Replace elements in vector

Code:

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std::vector<int> PutNumbers;

PutNumbers.push_back(1);
PutNumbers.push_back(2);
PutNumbers.push_back(3);

//Result
PutNumbers[0] = 1;
PutNumbers[1] = 2;
PutNumbers[2] = 3;

---

I have this code above where I push_back numbers to 3 elements.
I am reading these numbers from a file but when I will read the fourth number wich could be 4 I want to replace the vector so it will look like below.
I know I can do this manually by iterate trough the elements etc.. This is not a problem.
But I wonder if there is any function for this for the std::vector to easy achive this. I have searched around but havn´t found anything about that subject.
Thank You!

Code:

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PutNumbers.push_back(4);

//Result
PutNumbers[0] = 2;
PutNumbers[1] = 3;
PutNumbers[2] = 4;

---

Before push_back(), check if the size is equal to 3. If so, pop_front() first. std::deque would likely be better than std::vector for this, actually.

EDIT:
Oops, std::vector does not have pop_front(), so you'll have to just use erase() if you do not use a std::deque instead.

Yes this is a good idéa to check the size also.
Thanks laserlight, I will check this out.