Generic List - moving an item within the list

Question

So I have a generic list  and an oldIndex and a newIndex value   I want to move the item at oldIndex  to newIndex   as simply as possible   Any suggestions   Note  The item should be end up between the items at  newIndex - 1  and newIndex before it was removed

User · Accepted Answer

I know you said  generic list  but you didn t specify that you needed to use the List T  class so here is a shot at something different   The ObservableCollection T  class has a Move method that does exactly what you want   public void Move int oldIndex  int newIndex    Underneath it is basically implemented like this   T item   base oldIndex   base RemoveItem oldIndex   base InsertItem newIndex  item     So as you can see the swap method that others have suggested is essentially what the ObservableCollection does in it s own Move method    UPDATE 2015-12-30  You can see the source code for the Move and MoveItem methods in corefx now for yourself without using Reflector ILSpy since  NET is open source

User · Answer

Simplest way   list newIndex    list oldIndex   list RemoveAt oldIndex     EDIT  The question isn t very clear     Since we don t care where the list newIndex  item goes I think the simplest way of doing this is as follows  with or without an extension method        public static void Move lt T gt  this List lt T gt  list  int oldIndex  int newIndex                T aux   list newIndex           list newIndex    list oldIndex           list oldIndex    aux          This solution is the fastest because it doesn t involve list insertions removals

User · Answer

Is more simple guys just do this      public void MoveUp object item List Concepts            int ind   Concepts IndexOf item ToString              if  ind    0                        Concepts RemoveAt ind               Concepts Insert ind-1 item ToString                 obtenernombres                NotifyPropertyChanged  Concepts                 Do the same with MoveDown but change the if for  if  ind   Concepts Count     and the Concepts Insert ind 1 item ToString

User · Answer

var item   list oldIndex    list RemoveAt oldIndex    if  newIndex  gt  oldIndex  newIndex--      the actual index could have shifted due to the removal  list Insert newIndex  item    Put into Extension methods they look like      public static void Move lt T gt  this List lt T gt  list  int oldIndex  int newIndex                var item   list oldIndex            list RemoveAt oldIndex            if  newIndex  gt  oldIndex  newIndex--             the actual index could have shifted due to the removal          list Insert newIndex  item              public static void Move lt T gt  this List lt T gt  list  T item  int newIndex                if  item    null                        var oldIndex   list IndexOf item               if  oldIndex  gt  -1                                list RemoveAt oldIndex                    if  newIndex  gt  oldIndex  newIndex--                     the actual index could have shifted due to the removal                  list Insert newIndex  item

User · Answer

Insert the item currently at oldIndex to be at newIndex and then remove the original instance   list Insert newIndex  list oldIndex    if  newIndex  lt   oldIndex    oldIndex  list RemoveAt oldIndex     You have to take into account that the index of the item you want to remove may change due to the insertion

User · Answer

var item   list oldIndex    list RemoveAt oldIndex    if  newIndex  gt  oldIndex  newIndex--      the actual index could have shifted due to the removal  list Insert newIndex  item    Put into Extension methods they look like      public static void Move lt T gt  this List lt T gt  list  int oldIndex  int newIndex                var item   list oldIndex            list RemoveAt oldIndex            if  newIndex  gt  oldIndex  newIndex--             the actual index could have shifted due to the removal          list Insert newIndex  item              public static void Move lt T gt  this List lt T gt  list  T item  int newIndex                if  item    null                        var oldIndex   list IndexOf item               if  oldIndex  gt  -1                                list RemoveAt oldIndex                    if  newIndex  gt  oldIndex  newIndex--                     the actual index could have shifted due to the removal                  list Insert newIndex  item

