Possible to iterate backwards through a foreach

Question

I know I could use a for statement and achieve the same effect  but can I loop backwards through a foreach loop in C

User · Answer

Elaborateling slighty on the nice answer by Jon Skeet, this could be versatile:

public static IEnumerable<T> Directional<T>(this IList<T> items, bool Forwards) {
    if (Forwards) foreach (T item in items) yield return item;
    else for (int i = items.Count-1; 0<=i; i--) yield return items[i];
}

And then use as

foreach (var item in myList.Directional(forwardsCondition)) {
    .
    .
}

User · Answer

If you are on  NET 3 5 you can do this   IEnumerable lt int gt  enumerableThing        foreach  var x in enumerableThing Reverse      It isn t very efficient as it has to basically go through the enumerator forwards putting everything on a stack then pops everything back out in reverse order   If you have a directly-indexable collection  e g  IList  you should definitely use a for loop instead   If you are on  NET 2 0 and cannot use a for loop  i e  you just have an IEnumerable  then you will just have to write your own Reverse function  This should work   static IEnumerable lt T gt  Reverse lt T gt  IEnumerable lt T gt  input        return new Stack lt T gt  input       This relies on some behaviour which is perhaps not that obvious  When you pass in an IEnumerable to the stack constructor it will iterate through it and push the items onto the stack  When you then iterate through the stack it pops things back out in reverse order   This and the  NET 3 5 Reverse   extension method will obviously blow up if you feed it an IEnumerable which never stops returning items

User · Answer

This works pretty well  List lt string gt  list   new List lt string gt      list Add  Hello    list Add  Who    list Add  Are    list Add  You     foreach  String s in list        Console WriteLine list list Count - list IndexOf s  - 1

User · Answer

Before using foreach for iteration  reverse the list by the reverse method      myList Reverse        foreach  List listItem in myList               Console WriteLine listItem

User · Answer

It is possible if you can change the collection code that implements IEnumerable or IEnumerable  e g  your own implementation of IList    Create an Iterator doing this job for you  for example like the following implementation through the IEnumerable interface  assuming  items  is a List field in this sample    public IEnumerator lt TObject gt  GetEnumerator         for  var i   items Count - 1  i  gt   0  i--                 yield return items i            IEnumerator IEnumerable GetEnumerator         return GetEnumerator        Because of this your List will iterate in reverse order through your list   Just a hint  You should clearly state this special behaviour of your list within the documentation  even better by choosing a self-explaining class name like Stack or Queue  too

User · Answer

As 280Z28 says  for an IList lt T gt  you can just use the index  You could hide this in an extension method   public static IEnumerable lt T gt  FastReverse lt T gt  this IList lt T gt  items        for  int i   items Count-1  i  gt   0  i--                yield return items i             This will be faster than Enumerable Reverse   which buffers all the data first   I don t believe Reverse has any optimisations applied in the way that Count   does   Note that this buffering means that the data is read completely when you first start iterating  whereas FastReverse will  see  any changes made to the list while you iterate   It will also break if you remove multiple items between iterations    For general sequences  there s no way of iterating in reverse - the sequence could be infinite  for example   public static IEnumerable lt T gt  GetStringsOfIncreasingSize         string ret           while  true                yield return ret          ret   ret    x             What would you expect to happen if you tried to iterate over that in reverse

User · Answer

If you use a List lt T   you can also use this code   List lt string gt  list   new List lt string gt     list Add  1    list Add  2    list Add  3    list Reverse      This is a method that write the list reverse in itself   Now the foreach   foreach string s in list        Console WriteLine s       The output is   3 2 1

User · Answer

Sometimes you don t have the luxury of indexing  or perhaps you want to reverse the results of a Linq query  or maybe you don t want to modify the source collection  if any of these are true  Linq can help you   A Linq extension method using anonymous types with Linq Select to provide a sorting key for Linq OrderByDescending       public static IEnumerable lt T gt  Invert lt T gt  this IEnumerable lt T gt  source                var transform   source Select               o  i    gt  new                               Index   i                  Object   o                          return transform OrderByDescending o   gt  o Index                           Select o   gt  o Object           Usage       var eable   new     a    b    c          foreach var o in eable Invert                  Console WriteLine o                  c    b    a    It is named  Invert  because it is synonymous with  Reverse  and enables disambiguation with the List Reverse implementation   It is possible to reverse certain ranges of a collection too  since Int32 MinValue and Int32 MaxValue are out of the range of any kind of collection index  we can leverage them for the ordering process  if an element index is below the given range  it is assigned Int32 MaxValue so that its order doesn t change when using OrderByDescending  similarly  elements at an index greater than the given range  will be assigned Int32 MinValue  so that they appear to the end of the ordering process  All elements within the given range are assigned their normal index and are reversed accordingly       public static IEnumerable lt T gt  Invert lt T gt  this IEnumerable lt T gt  source  int index  int count                var transform   source Select               o  i    gt  new                               Index   i  lt  index   Int32 MaxValue   i  gt   index   count   Int32 MinValue   i                  Object   o                          return transform OrderByDescending o   gt  o Index                           Select o   gt  o Object           Usage       var eable   new     a    b    c    d          foreach var o in eable Invert 1  2                 Console WriteLine o                  a    c    b    d    I m not sure of the performance hits of these Linq implementations versus using a temporary List to wrap a collection for reversing     At time of writing  I was not aware of Linq s own Reverse implementation  still  it was fun working this out   https   msdn microsoft com en-us library vstudio bb358497 v vs 100  aspx

User · Answer

I have used this code which worked                  if  element HasAttributes                         foreach var attr in element Attributes   Reverse                                                   if  depth  gt  1                                                        elements upper hierarchy text                                   foreach  var ancest  in element Ancestors   Reverse                                                                  elements upper hierarchy text    ancest Name                                        foreach var ancest  in element Ancestors                                if  depth  gt  1                          xml taglist report          depth          elements upper hierarchy text  element Name         attr Name        attr Name                    attr Value     r n                           foreach var attr in element Attributes   Reverse                         if  element HasAttributes

User · Answer

When working with a list  direct indexing   you cannot do it as efficiently as using a for loop   Edit  Which generally means  when you are able to use a for loop  it s likely the correct method for this task  Plus  for as much as foreach is implemented in-order  the construct itself is built for expressing loops that are independent of element indexes and iteration order  which is particularly important in parallel programming  It is my opinion that iteration relying on order should not use foreach for looping

User · Answer

No  ForEach just iterates through collection for each item and order depends whether it uses IEnumerable or GetEnumerator

[c#] Possible to iterate backwards through a foreach?

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