Advantages of std for each over for loop

Question

Are there any advantages of std  for each over for loop  To me  std  for each only seems to hinder the readability of code  Why do then some coding standards recommend its use

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If you frequently use other algorithms from the STL, there are several advantages to for_each:

It will often be simpler and less error prone than a for loop, partly because you'll be used to functions with this interface, and partly because it actually is a little more concise in many cases.
Although a range-based for loop can be even simpler, it is less flexible (as noted by Adrian McCarthy, it iterates over a whole container).
Unlike a traditional for loop, for_each forces you to write code that will work for any input iterator. Being restricted in this way can actually be a good thing because:
1. You might actually need to adapt the code to work for a different container later.
2. At the beginning, it might teach you something and/or change your habits for the better.
3. Even if you would always write for loops which are perfectly equivalent, other people that modify the same code might not do this without being prompted to use for_each.
Using for_each sometimes makes it more obvious that you can use a more specific STL function to do the same thing. (As in Jerry Coffin's example; it's not necessarily the case that for_each is the best option, but a for loop is not the only alternative.)

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There are a lot of good reasons in other answers but all seem to forget that for each allows you to use reverse or pretty much any custom iterator when for loop always starts with begin   iterator  Example with reverse iterator  std  list lt int gt  l  1 2 3   std  for each l rbegin    l rend       auto o  std  cout lt  lt o      Example with some custom tree iterator  SomeCustomTree lt int gt  a 1 2 3 4 5 6 7   auto node   a find 4   std  for each node breadthFirstBegin    node breadthFirstEnd       auto o  std  cout lt  lt o

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You can have the iterator be a call to a function that is performed on each iteration through the loop   See here  http   www cplusplus com reference algorithm for each

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Here are some reasons   It seems to hinder readability just because you re not used to it and or not using the right tools around it to make it really easy   see boost  range and boost  bind boost  lambda for helpers  Many of these will go into C  0x and make for each and related functions more useful    It allows you to write an algorithm on top of for each that works with any iterator   It reduces the chance of stupid typing bugs   It also opens your mind to the rest of the STL-algorithms  like find if  sort  replace  etc and these won t look so strange anymore  This can be a huge win    Update 1  Most importantly  it helps you go beyond for each vs  for-loops like that s all there is  and look at the other STL-alogs  like find   sort   partition   copy replace if  parallel execution    or whatever  A lot of processing can be written very concisely using  quot the rest quot  of for each s siblings  but if all you do is to write a for-loop with various internal logic  then you ll never learn how to use those  and you ll end up inventing the wheel over and over  And  the soon-to-be available range-style for each    lambdas  for each monsters     auto amp  m    m think         is IMO more readable than  for  auto i   monsters begin    i    monsters end      i        i- gt think        Also this  for each bananas    amp   auto amp  b    my monkey eat b       Is more concise than  for  auto i   bananas begin    i    bananas end      i        my monkey- gt eat  i       But new range based for is probably the best   for  auto amp  b   bananas       my monkey eat b    But the for each could be useful  especially if you have several functions to call in order but need to run each method for all objects before next    but maybe that s just me     Update 2  I ve written my own one-liner wrappers of stl-algos that work with ranges instead of pair of iterators  boost  range ex  once released  will include that and maybe it will be there in C  0x too

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For loop can  break   I dont want to be a parrot for Herb Sutter so here is the link to his presentation  http   channel9 msdn com Events BUILD BUILD2011 TOOL-835T Be sure to read the comments also

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for each allow us to implement Fork-Join pattern   Other than that it supports fluent-interface    fork-join pattern  We can add implementation gpu  for each to use cuda gpu for heterogeneous-parallel computing by calling the lambda task in multiple workers   gpu  for each users begin   users end   update summary      all summary is complete now    go access the user-summary here    And gpu  for each may wait for the workers work on all the lambda-tasks to finish before executing the next statements   fluent-interface  It allow us to write human-readable code in concise manner   accounts  erase std  remove if accounts begin   accounts end   used this year    std  for each accounts begin   accounts end   mark dormant