User · Answer

I created an extension method for moving items in a list   An index should not shift if we are moving an existing item since we are moving an item to an existing index position in the list   The edge case that  Oliver refers to below  moving an item to the end of the list  would actually cause the tests to fail  but this is by design  To insert a new item at the end of the list we would just call List lt T gt  Add  list Move predicate  list Count  should fail since this index position does not exist before the move   In any case  I ve created two additional extension methods  MoveToEnd and MoveToBeginning  the source of which can be found here        lt summary gt      Extension methods for  lt see cref  System Collections Generic List T    gt       lt  summary gt  public static class ListExtensions            lt summary gt          Moves the item matching the  lt paramref name  itemSelector   gt  to the  lt paramref name  newIndex   gt  in a list           lt  summary gt      public static void Move lt T gt  this List lt T gt  list  Predicate lt T gt  itemSelector  int newIndex                Ensure Argument NotNull list   list            Ensure Argument NotNull itemSelector   itemSelector            Ensure Argument Is newIndex  gt   0   New index must be greater than or equal to zero              var currentIndex   list FindIndex itemSelector           Ensure That lt ArgumentException gt  currentIndex  gt   0   No item was found that matches the specified selector                 Copy the current item         var item   list currentIndex               Remove the item         list RemoveAt currentIndex               Finally add the item at the new index         list Insert newIndex  item             Subject typeof ListExtensions    Move    public class List Move       static List lt int gt  list       public class When no matching item is found               static Exception exception           Establish ctx        gt                list   new List lt int gt                         Because of                    gt  exception   Catch Exception      gt  list Move x   gt  x    10  10             It Should throw an exception                    gt  exception ShouldBeOfType lt ArgumentException gt                public class When new index is higher               Establish ctx        gt                list   new List lt int gt    1  2  3  4  5                        Because of                    gt  list Move x   gt  x    3  4      move 3 to end of list  index 4           It Should be moved to the specified index        gt                                list 0  ShouldEqual 1                   list 1  ShouldEqual 2                   list 2  ShouldEqual 4                   list 3  ShouldEqual 5                   list 4  ShouldEqual 3                             public class When new index is lower               Establish ctx        gt                list   new List lt int gt    1  2  3  4  5                        Because of                    gt  list Move x   gt  x    4  0      move 4 to beginning of list  index 0           It Should be moved to the specified index        gt                        list 0  ShouldEqual 4               list 1  ShouldEqual 1               list 2  ShouldEqual 2               list 3  ShouldEqual 3               list 4  ShouldEqual 5

User · Answer

List lt T gt  Remove   and List lt T gt  RemoveAt   do not return the item that is being removed   Therefore you have to use this   var item   list oldIndex   list RemoveAt oldIndex   list Insert newIndex  item

User · Answer

Is more simple guys just do this      public void MoveUp object item List Concepts            int ind   Concepts IndexOf item ToString              if  ind    0                        Concepts RemoveAt ind               Concepts Insert ind-1 item ToString                 obtenernombres                NotifyPropertyChanged  Concepts                 Do the same with MoveDown but change the if for  if  ind   Concepts Count     and the Concepts Insert ind 1 item ToString

User · Answer

Simplest way   list newIndex    list oldIndex   list RemoveAt oldIndex     EDIT  The question isn t very clear     Since we don t care where the list newIndex  item goes I think the simplest way of doing this is as follows  with or without an extension method        public static void Move lt T gt  this List lt T gt  list  int oldIndex  int newIndex                T aux   list newIndex           list newIndex    list oldIndex           list oldIndex    aux          This solution is the fastest because it doesn t involve list insertions removals

User · Answer

List lt T gt  Remove   and List lt T gt  RemoveAt   do not return the item that is being removed   Therefore you have to use this   var item   list oldIndex   list RemoveAt oldIndex   list Insert newIndex  item

User · Answer

I know this question is old but I adapted THIS response of javascript code to C   Hope it helps public static void Move lt T gt  this List lt T gt  list  int oldIndex  int newIndex           exit if positions are equal or outside array     if   oldIndex    newIndex      0  gt  oldIndex      oldIndex  gt   list Count      0  gt  newIndex              newIndex  gt   list Count   return         local variables     var i   0      T tmp   list oldIndex          move element down and shift other elements up     if  oldIndex  lt  newIndex                for  i   oldIndex  i  lt  newIndex  i                          list i    list i   1                              move element up and shift other elements down     else               for  i   oldIndex  i  gt  newIndex  i--                        list i    list i - 1                          put element from position 1 to destination     list newIndex    tmp

User · Answer

List lt T gt  Remove   and List lt T gt  RemoveAt   do not return the item that is being removed   Therefore you have to use this   var item   list oldIndex   list RemoveAt oldIndex   list Insert newIndex  item

User · Answer

var item   list oldIndex    list RemoveAt oldIndex    if  newIndex  gt  oldIndex  newIndex--      the actual index could have shifted due to the removal  list Insert newIndex  item    Put into Extension methods they look like      public static void Move lt T gt  this List lt T gt  list  int oldIndex  int newIndex                var item   list oldIndex            list RemoveAt oldIndex            if  newIndex  gt  oldIndex  newIndex--             the actual index could have shifted due to the removal          list Insert newIndex  item              public static void Move lt T gt  this List lt T gt  list  T item  int newIndex                if  item    null                        var oldIndex   list IndexOf item               if  oldIndex  gt  -1                                list RemoveAt oldIndex                    if  newIndex  gt  oldIndex  newIndex--                     the actual index could have shifted due to the removal                  list Insert newIndex  item

