Foreach loop determine which is the last iteration of the loop

Question

I have a foreach loop and need to execute some logic when the last item is chosen from the List  e g     foreach  Item result in Model Results             if current result is the last item in Model Results         then do something in the code      Can I know which loop is last without using for loop and counters

User · Answer

Jon Skeet created a SmartEnumerable lt T gt  type a while back to solve this exact issue  You can see it s implementation here   http   codeblog jonskeet uk 2007 07 27 smart-enumerations   To download  http   www yoda arachsys com csharp miscutil

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You can make an extension method specially dedicated to this   public static class EnumerableExtensions       public static bool IsLast lt T gt  this List lt T gt  items  T item                        if  items Count    0                  return false              T last   items items Count - 1               return item Equals last                     and you can use it like this   foreach  Item result in Model Results        if Model Results IsLast result                   do something in the code

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As Shimmy has pointed out  using Last   can be a performance problem  for instance if your collection is the live result of a LINQ expression  To prevent multiple iterations  you could use a  ForEach  extension method like this   var elements   new      A    B    C     elements ForEach  element  info    gt        if   info IsLast            Console WriteLine element         else           Console WriteLine  Last one      element               The extension method looks like this  as an added bonus  it will also tell you the index and if you re looking at the first element    public static class EnumerableExtensions       public delegate void ElementAction lt in T gt  T element  ElementInfo info        public static void ForEach lt T gt  this IEnumerable lt T gt  elements  ElementAction lt T gt  action            using  IEnumerator lt T gt  enumerator   elements GetEnumerator                          bool isFirst   true              bool hasNext   enumerator MoveNext                int index   0              while  hasNext                                T current   enumerator Current                  hasNext   enumerator MoveNext                    action current  new ElementInfo index  isFirst   hasNext                    isFirst   false                  index                                       public struct ElementInfo           public ElementInfo int index  bool isFirst  bool isLast                this                 Index   index              IsFirst   isFirst              IsLast   isLast                     public int Index   get  private set            public bool IsFirst   get  private set            public bool IsLast   get  private set

User · Answer

Using Last   on certain types will loop thru the entire collection  Meaning that if you make a foreach and call Last    you looped twice  which I m sure you d like to avoid in big collections   Then the solution is to use a do while loop   using var enumerator   collection GetEnumerator     var last    enumerator MoveNext    T current   while   last      current   enumerator Current               process item    last    enumerator MoveNext              if last            additional processing for last item         So unless the collection type is of type IList lt T gt  the Last   function will iterate thru all collection elements   Test  If your collection provides random access  e g  implements IList lt T gt    you can also check your item as follows   if collection is IList lt T gt  list    return collection  1     replace with collection Count -1 in pre-C 8 apps

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You could just use a for loop and there is no need to add an extra if inside the for body   for  int i   0  i  lt  Model Results Count - 1  i          var item   Model Results i       The -1 in the for condition takes care of skipping the last item

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If you just need to do something with the last element  as opposed to something different with the last element then using LINQ will help here   Item last   Model Results Last       do something with last   If you need to do something different with the last element then you d need something like   Item last   Model Results Last    foreach  Item result in Model Results           do something with each item     if  result Equals last                    do something different with the last item           else                  do something different with every item but the last           Though you d probably need to write a custom comparer to ensure that you could tell that the item was the same as the item returned by Last     This approach should be used with caution as Last may well have to iterate through the collection  While this might not be a problem for small collections  if it gets large it could have performance implications  It will also fail if the list contains duplicate items  In this cases something like this may be more appropriate   int totalCount   result Count    for  int count   0  count  lt  totalCount  count          Item result   Model Results count           do something with each item     if   count   1     totalCount                   do something different with the last item           else                  do something different with every item but the last

User · Answer

You can do like this    foreach  DataGridViewRow dgr in product list Rows        if  dgr Index    dgr DataGridView RowCount - 1                  do something

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Improving Daniel Wolf answer even further you could stack on another IEnumerable to avoid multiple iterations and lambdas such as   var elements   new      A    B    C     foreach  var e in elements Detailed          if   e IsLast            Console WriteLine e Value         else           Console WriteLine  Last one      e Value             The extension method implementation   public static class EnumerableExtensions       public static IEnumerable lt IterationElement lt T gt  gt  Detailed lt T gt  this IEnumerable lt T gt  source                if  source    null              throw new ArgumentNullException nameof source             using  var enumerator   source GetEnumerator                          bool isFirst   true              bool hasNext   enumerator MoveNext                int index   0              while  hasNext                                T current   enumerator Current                  hasNext   enumerator MoveNext                    yield return new IterationElement lt T gt  index  current  isFirst   hasNext                   isFirst   false                  index                                       public struct IterationElement lt T gt                public int Index   get            public bool IsFirst   get            public bool IsLast   get            public T Value   get             public IterationElement int index  T value  bool isFirst  bool isLast                        Index   index              IsFirst   isFirst              IsLast   isLast              Value   value

