Can you remove elements from a std list while iterating through it

Question

I ve got code that looks like this   for  std  list lt item  gt   iterator i items begin   i  items end   i          bool isActive     i - gt update          if   isActive           items remove  i          else        other code involving  i     items remove if CheckItemNotActive     I d like remove inactive items immediately after update them  inorder to avoid walking the list again   But if I add the commented-out lines  I get an error when I get to i     List iterator not incrementable    I tried some alternates which didn t increment in the for statement  but I couldn t get anything to work   What s the best way to remove items as you are walking a std  list

User · Answer

Use std  remove if algorithm  Edit  Work with collections should be like   prepare collection  process collection   Life will be easier if you won t mix this steps   std  remove if  or list  remove if   if you know that you work with list and not with the TCollection   std  for each

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do while loop  it s flexable and fast and easy to read and write  auto textRegion   m pdfTextRegions begin        while textRegion    m pdfTextRegions end                  if    textRegion - gt glyphs empty                          m pdfTextRegions erase textRegion               textRegion   m pdfTextRegions begin                      else             textRegion

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I have sumup it  here is the three method with example   1  using while loop  list lt int gt  lst 4  1  2  3  5    auto it   lst begin    while  it    lst end         if   it   2     1           it   lst erase it     erase and go to next       else            it      go to next          for auto it lst cout lt  lt it lt  lt      cout lt  lt endl     4 2   2  using remove if member funtion in list   list lt int gt  lst 4  1  2  3  5    lst remove if    int a  return a   2    1      for auto it lst cout lt  lt it lt  lt      cout lt  lt endl     4 2   3  using std  remove if funtion combining with erase member function   list lt int gt  lst 4  1  2  3  5    lst erase std  remove if lst begin    lst end       int a       return a   2    1      lst end      for auto it lst cout lt  lt it lt  lt      cout lt  lt endl     4 2   4  using for loop   should note update the iterator   list lt int gt  lst 4  1  2  3  5    for auto it   lst begin    it    lst end     it       if    it   2     1           it   lst erase it    erase and go to next erase will return the next iterator          --it      as it will be add again in for  so we go back one step          for auto it lst cout lt  lt it lt  lt      cout lt  lt endl     4 2

User · Answer

You have to increment the iterator first  with i    and then remove the previous element  e g   by using the returned value from i      You can change the code to a while loop like so   std  list lt item  gt   iterator i   items begin    while  i    items end          bool isActive     i - gt update        if   isActive                items erase i         alternatively  i   items erase i             else               other code involving  i             i

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Iterating backwards avoids the effect of erasing an element on the remaining elements to be traversed   typedef list lt item  gt  list t  for   list t  iterator it   items end     it    items begin             --it      bool remove    lt determine whether to remove gt      if   remove             items erase  it              PS  see this  e g   regarding backward iteration   PS2  I did not thoroughly tested if it handles well erasing elements at the ends

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You can write  std  list lt item  gt   iterator i   items begin    while  i    items end          bool isActive     i - gt update        if   isActive            i   items erase i          else           other code involving  i           i              You can write equivalent code with std  list  remove if  which is less verbose and more explicit   items remove if     item i        bool isActive     i - gt update        if   isActive           return true       other code involving  i       return false        The std  vector  erase std  remove if idiom should be used when items is a vector instead of a list to keep compexity at O n  - or in case you write generic code and items might be a container with no effective way to erase single items  like a vector   items erase std  remove if begin items   end items       item i        bool isActive     i - gt update        if   isActive           return true       other code involving  i       return false

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The alternative for loop version to Kristo s answer    You lose some efficiency  you go backwards and then forward again when deleting but in exchange for the extra iterator increment you can have the iterator declared in the loop scope and the code looking a bit cleaner  What to choose depends on priorities of the moment   The answer was totally out of time  I know     typedef std  list lt item  gt   iterator item iterator   for item iterator i   items begin    i    items end      i        bool isActive     i - gt update         if   isActive                items erase i--              else               other code involving  i

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Here s an example using a for loop that iterates the list and increments or revalidates the iterator in the event of an item being removed during traversal of the list   for auto i   items begin    i    items end           if bool isActive     i - gt update                  other code involving  i             i             else               i   items erase i              items remove if CheckItemNotActive

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If you think of the std  list like a queue  then you can dequeue and enqueue all the items that you want to keep  but only dequeue  and not enqueue  the item you want to remove  Here s an example where I want to remove 5 from a list containing the numbers 1-10     std  list lt int gt  myList   int size   myList size       The size needs to be saved to iterate through the whole thing  for  int i   0  i  lt  size    i        int val   myList back       myList pop back      dequeue     if  val    5                 myList push front val     enqueue if not 5           myList will now only have numbers 1-4 and 6-10

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I think you have a bug there  I code this way   for  std  list lt CAudioChannel   gt   iterator itAudioChannel   audioChannels begin                 itAudioChannel    audioChannels end            CAudioChannel  audioChannel    itAudioChannel      std  list lt CAudioChannel   gt   iterator itCurrentAudioChannel   itAudioChannel      itAudioChannel         if  audioChannel- gt destroyMe                audioChannels erase itCurrentAudioChannel           delete audioChannel          continue            audioChannel- gt Mix outBuffer  numSamples

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Removal invalidates only the iterators that point to the elements that are removed   So in this case after removing  i   i is invalidated and you cannot do increment on it   What you can do is first save the iterator of element that is to be removed   then increment the iterator and then remove the saved one

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You want to do   i  items erase i     That will correctly update the iterator to point to the location after the iterator you removed

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You need to do the combination of Kristo s answer and MSN s      Note  Using the pre-increment operator is preferred for iterators because          there can be a performance gain        Note  As long as you are iterating from beginning to end  without inserting          along the way you can safely save end once  otherwise get it at the          top of each loop   std  list lt  item    gt   iterator iter   items begin    std  list lt  item    gt   iterator end    items end     while  iter    end        item   pItem    iter       if  pItem- gt update      true                other code involving pItem             iter            else                  BTW  who is deleting pItem  a k a    iter           iter   items erase iter             Of course  the most efficient and SuperCool reg  STL savy thing would be something like this      This implementation of update executes other code involving Item    if    this instance needs updating        This method returns true if this still needs future updates     bool Item  update void        if  m needsUpdates    true                m needsUpdates   other code involving this              return  m needsUpdates         This call does everything the previous loop did     Including the fact    that it isn t deleting the items that are erased   items remove if std  not1 std  mem fun  amp Item  update

[c++] Can you remove elements from a std::list while iterating through it?

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