Iterate through a C Vector using a for loop

Question

I am new to the C   language  I have been starting to use vectors  and have noticed that in all of the code I see to iterate though a vector via indices  the first parameter of the for loop is always something based on the vector  In Java I might do something like this with an ArrayList   for int i 0  i  lt  vector size    i        vector i  doSomething        Is there a reason I don t see this in C    Is it bad practice

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If you use std  vector lt std  reference wrapper lt std  string gt  gt  names      Do not forget  when you use auto in the for loop  to use also get  like this  for  auto element in   names        element get    do something

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different declaration type     vector lt int gt v        vector lt int gt v2 5 30     size is 5 and fill up with 30     vector lt int gt v3  10 20 30              From C  11 and onwards     for auto itr v2          cout lt  lt  quot  n quot  lt  lt itr                pre c  11         for auto itr v3 begin    itr   v3 end    itr            cout lt  lt  quot  n quot  lt  lt  itr

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The reason why you don t see such practice is quite subjective and cannot have a definite answer  because I have seen many of the code which uses your mentioned way rather than iterator style code   Following can be reasons of people not considering vector size   way of looping    Being paranoid about calling size   every time in the loop condition  However either it s a non-issue or it can be trivially fixed Preferring std  for each   over the for loop itself Later changing the container from std  vector to other one  e g  map  list  will also demand the change of the looping mechanism  because not every container support size   style of looping   C  11 provides a good facility to move through the containers  That is called  range based for loop   or  enhanced for loop  in Java    With little code you can traverse through the full  mandatory   std  vector   vector lt int gt  vi      for int i   vi     cout  lt  lt   i      lt  lt  i  lt  lt  endl

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There s a couple of strong reasons to use iterators  some of which are mentioned here   Switching containers later doesn t invalidate your code   i e   if you go from a std  vector to a std  list  or std  set  you can t use numerical indices to get at your contained value   Using an iterator is still valid   Runtime catching of invalid iteration  If you modify your container in the middle of your loop  the next time you use your iterator it will throw an invalid iterator exception

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With STL  programmers use iterators for traversing through containers  since iterator is an abstract concept  implemented in all standard containers  For example  std  list has no operator    at all

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A correct way of iterating over the vector and printing its values is as follows   include lt vector gt      declare the vector of type int vector lt int gt  v      insert elements in the vector for  unsigned int i   0  i  lt  5    i       v push back i         print those elements for  auto it   v begin    it    v end      it       std  cout  lt  lt   it  lt  lt  std  endl     But at least in the present case it is nicer to use a range-based for loop  for  auto x  v  std  cout  lt  lt  x  lt  lt   quot  n quot    You may also add  amp  after auto to make x a reference to the elements rather than a copy of them  It is then very similar to the above iterator-based approach  but easier to read and write

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The cleanest way of iterating through a vector is via iterators   for  auto it   begin  vector   it    end  vector     it        it- gt doSomething         or  equivalent to the above   for  auto  amp  element   vector        element doSomething         Prior to C  0x  you have to replace auto by the iterator type and use member functions instead of global functions begin and end   This probably is what you have seen  Compared to the approach you mention  the advantage is that you do not heavily depend on the type of vector  If you change vector to a different  collection-type  class  your code will probably still work  You can  however  do something similar in Java as well  There is not much difference conceptually  C    however  uses templates to implement this  as compared to generics in Java   hence the approach will work for all types for which begin and end functions are defined  even for non-class types such as static arrays  See here  How does the range-based for work for plain arrays

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The right way to do that is   for std  vector lt T gt   iterator it   v begin    it    v end      it        it- gt doSomething         Where T is the type of the class inside the vector  For example if the class was CActivity  just write CActivity instead of T   This type of method will work on every STL  Not only vectors  which is a bit better    If you still want to use indexes  the way is   for std  vector lt T gt   size type i   0  i    v size    i          v i  doSomething

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I was surprised nobody mentioned that iterating through an array with an integer index makes it easy for you to write faulty code by subscripting an array with the wrong index  For example  if you have nested loops using i and j as indices  you might incorrectly subscript an array with j rather than i and thus introduce a fault into the program   In contrast  the other forms listed here  namely the range based for loop  and iterators  are a lot less error prone  The language s semantics and the compiler s type checking mechanism will prevent you from accidentally accessing an array using the wrong index

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Using the auto operator really makes it easy to use as one does not have to worry about the data type and the size of the vector or any other data structure  Iterating vector using auto and for loop  vector lt int gt  vec    1 2 3 4 5   for auto itr   vec      cout  lt  lt  itr  lt  lt         Output   1 2 3 4 5   You can also use this method to iterate sets and list  Using auto automatically detects the data type used in the template and lets you use it  So  even if we had a vector of string or char the same syntax will work just fine

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Is there any reason I don t see this in C    Is it bad practice    No  It is not a bad practice  but the following approach renders your code certain flexibility   Usually  pre-C  11 the code for iterating over container elements uses iterators  something like   std  vector lt int gt   iterator it   vector begin      This is because it makes the code more flexible   All standard library containers support and provide iterators  If at a later point of development you need to switch to another container  then this code does not need to be changed   Note  Writing code which works with every possible standard library container is not as easy as it might seem to be

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Here is a simpler way to iterate and print values in vector   for int x  A     for integer x in vector A     cout lt  lt  x  lt  lt

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