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To do something additional to each element except for the last one  function based approach can be used   delegate void DInner               Dinner inner delegate                 inner delegate                            do something additional                      foreach  DataGridViewRow dgr in product list Rows                inner             do something           This approach has apparent drawbacks  less code clarity for more complex cases  Calling delegates might be not very effective  Troubleshooting might be not quite easy  The bright side - coding is fun   Having said that  I would suggest using plain for loops in trivial cases  if you know that your collection s count is not terribly slow

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var last   objList LastOrDefault    foreach  var item in objList      if  item Equals last

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foreach  DataRow drow in ds Tables 0  Rows                                cnt sl1     lt div class  col-md-6  gt  lt div class  Slider-img  gt                                 lt div class  row  gt  lt img src      drow  images path   ToString        alt      gt                                 lt  div gt  lt  div gt  lt  div gt                    cnt sl2     lt div class  col-md-6  gt  lt div class  Slider-details  gt                                 lt p gt     drow  situation details   ToString       lt  p gt                                 lt  div gt  lt  div gt                    if  i    0                                        lblSituationName Text   drow  situation   ToString                                      if  drow  images position   ToString       0                                         content      lt div class  item  gt     cnt sl1   cnt sl2     lt  div gt                        cnt sl1                           cnt sl2                                         else if  drow  images position   ToString       1                                         content      lt div class  item  gt     cnt sl2   cnt sl1     lt  div gt                        cnt sl1                           cnt sl2                                         i

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The iterator implementation does not provide that  Your collection might be an IList that is accessible via an index in O 1   In that case you can use a normal for-loop   for int i   0  i  lt  Model Results Count  i        if i    Model Results Count - 1  doMagic        If you know the count  but cannot access via indices  thus  result is an ICollection   you can count yourself by incrementing an i in the foreach s body and comparing it to the length   All this isn t perfectly elegant  Chris s solution may be the nicest I ve seen so far

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As Chris shows  Linq will work  just use Last   to get a reference to the last one in the enumerable  and as long as you aren t working with that reference then do your normal code  but if you ARE working with that reference then do your extra thing  Its downside is that it will always be O N -complexity   You can instead use Count    which is O 1  if the IEnumerable is also an ICollection  this is true for most of the common built-in IEnumerables   and hybrid your foreach with a counter   var i 0  var count   Model Results Count    foreach  Item result in Model Results        if    i    count    this is the last item

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Last    didnt work for me  so I had to do something like this   Dictionary lt string  string gt  iterativeDictionary   someOtherDictionary  var index   0  iterativeDictionary ForEach kvp   gt       index      iterativeDictionary Count              it s the last item                it s not the last item

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using Linq and the foreach   foreach  Item result in Model Results               if  Model Results IndexOf result     Model Results Count - 1                    this is the last item            https   code i-harness com en q 7213ce

