Iterator Loop vs index loop

Question

Possible Duplicate    Why use iterators instead of array indices        I m reviewing my knowledge on C   and I ve stumbled upon iterators  One thing I want to know is what makes them so special and I want to know why this    using namespace std   vector lt int gt  myIntVector  vector lt int gt   iterator myIntVectorIterator      Add some elements to myIntVector myIntVector push back 1   myIntVector push back 4   myIntVector push back 8    for myIntVectorIterator   myIntVector begin             myIntVectorIterator    myIntVector end            myIntVectorIterator          cout lt  lt  myIntVectorIterator lt  lt            Should output 1 4 8     is better than this    using namespace std   vector lt int gt  myIntVector     Add some elements to myIntVector myIntVector push back 1   myIntVector push back 4   myIntVector push back 8    for int y 0  y lt myIntVector size    y          cout lt  lt myIntVector y  lt  lt            Should output 1 4 8     And yes I know that I shouldn t be using the std namespace  I just took this example off of the cprogramming website  So can you please tell me why the latter is worse  What s the big difference

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its a matter of speed  using the iterator accesses the elements faster  a similar question was answered here   What  39 s faster  iterating an STL vector with vector  iterator or with at     Edit  speed of access varies with each cpu and compiler

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The nice thing about iterator is that later on if you wanted to switch your vector to a another STD container  Then the forloop will still work

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It always depends on what you need   You should use operator   when you need direct access to elements in the vector  when you need to index a specific element in the vector   There is nothing wrong in using it over iterators  However  you must decide for yourself which  operator   or iterators  suits best your needs   Using iterators would enable you to switch to other container types without much change in your code  In other words  using iterators would make your code more generic  and does not depend on a particular type of container

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By writing your client code in terms of iterators you abstract away the container completely   Consider this code   class ExpressionParser    some generic arbitrary expression parser   public      template lt typename It gt      void parse It begin  const It end                using namespace std          using namespace std  placeholders          for each begin  end               bind  amp ExpressionParser  process next  this   1                process next char in a stream  defined elsewhere      void process next char c        client code   ExpressionParser p   std  string expression  SUM A  FOR A in  1  2  3  4     p parse expression begin    expression end      std  istringstream file  expression txt    p parse std  istringstream lt char gt  file   std  istringstream lt char gt       char expr       12a 2   13a - 5  with a 108   p parse std  begin expr   std  end expr      Edit  Consider your original code example  implemented with    using namespace std   vector lt int gt  myIntVector     Add some elements to myIntVector myIntVector push back 1   myIntVector push back 4   myIntVector push back 8    copy myIntVector begin    myIntVector end         std  ostream iterator lt int gt  cout

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Iterators make your code more generic  Every standard library container provides an iterator hence if you change your container class in future the loop wont be affected

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Iterators are first choice over operator    C  11 provides std  begin    std  end   functions    As your code uses just std  vector  I can t say there is much difference in both codes  however  operator    may not operate as you intend to  For example if you use map  operator   will insert an element if not found   Also  by using iterator your code becomes more portable between containers  You can switch containers from std  vector to std  list or other container freely without changing much if you use iterator such rule doesn t apply to operator

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The special thing about iterators is that they provide the glue between algorithms and containers  For generic code  the recommendation would be to use a combination of STL algorithms  e g  find  sort  remove  copy  etc  that carries out the computation that you have in mind on your data structure  vector  list  map etc    and to supply that algorithm with iterators into your container   Your particular example could be written as a combination of the for each algorithm and the vector container  see option 3  below   but it s only one out of four distinct ways to iterate over a std  vector   1  index-based iteration  for  std  size t i   0  i    v size      i           access element as v i          any code including continue  break  return     Advantages  familiar to anyone familiar with C-style code  can loop using different strides  e g  i    2    Disadvantages  only for sequential random access containers  vector  array  deque   doesn t work for list  forward list or the associative containers  Also the loop control is a little verbose  init  check  increment   People need to be aware of the 0-based indexing in C     2  iterator-based iteration  for  auto it   v begin    it    v end      it           if the current index is needed      auto i   std  distance v begin    it            access element as  it         any code including continue  break  return     Advantages  more generic  works for all containers  even the new unordered associative containers  can also use different strides  e g  std  advance it  2     Disadvantages  need extra work to get the index of the current element  could be O N  for list or forward list   Again  the loop control is a little verbose  init  check  increment     3  STL for each algorithm   lambda  std  for each v begin    v end       T const amp  elem            if the current index is needed       auto i    amp elem -  amp v 0            cannot continue  break or return out of the loop       Advantages  same as 2  plus small reduction in loop control  no check and increment   this can greatly reduce your bug rate  wrong init  check or increment  off-by-one errors    Disadvantages  same as explicit iterator-loop plus restricted possibilities for flow control in the loop  cannot use continue  break or return  and no option for different strides  unless you use an iterator adapter that overloads operator      4  range-for loop  for  auto amp  elem  v            if the current index is needed       auto i    amp elem -  amp v 0           any code including continue  break  return     Advantages  very compact loop control  direct access to the current element   Disadvantages  extra statement to get the index  Cannot use different strides   What to use    For your particular example of iterating over std  vector  if you really need the index  e g  access the previous or next element  printing logging the index inside the loop etc   or you need a stride different than 1  then I would go for the explicitly indexed-loop  otherwise I d go for the range-for loop    For generic algorithms on generic containers I d go for the explicit iterator loop unless the code contained no flow control inside the loop and needed stride 1  in which case I d go for the STL for each   a lambda

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With a vector iterators do no offer any real advantage  The syntax is uglier  longer to type and harder to read   Iterating over a vector using iterators is not faster and is not safer  actually if the vector is possibly resized during the iteration using iterators will put you in big troubles    The idea of having a generic loop that works when you will change later the container type is also mostly nonsense in real cases  Unfortunately the dark side of a strictly typed language without serious typing inference  a bit better now with C  11  however  is that you need to say what is the type of everything at each step  If you change your mind later you will still need to go around and change everything  Moreover different containers have very different trade-offs and changing container type is not something that happens that often   The only case in which iteration should be kept if possible generic is when writing template code  but that  I hope for you  is not the most frequent case   The only problem present in your explicit index loop is that size returns an unsigned value  a design bug of C    and comparison between signed and unsigned is dangerous and surprising  so better avoided  If you use a decent compiler with warnings enabled there should be a diagnostic on that   Note that the solution is not to use an unsiged as the index  because arithmetic between unsigned values is also apparently illogical  it s modulo arithmetic  and x-1 may be bigger than x   You instead should cast the size to an integer before using it  It may make some sense to use unsigned sizes and indexes  paying a LOT of attention to every expression you write  only if you re working on a 16 bit C   implementation  16 bit was the reason for having unsigned values in sizes    As a typical mistake that unsigned size may introduce consider   void drawPolyline const std  vector lt P2d gt  amp  points        for  int i 0  i lt points size  -1  i            drawLine points i   points i 1        Here the bug is present because if you pass an empty points vector the value points size  -1 will be a huge positive number  making you looping into a segfault  A working solution could be  for  int i 1  i lt points size    i        drawLine points i - 1   points i      but I personally prefer to always remove unsinged-ness with int v size      PS  If you really don t want to think by to yourself to the implications and simply want an expert to tell you then consider that a quite a few world recognized C   experts agree and expressed opinions on that unsigned values are a bad idea except for bit manipulations   Discovering the ugliness of using iterators in the case of iterating up to second-last is left as an exercise for the reader

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