Counter in foreach loop in C

Question

Working of foreach  As I know       foreach is a loop which iterates through a collection or array one by   one  starting from 0 index till the   last item of the collection    So  if I have n items in an array   foreach  var item in arr           then   In  1st iteration  item arr 0   then  in 2nd       item arr 1         in last  nth    item arr n-1    Conclusion  from working it seems that at each iteration it knows that which value to be taken from array or it knows the index of the item to be taken from array   Now my question  How can I get index of an item without using a new variable   foreach  string item in mylist       if  item     myitem                  get index of item        break

User · Answer

The sequence being iterated in a foreach loop might not support indexing or know such a concept it just needs to implement a method called GetEnumerator that returns an object that as a minimum has the interface of IEnumerator though implmenting it is not required. If you know that what you iterate does support indexing and you need the index then I suggest to use a for loop instead.

An example class that can be used in foreach:

    class Foo {

        public iterator GetEnumerator() {
            return new iterator();
        }

        public class iterator {
            public Bar Current {
                get{magic}
            }

            public bool MoveNext() {
                incantation
            }
        }
    }

User · Answer

Use for instead of foreach  foreach doesn t expose its inner workings  it enumerates anything that is IEnumerable  which doesn t have to have an index at all    for  int i 0  i lt arr Length  i                  Besides  if what you re trying to do is find the index of a particular item in the list  you don t have to iterate it at all by yourself  Use Array IndexOf item  instead

User · Answer

Your understanding of foreach is incomplete   It works with any type that exposes IEnumerable  or implements a GetEnumerable method  and uses the returned IEnumerator to iterate over the items in the collection   How the Enumerator does this  using an index  yield statement or magic  is an implementation detail   In order to achieve what you want  you should use a for loop   for  int i   0  i  lt  mylist Count  i            Note   Getting the number of items in a list is slightly different depending on the type of list  For Collections  Use Count    property  For Arrays       Use Length   property  For IEnumerable  Use Count    Linq method

User · Answer

This is only true if you re iterating through an array  what if you were iterating through a different kind of collection that has no notion of accessing by index   In the array case  the easiest way to retain the index is to simply use a vanilla for loop

User · Answer

Not all collections have indexes  For instance  I can use a Dictionary with foreach  and iterate through all the keys and values   but I can t write get at individual elements using dictionary 0   dictionary 1  etc   If I did want to iterate through a dictionary and keep track of an index  I d have to use a separate variable that I incremented myself

User · Answer

From MSDN      The foreach statement repeats a group   of embedded statements for each   element in an array or an object   collection that implements the   System Collections IEnumerable or   System Collections Generic IEnumerable Of   T  interface    So  it s not necessarily Array  It could even be a lazy collection with no idea about the count of items in the collection

User · Answer

Or even more simple if you don t want to use a lot of linq and for some reason don t want to use a for loop   int i   0  foreach var x in arr         Do some stuff    i

User · Answer

It depends what you mean by  it   The iterator knows what index it s reached  yes - in the case of a List lt T gt  or an array  But there s no general index within IEnumerator lt T gt   Whether it s iterating over an indexed collection or not is up to the implementation  Plenty of collections don t support direct indexing    In fact  foreach doesn t always use an iterator at all  If the compile-time type of the collection is an array  the compiler will iterate over it using array 0   array 1  etc  Likewise the collection can have a method called GetEnumerator   which returns a type with the appropriate members  but without any implementation of IEnumerable IEnumerator in sight    Options for maintaining an index    Use a for loop Use a separate variable Use a projection which projects each item to an index value pair  e g    foreach  var x in list Select  value  index    gt  new   value  index                Use x value and x index in here     Use my SmartEnumerable class which is a little bit like the previous option   All but the first of these options will work whether or not the collection is naturally indexed

User · Answer

Without Custom Foreach Version   datas Where  data  index    gt          Your Logic     return false     Any      In some simple case my way is using where   false   any  It is fater a little than foreach   select  data index   gt new data index   and without custom Foreach method     MyLogic           use statement body run your logic      because return false new Enumrable data count is zero      use Any   let yeild run         Benchmark Test Code   RPlotExporter  RankColumn  public class BenchmarkTest       public static IEnumerable lt dynamic gt  TestDatas   Enumerable Range 1  10  Select  data  index    gt    item no  index           Benchmark      public static void ToArrayAndFor                 var datats   TestDatas ToArray            for  int index   0  index  lt  datats Length  index                          var result      datats index   index                          Benchmark      public static void IEnumrableAndForach                 var index   0          foreach  var item in TestDatas                        index                var result      item  index                          Benchmark      public static void LinqSelectForach                 foreach  var item in TestDatas Select  data  index    gt  new   index  data                           var result      item data  item index                          Benchmark      public static void LinqSelectStatementBodyToList                 TestDatas Select  data  index    gt                        var result      data  index                return true             ToList                Benchmark      public static void LinqSelectStatementBodyToArray                 TestDatas Select  data  index    gt                        var result      data  index                return true             ToArray                Benchmark      public static void LinqWhereStatementBodyAny                 TestDatas Where  data  index    gt                        var result      data  index                return false             Any             class Program       static void Main string   args                var summary   BenchmarkRunner Run lt BenchmarkTest gt              System Console Read              Benchmark Result                             Method       Mean       Error      StdDev   Rank   -------------------------------  ---------  ----------  ----------  -----                     ToArrayAndFor   4 027 us   0 0797 us   0 1241 us      4               IEnumrableAndForach   3 494 us   0 0321 us   0 0285 us      1                  LinqSelectForach   3 842 us   0 0503 us   0 0471 us      3     LinqSelectStatementBodyToList   3 822 us   0 0416 us   0 0389 us      3    LinqSelectStatementBodyToArray   3 857 us   0 0764 us   0 0785 us      3         LinqWhereStatementBodyAny   3 643 us   0 0693 us   0 0712 us      2

User · Answer

Probably pointless  but     foreach  var item in yourList Select  Value  Index    gt  new   Value  Index           Console WriteLine  Value     item Value      Index     item Index

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