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Making some small adjustments to the excelent code of Jon Skeet  you can even make it smarter by allowing access to the previous and next item  Of course this means you ll have to read ahead 1 item in the implementation  For performance reasons  the previous and next item are only retained for the current iteration item  It goes like this   using System  using System Collections  using System Collections Generic  using System Linq  using System Text     Based on source  http   jonskeet uk csharp miscutil   namespace Generic Utilities            lt summary gt          Static class to make creation easier  If possible though  use the extension         method in SmartEnumerableExt           lt  summary gt      public static class SmartEnumerable                    lt summary gt              Extension method to make life easier               lt  summary gt               lt typeparam name  T  gt Type of enumerable lt  typeparam gt               lt param name  source  gt Source enumerable lt  param gt               lt returns gt A new SmartEnumerable of the appropriate type lt  returns gt          public static SmartEnumerable lt T gt  Create lt T gt  IEnumerable lt T gt  source                        return new SmartEnumerable lt T gt  source                             lt summary gt          Type chaining an IEnumerable amp lt T amp gt  to allow the iterating code         to detect the first and last entries simply           lt  summary gt           lt typeparam name  T  gt Type to iterate over lt  typeparam gt      public class SmartEnumerable lt T gt    IEnumerable lt SmartEnumerable lt T gt  Entry gt                      lt summary gt              Enumerable we proxy to              lt  summary gt          readonly IEnumerable lt T gt  enumerable                lt summary gt              Constructor               lt  summary gt               lt param name  enumerable  gt Collection to enumerate  Must not be null  lt  param gt          public SmartEnumerable IEnumerable lt T gt  enumerable                        if  enumerable    null                                throw new ArgumentNullException  enumerable                              this enumerable   enumerable                          lt summary gt              Returns an enumeration of Entry objects  each of which knows             whether it is the first last of the enumeration  as well as the             current value and next previous values               lt  summary gt          public IEnumerator lt Entry gt  GetEnumerator                         using  IEnumerator lt T gt  enumerator   enumerable GetEnumerator                                  if   enumerator MoveNext                                          yield break                                    bool isFirst   true                  bool isLast   false                  int index   0                  Entry previous   null                   T current   enumerator Current                  isLast    enumerator MoveNext                    var entry   new Entry isFirst  isLast  current  index    previous                                   isFirst   false                  previous   entry                   while   isLast                                        T next   enumerator Current                      isLast    enumerator MoveNext                        var entry2   new Entry isFirst  isLast  next  index    entry                       entry SetNext entry2                       yield return entry                       previous UnsetLinks                        previous   entry                      entry   entry2                                                         yield return entry                  previous UnsetLinks                                          lt summary gt              Non-generic form of GetEnumerator               lt  summary gt          IEnumerator IEnumerable GetEnumerator                         return GetEnumerator                            lt summary gt              Represents each entry returned within a collection              containing the value and whether it is the first and or             the last entry in the collection s  enumeration              lt  summary gt          public class Entry                        region Fields             private readonly bool isFirst              private readonly bool isLast              private readonly T value              private readonly int index              private Entry previous              private Entry next   null               endregion               region Properties                  lt summary gt                  The value of the entry                   lt  summary gt              public T Value   get   return value                        lt summary gt                  Whether or not this entry is first in the collection s enumeration                   lt  summary gt              public bool IsFirst   get   return isFirst                        lt summary gt                  Whether or not this entry is last in the collection s enumeration                   lt  summary gt              public bool IsLast   get   return isLast                        lt summary gt                  The 0-based index of this entry  i e  how many entries have been returned before this one                   lt  summary gt              public int Index   get   return index                        lt summary gt                  Returns the previous entry                  Only available for the CURRENT entry                   lt  summary gt              public Entry Previous   get   return previous                        lt summary gt                  Returns the next entry for the current iterator                  Only available for the CURRENT entry                   lt  summary gt              public Entry Next   get   return next                   endregion               region Constructors             internal Entry bool isFirst  bool isLast  T value  int index  Entry previous                                this isFirst   isFirst                  this isLast   isLast                  this value   value                  this index   index                  this previous   previous                             endregion               region Methods                  lt summary gt                  Fix the link to the next item of the IEnumerable                  lt  summary gt                   lt param name  entry  gt  lt  param gt              internal void SetNext Entry entry                                next   entry                                  lt summary gt                  Allow previous and next Entry to be garbage collected by setting them to null                  lt  summary gt              internal void UnsetLinks                                 previous   null                  next   null                                  lt summary gt                  Returns   index value                   lt  summary gt                   lt returns gt  lt  returns gt              public override string ToString                                 return String Format    0   1    Index  Value                              endregion

User · Answer

How to convert foreach to react to the last element   List lt int gt  myList   new List lt int gt     1  2  3  4  5   Console WriteLine  foreach version          foreach  var current in myList                Console WriteLine current           Console WriteLine  equivalent that reacts to last element          var enumerator   myList GetEnumerator        if  enumerator MoveNext      true     Corner case  empty list                while  true                        int current   enumerator Current                  Handle current element here              Console WriteLine current                bool ifLastElement    enumerator MoveNext      false               if  ifLastElement                                   Cleanup after last element                 Console WriteLine   last element                     break                                    enumerator Dispose

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The best approach would probably be just to execute that step after the loop  e g   foreach Item result in Model Results         loop logic      Post execution logic   Or if you need to do something to the last result  foreach Item result in Model Results         loop logic    Item lastItem   Model Results Model Results Count - 1      Execute logic on lastItem here

