Editing dictionary values in a foreach loop

Question

I am trying to build a pie chart from a dictionary  Before I display the pie chart  I want to tidy up the data  I m removing any pie slices that would be less than 5  of the pie and putting them in a  Other  pie slice  However I m getting a Collection was modified  enumeration operation may not execute exception at runtime   I understand why you can not add or remove items from a dictionary while iterating over them  However I don t understand why you can t simply change a value for an existing key within the foreach loop   Any suggestions re  fixing my code  would be appreciated   Dictionary lt string  int gt  colStates   new Dictionary lt string int gt               Some code to populate colStates dictionary         int OtherCount   0   foreach string key in colStates Keys         double  Percent   colStates key    TotalCount       if  Percent  lt  0 05                OtherCount    colStates key           colStates key    0           colStates Add  Other   OtherCount

User · Answer

Disclaimer  I don t do much C   You are trying to modify the DictionaryEntry object which is stored in the HashTable  The Hashtable only stores one object -- your instance of DictionaryEntry  Changing the Key or the Value is enough to change the HashTable and cause the enumerator to become invalid   You can do it outside of the loop   if hashtable Contains key         hashtable key    value      by first creating a list of all the keys of the values you wish to change and iterate through that list instead

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Starting with  NET 4 5 You can do this with ConcurrentDictionary   using System Collections Concurrent   var colStates   new ConcurrentDictionary lt string int gt     colStates  foo     1  colStates  bar     2  colStates  baz     3   int OtherCount   0  int TotalCount   100   foreach string key in colStates Keys        double Percent    double colStates key    TotalCount       if  Percent  lt  0 05                OtherCount    colStates key           colStates key    0           colStates TryAdd  Other   OtherCount     Note however that its performance is actually much worse that a simple foreach dictionary Kes ToArray     using System  using System Collections Concurrent  using System Collections Generic  using System Linq  using BenchmarkDotNet Attributes  using BenchmarkDotNet Running   public class ConcurrentVsRegularDictionary       private readonly Random  rand      private const int Count   1 000       public ConcurrentVsRegularDictionary                  rand   new Random                Benchmark      public void ConcurrentDictionary                 var dict   new ConcurrentDictionary lt int  int gt             Populate dict            foreach  var key in dict Keys                        dict key     rand Next                          Benchmark      public void Dictionary                 var dict   new Dictionary lt int  int gt             Populate dict            foreach  var key in dict Keys ToArray                          dict key     rand Next                         private void Populate IDictionary lt int  int gt  dictionary                for  int i   0  i  lt  Count  i                          dictionary i    0                     public class Program       public static void Main string   args                BenchmarkRunner Run lt ConcurrentVsRegularDictionary gt               Result                 Method        Mean       Error      StdDev   ---------------------  ----------  ----------  ----------    ConcurrentDictionary   182 24 us   3 1507 us   2 7930 us              Dictionary    47 01 us   0 4824 us   0 4512 us

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Along with the other answers  I thought I d note that if you get sortedDictionary Keys or sortedDictionary Values and then loop over them with foreach  you also go through in sorted order  This is because those methods return  System Collections Generic SortedDictionary lt TKey TValue gt  KeyCollection or SortedDictionary lt TKey TValue gt  ValueCollection objects  which maintain the sort of the original dictionary

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This answer is for comparing two solutions  not a suggested solution  Instead of creating another list as other answers suggested  you can used a for loop using the dictionary Count for the loop stop condition and Keys ElementAt i  to get the key  for  int i   0  i  lt  dictionary Count  i          dictionary dictionary Keys ElementAt i     0     At firs I thought this would be more efficient because we do not need to create a key list  After running a test I found that the for loop solution is much less efficient  The reason is because ElementAt is O n  on the dictionary Keys property  it searches from the beginning of the collection until it gets to the nth item  Test  int iterations   10  int dictionarySize   10000  Stopwatch sw   new Stopwatch     Console WriteLine  quot Creating dictionary    quot    Dictionary lt string  int gt  dictionary   new Dictionary lt string  int gt  dictionarySize   for  int i   0  i  lt  dictionarySize  i          dictionary Add i ToString    i     Console WriteLine  quot Done quot     Console WriteLine  quot Starting tests    quot        for loop test sw Restart    for  int i   0  i  lt  iterations  i          for  int j   0  j  lt  dictionary Count  j                  dictionary dictionary Keys ElementAt j     3          sw Stop    Console WriteLine   quot for loop Test       sw ElapsedMilliseconds  ms quot        foreach loop test sw Restart    for  int i   0  i  lt  iterations  i          foreach  string key in dictionary Keys ToList                  dictionary key    3          sw Stop    Console WriteLine   quot foreach loop Test   sw ElapsedMilliseconds  ms quot     Console WriteLine  quot Done quot     Results  Creating dictionary    Done Starting tests    for loop Test      2367 ms foreach loop Test  3 ms Done