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The advantage of writing functional for beeing more readable  might not show up when for      and for each        If you utilize all algorithms in functional h  instead of using for-loops  the code gets a lot more readable   iterator longest tree   std  max element forest begin    forest end          iterator first leaf tree   std  find if forest begin    forest end          std  transform forest begin    forest end    firewood begin          std  for each forest begin    forest end    make plywood     is much more readable than   Forest  iterator longest tree   it begin    for  Forest  const iterator it   forest begin    it    forest end      it     if   it  gt   longest tree         longest tree   it          Forest  iterator leaf tree   it begin    for  Forest  const iterator it   forest begin    it    forest end      it     if  it- gt type      LEAF TREE         leaf tree    it       break          for  Forest  const iterator it   forest begin    jt   firewood begin          it    forest end          it    jt                 jt   boost  transformtowood  it          for  Forest  const iterator it   forest begin    it    forest end      it      std  makeplywood  it       And that is what I think is so nice  generalize the for-loops to one line functions

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Mostly you ll have to iterate over the whole collection  Therefore I suggest you write your own for each   variant  taking only 2 parameters  This will allow you to rewrite Terry Mahaffey s example as   for each container     int amp  i        i    10        I think this is indeed more readable than a for loop  However  this requires the C  0x compiler extensions

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Aside from readability and performance  one aspect commonly overlooked is consistency  There are many ways to implement a for  or while  loop over iterators  from   for  C  iterator iter   c begin    iter    c end    iter          do something  iter       to   C  iterator iter   c begin    C  iterator end   c end    while  iter    end        do something  iter         iter      with many examples in between at varying levels of efficiency and bug potential   Using for each  however  enforces consistency by abstracting away the loop   for each c begin    c end    do something     The only thing you have to worry about now is  do you implement the loop body as function  a functor  or a lambda using Boost or C  0x features   Personally  I d rather worry about that than how to implement or read a random for while loop

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The nice thing with C  11  previously called C  0x   is that this tiresome debate will be settled   I mean  no one in their right mind  who wants to iterate over a whole collection  will still use this  for auto it   collection begin    it    collection end       it       foo  it       Or this  for each collection begin    collection end       Element amp  e       foo e         when the range-based for loop syntax is available   for Element amp  e   collection       foo e       This kind of syntax has been available in Java and C  for some time now  and actually there are way more foreach loops than classical for loops in every recent Java or C  code I saw

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With C  11 and two simple templates  you can write          for   auto x  range v1 4 v1 6                      x  2                  cout lt  lt  x  lt  lt                  as a replacement for for each or a loop  Why choose it boils down to brevity and safety  there s no chance of error in an expression that s not there   For me  for each was always better on the same grounds when the loop body is already a functor  and I ll take any advantage I can get   You still use the three-expression for  but now when you see one you know there s something to understand there  it s not boilerplate   I hate boilerplate   I resent its existence   It s not real code  there s nothing to learn by reading it  it s just one more thing that needs checking   The mental effort can be measured by how easy it is to get rusty at checking it   The templates are  template lt typename iter gt  struct range                     iter begin    return   beg      iter end   return   end               range  iter const amp beg iter const amp end      beg beg    end end                 iter   beg    end      template lt typename iter gt  range  lt iter gt  range iter const  amp begin  iter const  amp end        return range  lt iter gt  begin end