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List lt int gt  ListInt   new List lt int gt    1  2  3  4  5  6  7  8  9  10                      int count   ListInt Count                  int index   1                  foreach  var item in ListInt                                        if  index    count                                                Console WriteLine  do something at index number      index                                             else                                               Console WriteLine  Foreach loop  this is the last iteration of the loop     index                                             index                          OR                 int count   ListInt Count                  int index   1                  foreach  var item in ListInt                                        if  index  lt  count                                                Console WriteLine  do something at index number      index                                             else                                               Console WriteLine  Foreach loop  this is the last iteration of the loop     index                                             index

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What about little simpler approach   Item last   null  foreach  Item result in Model Results           do something with each item      last   result       Here Item  last  contains the last object that came in the last of foreach loop  DoSomethingOnLastElement last

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Another way  which I didn t see posted  is to use a Queue   It s analogous to a way to implement a SkipLast   method without iterating more than necessary   This way will also allow you to do this on any number of last items   public static void ForEachAndKnowIfLast lt T gt       this IEnumerable lt T gt  source      Action lt T  bool gt  a      int numLastItems   1        int bufferMax   numLastItems   1      var buffer   new Queue lt T gt  bufferMax       foreach  T x in source                buffer Enqueue x           if  buffer Count  lt  bufferMax              continue    Until the buffer is full  just add to it          a buffer Dequeue    false             foreach  T item in buffer          a item  true       To call this you d do the following   Model Results ForEachAndKnowIfLast       result  isLast    gt                  your logic goes here  using isLast to do things differently for last item s

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Based on  Shimmy s response  I created an extension method that is the solution that everyone wants  It is simple  easy to use  and only loops through the collection once   internal static class EnumerableExtensions       public static void ForEachLast lt T gt  this IEnumerable lt T gt  collection  Action lt T gt   actionExceptLast   null  Action lt T gt   actionOnLast   null                using var enumerator   collection GetEnumerator            var isNotLast   enumerator MoveNext            while  isNotLast                        var current   enumerator Current              isNotLast   enumerator MoveNext                var action   isNotLast   actionExceptLast   actionOnLast              action  Invoke current                       This works on any IEnumerable lt T gt   Usage looks like this   var items   new    1  2  3  4  5   items ForEachLast i   gt  Console WriteLine    i      i   gt  Console WriteLine i      Output looks like   1  2  3  4  5   Additionally  you can make this into a Select style method  Then  reuse that extension in the ForEach  That code looks like this   internal static class EnumerableExtensions       public static void ForEachLast lt T gt  this IEnumerable lt T gt  collection  Action lt T gt   actionExceptLast   null  Action lt T gt   actionOnLast   null    gt             ReSharper disable once IteratorMethodResultIsIgnored         collection SelectLast i   gt    actionExceptLast  Invoke i   return true     i   gt    actionOnLast  Invoke i   return true     ToArray         public static IEnumerable lt TResult gt  SelectLast lt T  TResult gt  this IEnumerable lt T gt  collection  Func lt T  TResult gt   selectorExceptLast   null  Func lt T  TResult gt   selectorOnLast   null                using var enumerator   collection GetEnumerator            var isNotLast   enumerator MoveNext            while  isNotLast                        var current   enumerator Current              isNotLast   enumerator MoveNext                var selector   isNotLast   selectorExceptLast   selectorOnLast                https   stackoverflow com a 32580613 294804             if  selector    null                                yield return selector Invoke current

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Just store the previous value and work with it inside the loop  Then at the end the  previous  value will be the last item  letting you handle it differently  No counting or special libraries required   bool empty   true  Item previousItem   foreach  Item result in Model Results        if   empty                   We know this isn t the last item because it came from the previous iteration         handleRegularItem previousItem              previousItem   result      empty   false     if   empty           We know this is the last item because the loop is finished     handleLastItem previousItem

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How about a good old fashioned for loop   for  int i   0  i  lt  Model Results Count  i            if  i    Model Results Count - 1                  this is the last item            Or using Linq and the foreach   foreach  Item result in Model Results               if  Model Results IndexOf result     Model Results Count - 1                    this is the last item

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We can check last item in loop   foreach  Item result in Model Results        if  result  Model Results Last                     do something different with the last item

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The accepted answer will not work for duplicates in the collection  If you re set on the foreach  you can just add your own indexing variable s    int last   Model Results Count - 1  int index   0  foreach  Item result in Model Results          Do Things      if  index    last            Do Things with the last result      index

[c#] Foreach loop, determine which is the last iteration of the loop

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