How to get a certain element in a list given the position

Question

So I ve got a list   list lt Object gt  myList  myList push back Object myObject     I m not sure but I m confident that this would be the  0th  element in the array  Is there any function I can use that will return  myObject    Object copy   myList find element 0

User · Answer

std  list doesn t provide any function to get element given an index  You may try to get it by writing some code  which I wouldn t recommend  because that would be inefficient if you frequently need to do so   What you need is   std  vector  Use it as   std  vector lt Object gt  objects  objects push back myObject    Object const  amp  x   objects 0        index isn t checked Object const  amp  y   objects at 0     index is checked

User · Answer

std  list lt Object gt  l   std  list lt Object gt   iterator ptr  int i   for  i   0   ptr   l begin     i  lt  N  amp  amp  ptr    l end     i     ptr       if  ptr    l end              list too short     else           ptr  points to N-th element of list

User · Answer

Maybe not the most efficient way   But you could convert the list into a vector    include  lt list gt   include  lt vector gt   list lt Object gt  myList   vector lt Object gt  myVector myList begin    myList end       Then access the vector using the  x  operator   auto x   MyVector 0     You could put that in a helper function    include  lt memory gt   include  lt vector gt   include  lt list gt   template lt class T gt  shared ptr lt vector lt T gt  gt   ListToVector list lt T gt  List    shared ptr lt vector lt T gt  gt  Vector           new vector lt string gt  List begin    List end      return Vector      Then use the helper funciton like this   auto MyVector   ListToVector Object   auto x   MyVector 0

User · Answer

If you frequently need to access the Nth element of a sequence  std  list  which is implemented as a doubly linked list  is probably not the right choice   std  vector or std  deque would likely be better   That said  you can get an iterator to the Nth element using std  advance   std  list lt Object gt  l     add elements to list  l      unsigned N      index of the element you want to retrieve     if  l size    gt  N        std  list lt Object gt   iterator it   l begin        std  advance it  N           it  points to the element at index  N      For a container that doesn t provide random access  like std  list  std  advance calls operator   on the iterator N times   Alternatively  if your Standard Library implementation provides it  you may call std  next   if  l size    gt  N        std  list lt Object gt   iterator it   std  next l begin    N       std  next is effectively wraps a call to std  advance  making it easier to advance an iterator N times with fewer lines of code and fewer mutable variables   std  next was added in C  11

User · Answer

Not very efficient  but if you must use a list  you can deference the iterator  myList begin   N

[c++] How to get a certain element in a list, given the position?

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