User · Answer

I would expect either      Makes sure item is at newIndex after the operation T item   list oldIndex   list RemoveAt oldIndex   list Insert newIndex  item         or      Makes sure relative ordering of newIndex is preserved after the operation      meaning that the item may actually be inserted at newIndex - 1  T item   list oldIndex   list RemoveAt oldIndex   newIndex    newIndex  gt  oldIndex   newIndex - 1  newIndex  list Insert newIndex  item         would do the trick  but I don t have VS on this machine to check

User · Answer

This is how I implemented a move element extension method   It handles moving before after and to the extremes for elements pretty well   public static void MoveElement lt T gt  this IList lt T gt  list  int fromIndex  int toIndex      if   fromIndex InRange 0  list Count - 1           throw new ArgumentException  From index is invalid          if   toIndex InRange 0  list Count - 1           throw new ArgumentException  To index is invalid           if  fromIndex    toIndex  return     var element   list fromIndex      if  fromIndex  gt  toIndex          list RemoveAt fromIndex       list Insert toIndex  element         else         list Insert toIndex   1  element       list RemoveAt fromIndex

User · Answer

I would expect either      Makes sure item is at newIndex after the operation T item   list oldIndex   list RemoveAt oldIndex   list Insert newIndex  item         or      Makes sure relative ordering of newIndex is preserved after the operation      meaning that the item may actually be inserted at newIndex - 1  T item   list oldIndex   list RemoveAt oldIndex   newIndex    newIndex  gt  oldIndex   newIndex - 1  newIndex  list Insert newIndex  item         would do the trick  but I don t have VS on this machine to check

User · Answer

I would expect either      Makes sure item is at newIndex after the operation T item   list oldIndex   list RemoveAt oldIndex   list Insert newIndex  item         or      Makes sure relative ordering of newIndex is preserved after the operation      meaning that the item may actually be inserted at newIndex - 1  T item   list oldIndex   list RemoveAt oldIndex   newIndex    newIndex  gt  oldIndex   newIndex - 1  newIndex  list Insert newIndex  item         would do the trick  but I don t have VS on this machine to check

User · Answer

var item   list oldIndex    list RemoveAt oldIndex    if  newIndex  gt  oldIndex  newIndex--      the actual index could have shifted due to the removal  list Insert newIndex  item    Put into Extension methods they look like      public static void Move lt T gt  this List lt T gt  list  int oldIndex  int newIndex                var item   list oldIndex            list RemoveAt oldIndex            if  newIndex  gt  oldIndex  newIndex--             the actual index could have shifted due to the removal          list Insert newIndex  item              public static void Move lt T gt  this List lt T gt  list  T item  int newIndex                if  item    null                        var oldIndex   list IndexOf item               if  oldIndex  gt  -1                                list RemoveAt oldIndex                    if  newIndex  gt  oldIndex  newIndex--                     the actual index could have shifted due to the removal                  list Insert newIndex  item

User · Answer

Simplest way   list newIndex    list oldIndex   list RemoveAt oldIndex     EDIT  The question isn t very clear     Since we don t care where the list newIndex  item goes I think the simplest way of doing this is as follows  with or without an extension method        public static void Move lt T gt  this List lt T gt  list  int oldIndex  int newIndex                T aux   list newIndex           list newIndex    list oldIndex           list oldIndex    aux          This solution is the fastest because it doesn t involve list insertions removals

User · Answer

This is how I implemented a move element extension method   It handles moving before after and to the extremes for elements pretty well   public static void MoveElement lt T gt  this IList lt T gt  list  int fromIndex  int toIndex      if   fromIndex InRange 0  list Count - 1           throw new ArgumentException  From index is invalid          if   toIndex InRange 0  list Count - 1           throw new ArgumentException  To index is invalid           if  fromIndex    toIndex  return     var element   list fromIndex      if  fromIndex  gt  toIndex          list RemoveAt fromIndex       list Insert toIndex  element         else         list Insert toIndex   1  element       list RemoveAt fromIndex

User · Answer

Insert the item currently at oldIndex to be at newIndex and then remove the original instance   list Insert newIndex  list oldIndex    if  newIndex  lt   oldIndex    oldIndex  list RemoveAt oldIndex     You have to take into account that the index of the item you want to remove may change due to the insertion

User · Answer

I would expect either      Makes sure item is at newIndex after the operation T item   list oldIndex   list RemoveAt oldIndex   list Insert newIndex  item         or      Makes sure relative ordering of newIndex is preserved after the operation      meaning that the item may actually be inserted at newIndex - 1  T item   list oldIndex   list RemoveAt oldIndex   newIndex    newIndex  gt  oldIndex   newIndex - 1  newIndex  list Insert newIndex  item         would do the trick  but I don t have VS on this machine to check