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The for each loop is meant to hide the iterators  detail of how a loop is implemented  from the user code and define clear semantics on the operation  each element will be iterated exactly once   The problem with readability in the current standard is that it requires a functor as the last argument instead of a block of code  so in many cases you must write specific functor type for it  That turns into less readable code as functor objects cannot be defined in-place  local classes defined within a function cannot be used as template arguments  and the implementation of the loop must be moved away from the actual loop   struct myfunctor      void operator    int arg1     code      void apply  std  vector lt int gt  const  amp  v           code    std  for each  v begin    v end    myfunctor            more code     Note that if you want to perform an specific operation on each object  you can use std  mem fn  or boost  bind  std  bind in the next standard   or boost  lambda  lambdas in the next standard  to make it simpler   void function  int value    void apply  std  vector lt X gt  const  amp  v           code    std  for each  v begin    v end    boost  bind  function   1            code     Which is not less readable and more compact than the hand rolled version if you do have function method to call in place  The implementation could provide other implementations of the for each loop  think parallel processing    The upcoming standard takes care of some of the shortcomings in different ways  it will allow for locally defined classes as arguments to templates   void apply  std  vector lt int gt  const  amp  v           code    struct myfunctor         void operator    int     code            std  for each  v begin    v end    myfunctor            code     Improving the locality of code  when you browse you see what it is doing right there  As a matter of fact  you don t even need to use the class syntax to define the functor  but use a lambda right there   void apply  std  vector lt int gt  const  amp  v           code    std  for each  v begin    v end               int        code            code     Even if for the case of for each there will be an specific construct that will make it more natural   void apply  std  vector lt int gt  const  amp  v           code    for   int i   v              code            code     I tend to mix the for each construct with hand rolled loops  When only a call to an existing function or method is what I need  for each  v begin    v end    boost  bind   amp Type  update   1      I go for the for each construct that takes away from the code a lot of boiler plate iterator stuff  When I need something more complex and I cannot implement a functor just a couple of lines above the actual use  I roll my own loop  keeps the operation in place   In non-critical sections of code I might go with BOOST FOREACH  a co-worker got me into it

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I used to dislike std  for each and thought that without lambda  it was done utterly wrong  However I did change my mind some time ago  and now I actually love it  And I think it even improves readability  and makes it easier to test your code in a TDD way   The std  for each algorithm can be read as do something with all elements in range  which can improve readability  Say the action that you want to perform is 20 lines long  and the function where the action is performed is also about 20 lines long  That would make a function 40 lines long with a conventional for loop  and only about 20 with std  for each  thus likely easier to comprehend   Functors for std  for each are more likely to be more generic  and thus reusable  e g   struct DeleteElement       template  lt typename T gt      void operator   const T  ptr                delete ptr             And in the code you d only have a one-liner like std  for each v begin    v end    DeleteElement    which is slightly better IMO than an explicit loop   All of those functors are normally easier to get under unit tests than an explicit for loop in the middle of a long function  and that alone is already a big win for me   std  for each is also generally more reliable  as you re less likely to make a mistake with range   And lastly  compiler might produce slightly better code for std  for each than for certain types of hand-crafted for loop  as it  for each  always looks the same for compiler  and compiler writers can put all of their knowledge  to make it as good as they can   Same applies to other std algorithms like find if  transform etc

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Personally  any time I d need to go out of my way to use std  for each  write special-purpose functors   complicated boost  lambdas   I find BOOST FOREACH and C  0x s range-based for clearer   BOOST FOREACH Monster  m  monsters         if  m- gt has plan              m- gt act        vs  std  for each monsters begin    monsters end       if then bind  amp Monster  has plan   1        bind  amp Monster  act   1

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Like many of the algorithm functions  an initial reaction is to think it s more unreadable to use foreach than a loop  It s been a topic of many flame wars   Once you get used to the idiom you may find it useful  One obvious advantage is that it forces the coder to separate the inner contents of the loop from the actual iteration functionality   OK  I think it s an advantage  Other s say you re just chopping up the code with no real benifit    One other advantage is that when I see foreach  I know that either every item will be processed or an exception will be thrown   A for loop allows several options for terminating the loop  You can let the loop run its full course  or you can use the break keyword to explicitly jump out of the loop  or use the return keyword to exit the entire function mid-loop  In contrast  foreach does not allow these options  and this makes it more readable  You can just glance at the function name and you know the full nature of the iteration   Here s an example of a confusing for loop   for std  vector lt widget gt   iterator i   v begin    i    v end      i                                                                                   Imagine a page of code here by programmers who don t refactor                                                                               if widget- gt Cost  lt  calculatedAmountSofar               break                                                                                         And then some more code added by a stressed out juniour developer          amp       amp           amp    amp                amp        amp       amp    amp                                                                                            for std  vector lt widgetPart gt   iterator ip   widget GetParts   begin    ip    widget GetParts   end      ip             if ip- gt IsBroken                     return false

