CODEWITHSUNDEEP
c++vectoriterator
user11508332
user11508332

Reputation: 667

including an iterator in c++ vector construction

In the code below, which is supposed to be an implementation of ranges::rotate:

auto second(std::vector<int>& v, std::vector<int>::iterator new_first) -> std::vector<int>::const_iterator {
  auto copy = std::vector<int>(v.begin(), new_first);
  v.erase(v.begin(), new_first);
  return v.insert(v.end(), copy.begin(), copy.end());
}

what is actually happening in the first 2 lines of the function where the new_first iterator isn't necessarily at the end? I've only seen examples where the second parameter is specifically at the end.

Upvotes: 0

Views: 245

Answers (2)

theWiseBro
theWiseBro

Reputation: 1529

auto copy = std::vector<int>(v.begin(), new_first);

You are copying first new_first - v.begin() elements to a new vector named copy.

v.erase(v.begin(), new_first);

You are erasing the same new_first - v.begin() elements from the original vector.

v.insert(v.end(), copy.begin(), copy.end());

You are inserting all the elements of copy into the original vector. So basically you've rotated the vector v to the right by new_first - v.begin() elements.

Therefore, if your v was {1,2,3,4,5} in the beginning and new_first points to say 4, then the return vector is {4,5,1,2,3}

Upvotes: 1

cigien
cigien

Reputation: 60208

Let's say the arguments to the function look like this:

v : { 1, 2, 3, 4, 5}
  new_first ^

Then this line:

auto copy = std::vector<int>(v.begin(), new_first);

uses the constructor of vector that takes 2 iterators to construct the copy variable:

v : { 1, 2, 3, 4, 5}
  new_first ^
copy : {1, 2}

Then this line:

v.erase(v.begin(), new_first);

uses the erase method of vector to remove the initial elements:

v : { 3, 4, 5}
copy : {1, 2}

And finally, this line:

return v.insert(v.end(), copy.begin(), copy.end());

uses the insert method of vector to copy the initial elements (stored in copy), to the end of v, and returns an iterator to the first inserted element:

v : { 3, 4, 5, 1, 2 }
        return ^

effectively implementing rotate.

Upvotes: 2

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