vector.insert doesn't resize vector?

Question

i'm trying to create a function that takes in a vector by reference and if it sees a vector entry 2, it would take delete the entry and replace it with 4 copies of 5.

so if the vector is 222 (then it is size n = 3), i want 555555555555 as the new vector

however, it only works properly for the first few, when the index is < n.

so right now, it would change vector a to be 555522 any ideas how to make the vector resize?

void replace2 (vector <int>* a, int n){

  for (int i = 0; i < n; ++i){
    if ((*a)[i] == 2){
      (*a).erase((*a).begin() + i);
      for(int j = 0; j < 4; ++j){
      (*a).insert((*a).begin() + i, 5);
            }
        }
      }

}

Answer 1

The problem with using

for ( int i = 0; i < n; ++i ) { ... }

has already been pointed out in one of the comments:

After the first iteration of the loop, (*a)[i] no longer equals 2. Remaining iterations do nothing.

Your function will be simplified if you iterate from the end of the vector and go back.

Also, pass a reference to the vector instead of a pointer.

Here's a complete program:

#include <vector>
#include <iostream>

void replace2(std::vector <int>& a, int n){
   for (int i = n-1; i >= 0; --i){
      if (a[i] == 2){
         a.erase(a.begin() + i);
         for(int j = 0; j < 4; ++j){
            a.insert(a.begin() + i, 5);
         }
      }
   }
}

int main()
{
   std::vector<int> a{2, 2, 2};
   replace2(a, 3);

   for(auto item : a )
   {
      std::cout << item;
   }

   std::cout << std::endl;
}

See it working at https://ideone.com/0Lip5j .