  operation  where n is Count    If the item is not found  it will return -1

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EDIT  If you re only using a List lt  gt  and you only need the index  then List FindIndex is indeed the best approach  I ll leave this answer here for those who need anything different  e g  on top of any IEnumerable lt  gt     Use the overload of Select which takes an index in the predicate  so you transform your list into an  index  value  pair   var pair   myList Select  Value  Index    gt  new   Value  Index                      Single p   gt  p Value Prop    oProp     Then   Console WriteLine  Index  0   Value   1    pair Index  pair Value     Or if you only want the index and you re using this in multiple places  you could easily write your own extension method which was like Where  but instead of returning the original items  it returned the indexes of those items which matched the predicate

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That s all fine and good -- but what if you want to select an existing element as the default   In my issue there is no  quot --select a value-- quot  option  Here s my code -- you could make it into a one liner if you didn t want to check for no results I suppose    private void LoadCombo ComboBox cb  string itemType  string defVal    quot  quot         cb DisplayMember    quot Name quot       cb ValueMember    quot ItemCode quot       cb DataSource   db Items Where q   gt  q ItemTypeId    itemType  ToList         if   string IsNullOrEmpty defVal                 var i     List lt GCC Pricing Models Item gt  cb DataSource  FindIndex q   gt  q ItemCode    defVal           if  i gt  0  cb SelectedIndex   i

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If anyone wonders for the Array version  it goes like this   int i   Array FindIndex yourArray  x   gt  x    itemYouWant

User · Answer

Here s a copy paste-able extension method for IEnumerable  public static class EnumerableExtensions            lt summary gt          Searches for an element that matches the conditions defined by the specified predicate          and returns the zero-based index of the first occurrence within the entire  lt see cref  IEnumerable T    gt            lt  summary gt           lt typeparam name  T  gt  lt  typeparam gt           lt param name  list  gt The list  lt  param gt           lt param name  predicate  gt The predicate  lt  param gt           lt returns gt          The zero-based index of the first occurrence of an element that matches the conditions defined by  lt paramref name  predicate   gt   if found  otherwise it ll throw           lt  returns gt      public static int FindIndex lt T gt  this IEnumerable lt T gt  list  Func lt T  bool gt  predicate                var idx   list Select  value  index    gt  new  value  index   Where x   gt  predicate x value   Select x   gt  x index  First            return idx            Enjoy

[c#] How to get the index of an item in a list in a single step?

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