Replace an element into a specific position of a vector

Question

I want to replace an element into a specific position of a vector, can I just use an assignment:

// vec1 and 2 have the same length & filled in somehow
vec1;
vec2;

vec1[i] = vec2[i] // insert vec2[i] at position i of vec1

or I have to use insert():

vector<sometype>::iterator iterator = vec1.begin();

vec1.insert(iterator+(i+1), vec2[i]);

Answer 1

vec1[i] = vec2[i] 

will set the value of vec1[i] to the value of vec2[i]. Nothing is inserted. Your second approach is almost correct. Instead of +i+1 you need just +i

v1.insert(v1.begin()+i, v2[i]) 

Answer 2

You can do that using at. You can try out the following simple example:

const size_t N = 20; std::vector<int> vec(N); try {     vec.at(N - 1) = 7; } catch (std::out_of_range ex) {     std::cout << ex.what() << std::endl; } assert(vec.at(N - 1) == 7); 

Notice that method at returns an allocator_type::reference, which is that case is a int&. Using at is equivalent to assigning values like vec[i]=....

---

There is a difference between at and insert as it can be understood with the following example:

const size_t N = 8; std::vector<int> vec(N); for (size_t i = 0; i<5; i++){     vec[i] = i + 1; }  vec.insert(vec.begin()+2, 10); 

If we now print out vec we will get:

1 2 10 3 4 5 0 0 0 

If, instead, we did vec.at(2) = 10, or vec[2]=10, we would get

1 2 10 4 5 0 0 0 

Answer 3

See an example here: http://www.cplusplus.com/reference/stl/vector/insert/ eg.:

...
vector::iterator iterator1;

  iterator1= vec1.begin();
  vec1.insert ( iterator1+i , vec2[i] );

// This means that at position "i" from the beginning it will insert the value from vec2 from position i

Your first approach was replacing the values from vec1[i] with the values from vec2[i]