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You can make a list copy of the dict Values  then you can use the List ForEach lambda function for iteration   or a foreach loop  as suggested before    new List lt string gt  myDict Values  ForEach str   gt        Use str in any other way you need here    Console WriteLine str

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If you re feeling creative you could do something like this  Loop backwards through the dictionary to make your changes   Dictionary lt string  int gt  collection   new Dictionary lt string  int gt     collection Add  value1   9   collection Add  value2   7   collection Add  value3   5   collection Add  value4   3   collection Add  value5   1    for  int i   collection Keys Count  i--  gt  0          if  collection Values ElementAt i   lt  5            collection Remove collection Keys ElementAt i                 Certainly not identical  but you might be interested anyways

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How about just doing some linq queries against your dictionary  and then binding your graph to the results of those      var under   colStates Where c   gt   decimal c Value    decimal totalCount  lt   05M   var over   colStates Where c   gt   decimal c Value    decimal totalCount  gt    05M   var newColStates   over Union new Dictionary lt string  int gt         Other   under Sum c   gt  c Value         foreach  var item in newColStates        Console WriteLine   0   1    item Key  item Value

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You are modifying the collection in this line      colStates key    0    By doing so  you are essentially deleting and reinserting something at that point  as far as IEnumerable is concerned anyways   If you edit a member of the value you are storing  that would be OK  but you are editing the value itself and IEnumberable doesn t like that   The solution I ve used is to eliminate the foreach loop and just use a for loop  A simple for loop won t check for changes that you know won t effect the collection   Here s how you could do it   List lt string gt  keys   new List lt string gt  colStates Keys   for int i   0  i  lt  keys Count  i          string key   keys i       double  Percent   colStates key    TotalCount      if  Percent  lt  0 05                            OtherCount    colStates key           colStates key    0

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Setting a value in a dictionary updates its internal  version number  - which invalidates the iterator  and any iterator associated with the keys or values collection   I do see your point  but at the same time it would be odd if the values collection could change mid-iteration - and for simplicity there s only one version number   The normal way of fixing this sort of thing is to either copy the collection of keys beforehand and iterate over the copy  or iterate over the original collection but maintain a collection of changes which you ll apply after you ve finished iterating   For example   Copying keys first  List lt string gt  keys   new List lt string gt  colStates Keys   foreach string key in keys        double percent   colStates key    TotalCount          if  percent  lt  0 05                OtherCount    colStates key           colStates key    0            Or     Creating a list of modifications  List lt string gt  keysToNuke   new List lt string gt     foreach string key in colStates Keys        double percent   colStates key    TotalCount          if  percent  lt  0 05                OtherCount    colStates key           keysToNuke Add key           foreach  string key in keysToNuke        colStates key    0

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You need to create a new Dictionary from the old rather than modifying in place  Somethine like  also iterate over the KeyValuePair lt    rather than using a key lookup   int otherCount   0  int totalCounts   colStates Values Sum    var newDict   new Dictionary lt string int gt     foreach  var kv in colStates      if  kv Value  double totalCounts  lt  0 05        otherCount    kv Value      else       newDict Add kv Key  kv Value         if  otherCount  gt  0      newDict Add  Other   otherCount      colStates   newDict

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You can t modify the keys nor the values directly in a ForEach  but you can modify their members   E g   this should work   public class State       public int Value          Dictionary lt string  State gt  colStates   new Dictionary lt string State gt      int OtherCount   0  foreach string key in colStates Keys        double  Percent   colStates key  Value   TotalCount       if  Percent  lt  0 05                OtherCount    colStates key  Value          colStates key  Value   0           colStates Add  Other   new State   Value    OtherCount

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You can t modify the collection  not even the values  You could save these cases and remove them later  It would end up like this  Dictionary lt string  int gt  colStates   new Dictionary lt string  int gt               Some code to populate colStates dictionary         int OtherCount   0  List lt string gt  notRelevantKeys   new List lt string gt      foreach  string key in colStates Keys         double Percent   colStates key    colStates Count       if  Percent  lt  0 05                OtherCount    colStates key           notRelevantKeys Add key            foreach  string key in notRelevantKeys        colStates key    0     colStates Add  quot Other quot   OtherCount

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Call the ToList   in the foreach loop  This way we dont need a temp variable copy  It depends on Linq which is available since  Net 3 5   using System Linq   foreach string key in colStates Keys ToList        double  Percent   colStates key    TotalCount       if  Percent  lt  0 05                OtherCount    colStates key           colStates key    0

[c#] Editing dictionary values in a foreach loop

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