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its very subjective   some will say that using for each  will make the code more readable  as it allows to treat different collections with the same conventions   for each itslef is implemented as a loop  template lt class InputIterator  class Function gt    Function for each InputIterator first  InputIterator last  Function f          for     first  last    first   f  first       return f        so its up to you to choose what is right for you

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I find for each to be bad for readability  The concept is a good one but c   makes it very hard to write readable  at least for me  c  0x lamda expressions will help  I really like the idea of lamdas  However on first glance I think the syntax is very ugly and I m not 100  sure I ll ever get used to it  Maybe in 5 years I ll have got used to it and not give it a second thought  but maybe not  Time will tell     I prefer to use  vector lt thing gt   iterator istart   container begin    vector lt thing gt   iterator iend   container end    for vector lt thing gt   iterator i   istart  i    iend    i         Do stuff     I find an explicit for loop clearer to read and explicity using named variables for the start and end iterators reduces the clutter in the for loop   Of course cases vary  this is just what I usually find best

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for each is more generic  You can use it to iterate over any type of container  by passing in the begin end iterators   You can potentially swap out containers underneath a function which uses for each without having to update the iteration code  You need to consider that there are other containers in the world than std  vector and plain old C arrays to see the advantages of for each   The major drawback of for each is that it takes a functor  so the syntax is clunky  This is fixed in C  11  formerly C  0x  with the introduction of lambdas   std  vector lt int gt  container      std  for each container begin    container end       int amp  i       i   10        This will not look weird to you in 3 years

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You re mostly correct  most of the time  std  for each is a net loss  I d go so far as to compare for each to goto  goto provides the most versatile flow-control possible -- you can use it to implement virtually any other control structure you can imagine  That very versatility  however  means that seeing a goto in isolation tells you virtually nothing about what s it s intended to do in this situation  As a result  almost nobody in their right mind uses goto except as a last resort   Among the standard algorithms  for each is much the same way -- it can be used to implement virtually anything  which means that seeing for each tells you virtually nothing about what it s being used for in this situation  Unfortunately  people s attitude toward for each is about where their attitude toward goto was in  say  1970 or so -- a few people had caught onto the fact that it should be used only as a last resort  but many still consider it the primary algorithm  and rarely if ever use any other  The vast majority of the time  even a quick glance would reveal that one of the alternatives was drastically superior   Just for example  I m pretty sure I ve lost track of how many times I ve seen people writing code to print out the contents of a collection using for each  Based on posts I ve seen  this may well be the single most common use of for each  They end up with something like   class XXX           public       std  ostream  amp print std  ostream  amp os    return os  lt  lt   my data n          And their post is asking about what combination of bind1st  mem fun  etc  they need to make something like   std  vector lt XXX gt  coll   std  for each coll begin    coll end    XXX  print     work  and print out the elements of coll  If it really did work exactly as I ve written it there  it would be mediocre  but it doesn t -- and by the time you ve gotten it to work  it s difficult to find those few bits of code related to what s going on among the pieces that hold it together   Fortunately  there is a much better way  Add a normal stream inserter overload for XXX   std  ostream  amp operator lt  lt  std  ostream  os  XXX const  amp x        return x print os       and use std  copy   std  copy coll begin    coll end    std  ostream iterator lt XXX gt  std  cout    n       That does work -- and takes virtually no work at all to figure out that it prints the contents of coll to std  cout

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for is for loop that can iterate each element or every third etc  for each is for iterating only each element  It is clear from its name  So it is more clear what you are intending to do in your code

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Easy  for each is useful when you already have a function to handle every array item  so you don t have to write a lambda  Certainly  this  for each a begin    a end    a item handler     is better than  for auto amp  item  a        a item handler a       Also  ranged for loop only iterates over whole containers from start to end  whilst for each is more flexible

[c++] Advantages of std::for_each over for loop

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