How to get index using LINQ

Question

Given a datasource like that   var c   new Car       new Car  Color  Blue   Price 28000     new Car  Color  Red   Price 54000     new Car  Color  Pink   Price 9999                How can I find the index of the first car satisfying a certain condition with LINQ   EDIT   I could think of something like this but it looks horrible   int firstItem   someItems Select  item  index    gt  new               ItemName   item Color          Position   index        Where i   gt  i ItemName     purple          First          Position    Will it be the best to solve this with a plain old loop

User · Answer

myCars Select  v  i    gt  new  car   v  index   i   First myCondition  index    or the slightly shorter  myCars Select  car  index    gt  new  car  index   First myCondition  index

User · Answer

Simply do     int index   List FindIndex your condition     E g    int index   cars FindIndex c   gt  c ID    150

User · Answer

I will make my contribution here    why  just because  p Its a different implementation  based on the Any LINQ extension  and a delegate  Here it is   public static class Extensions       public static int IndexOf lt T gt               this IEnumerable lt T gt  list               Predicate lt T gt  condition                           int i   -1          return list Any x   gt    i    return condition x        i   -1           void Main         TestGetsFirstItem        TestGetsLastItem        TestGetsMinusOneOnNotFound        TestGetsMiddleItem           TestGetsMinusOneOnEmptyList       void TestGetsFirstItem            Arrange     var list   new string      a    b    c    d             Act     int index   list IndexOf item   gt  item Equals  a             Assert     if index    0                throw new Exception  Index should be 0 but is      index               Test Successful  Dump       void TestGetsLastItem            Arrange     var list   new string      a    b    c    d             Act     int index   list IndexOf item   gt  item Equals  d             Assert     if index    3                throw new Exception  Index should be 3 but is      index               Test Successful  Dump       void TestGetsMinusOneOnNotFound            Arrange     var list   new string      a    b    c    d             Act     int index   list IndexOf item   gt  item Equals  e             Assert     if index    -1                throw new Exception  Index should be -1 but is      index               Test Successful  Dump       void TestGetsMinusOneOnEmptyList            Arrange     var list   new string                 Act     int index   list IndexOf item   gt  item Equals  e             Assert     if index    -1                throw new Exception  Index should be -1 but is      index               Test Successful  Dump       void TestGetsMiddleItem            Arrange     var list   new string      a    b    c    d    e             Act     int index   list IndexOf item   gt  item Equals  c             Assert     if index    2                throw new Exception  Index should be 2 but is      index               Test Successful  Dump

User · Answer

Here is a little extension I just put together   public static class PositionsExtension       public static Int32 Position lt TSource gt  this IEnumerable lt TSource gt  source                                            Func lt TSource  bool gt  predicate                return Positions lt TSource gt  source  predicate  FirstOrDefault              public static IEnumerable lt Int32 gt  Positions lt TSource gt  this IEnumerable lt TSource gt  source                                                           Func lt TSource  bool gt  predicate                if  typeof TSource  is IDictionary                        throw new Exception  Dictionaries aren t supported                       if  source    null                        throw new ArgumentOutOfRangeException  source is null                      if  predicate    null                        throw new ArgumentOutOfRangeException  predicate is null                      var found   source Where predicate  First            var query   source Select  item  index    gt  new                               Found   ReferenceEquals item  found                   Index   index                 Where  it   gt  it Found  Select  it   gt  it Index           return query            Then you can call it like this   IEnumerable lt Int32 gt  indicesWhereConditionIsMet          ListItems Positions item   gt  item    this    Int32 firstWelcomeMessage ListItems Position msg   gt                       msg WelcomeMessage Contains  Hello

User · Answer

An IEnumerable is not an ordered set  Although most IEnumerables are ordered  some  such as Dictionary or HashSet  are not   Therefore  LINQ does not have an IndexOf method   However  you can write one yourself       lt summary gt Finds the index of the first item matching an expression in an enumerable  lt  summary gt      lt param name  items  gt The enumerable to search  lt  param gt      lt param name  predicate  gt The expression to test the items against  lt  param gt      lt returns gt The index of the first matching item  or -1 if no items match  lt  returns gt  public static int FindIndex lt T gt  this IEnumerable lt T gt  items  Func lt T  bool gt  predicate        if  items    null  throw new ArgumentNullException  items        if  predicate    null  throw new ArgumentNullException  predicate         int retVal   0      foreach  var item in items            if  predicate item   return retVal          retVal              return -1        lt summary gt Finds the index of the first occurrence of an item in an enumerable  lt  summary gt      lt param name  items  gt The enumerable to search  lt  param gt      lt param name  item  gt The item to find  lt  param gt      lt returns gt The index of the first matching item  or -1 if the item was not found  lt  returns gt  public static int IndexOf lt T gt  this IEnumerable lt T gt  items  T item    return items FindIndex i   gt  EqualityComparer lt T gt  Default Equals item  i

User · Answer

myCars TakeWhile car   gt   myCondition car   Count      It works  Think about it  The index of the first matching item equals the number of  not matching  item before it   Story time  I too dislike the horrible standard solution you already suggested in your question  Like the accepted answer I went for a plain old loop although with a slight modification   public static int FindIndex lt T gt  this IEnumerable lt T gt  items  Predicate lt T gt  predicate        int index   0      foreach  var item in items            if  predicate item   break          index              return index      Note that it will return the number of items instead of -1 when there is no match  But let s ignore this minor annoyance for now  In fact the horrible standard solution crashes in that case and I consider returning an index that is out-of-bounds superior   What happens now is ReSharper telling me Loop can be converted into LINQ-expression  While most of the time the feature worsens readability  this time the result was awe-inspiring  So Kudos to the JetBrains   Analysis  Pros   Concise Combinable with other LINQ Avoids newing anonymous objects Only evaluates the enumerable until the predicate matches for the first time   Therefore I consider it optimal in time and space while remaining readable   Cons   Not quite obvious at first Does not return -1 when there is no match   Of course you can always hide it behind an extension method  And what to do best when there is no match heavily depends on the context

User · Answer

Here s an implementation of the highest-voted answer that returns -1 when the item is not found   public static int FindIndex lt T gt  this IEnumerable lt T gt  items  Func lt T  bool gt  predicate        var itemsWithIndices   items Select  item  index    gt  new   Item   item  Index   index         var matchingIndices           from itemWithIndex in itemsWithIndices         where predicate itemWithIndex Item          select  int  itemWithIndex Index       return matchingIndices FirstOrDefault      -1

[c#] How to get index using LINQ?

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