User · Answer

List lt T gt  Remove   and List lt T gt  RemoveAt   do not return the item that is being removed   Therefore you have to use this   var item   list oldIndex   list RemoveAt oldIndex   list Insert newIndex  item

User · Answer

Insert the item currently at oldIndex to be at newIndex and then remove the original instance   list Insert newIndex  list oldIndex    if  newIndex  lt   oldIndex    oldIndex  list RemoveAt oldIndex     You have to take into account that the index of the item you want to remove may change due to the insertion

User · Answer

I created an extension method for moving items in a list   An index should not shift if we are moving an existing item since we are moving an item to an existing index position in the list   The edge case that  Oliver refers to below  moving an item to the end of the list  would actually cause the tests to fail  but this is by design  To insert a new item at the end of the list we would just call List lt T gt  Add  list Move predicate  list Count  should fail since this index position does not exist before the move   In any case  I ve created two additional extension methods  MoveToEnd and MoveToBeginning  the source of which can be found here        lt summary gt      Extension methods for  lt see cref  System Collections Generic List T    gt       lt  summary gt  public static class ListExtensions            lt summary gt          Moves the item matching the  lt paramref name  itemSelector   gt  to the  lt paramref name  newIndex   gt  in a list           lt  summary gt      public static void Move lt T gt  this List lt T gt  list  Predicate lt T gt  itemSelector  int newIndex                Ensure Argument NotNull list   list            Ensure Argument NotNull itemSelector   itemSelector            Ensure Argument Is newIndex  gt   0   New index must be greater than or equal to zero              var currentIndex   list FindIndex itemSelector           Ensure That lt ArgumentException gt  currentIndex  gt   0   No item was found that matches the specified selector                 Copy the current item         var item   list currentIndex               Remove the item         list RemoveAt currentIndex               Finally add the item at the new index         list Insert newIndex  item             Subject typeof ListExtensions    Move    public class List Move       static List lt int gt  list       public class When no matching item is found               static Exception exception           Establish ctx        gt                list   new List lt int gt                         Because of                    gt  exception   Catch Exception      gt  list Move x   gt  x    10  10             It Should throw an exception                    gt  exception ShouldBeOfType lt ArgumentException gt                public class When new index is higher               Establish ctx        gt                list   new List lt int gt    1  2  3  4  5                        Because of                    gt  list Move x   gt  x    3  4      move 3 to end of list  index 4           It Should be moved to the specified index        gt                                list 0  ShouldEqual 1                   list 1  ShouldEqual 2                   list 2  ShouldEqual 4                   list 3  ShouldEqual 5                   list 4  ShouldEqual 3                             public class When new index is lower               Establish ctx        gt                list   new List lt int gt    1  2  3  4  5                        Because of                    gt  list Move x   gt  x    4  0      move 4 to beginning of list  index 0           It Should be moved to the specified index        gt                        list 0  ShouldEqual 4               list 1  ShouldEqual 1               list 2  ShouldEqual 2               list 3  ShouldEqual 3               list 4  ShouldEqual 5

User · Answer

Insert the item currently at oldIndex to be at newIndex and then remove the original instance   list Insert newIndex  list oldIndex    if  newIndex  lt   oldIndex    oldIndex  list RemoveAt oldIndex     You have to take into account that the index of the item you want to remove may change due to the insertion

User · Answer

Simplest way   list newIndex    list oldIndex   list RemoveAt oldIndex     EDIT  The question isn t very clear     Since we don t care where the list newIndex  item goes I think the simplest way of doing this is as follows  with or without an extension method        public static void Move lt T gt  this List lt T gt  list  int oldIndex  int newIndex                T aux   list newIndex           list newIndex    list oldIndex           list oldIndex    aux          This solution is the fastest because it doesn t involve list insertions removals

User · Answer

I know this question is old but I adapted THIS response of javascript code to C   Hope it helps public static void Move lt T gt  this List lt T gt  list  int oldIndex  int newIndex           exit if positions are equal or outside array     if   oldIndex    newIndex      0  gt  oldIndex      oldIndex  gt   list Count      0  gt  newIndex              newIndex  gt   list Count   return         local variables     var i   0      T tmp   list oldIndex          move element down and shift other elements up     if  oldIndex  lt  newIndex                for  i   oldIndex  i  lt  newIndex  i                          list i    list i   1                              move element up and shift other elements down     else               for  i   oldIndex  i  gt  newIndex  i--                        list i    list i - 1                          put element from position 1 to destination     list newIndex    tmp

[c#] Generic List - moving an item within the list

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