Can't push_back() after 1000th element in a vector in c++11

Question

#include <iostream>
#include <vector>
#include <bits/stdc++.h> 

using std::vector;
using std::cin;
using std::cout;

int main(){
    vector <long long> vec(2000,1);
    vec.reserve(2000);
    int count = 0;
    vec.push_back(31);
    for (int i = 0; i < vec.size(); ++i){
        count ++;
    }
    cout << count << " " << vec.max_size() << " " << vec.at(1543) << " " << vec.at(1000);

    return 0;
}

In this code I can't get 1000th element which is 31. I get 1 instead of it. I can use push_back() for elements which index is less than 1000.

Thanks in advance

You created 2000-element vector whose all elements are `1` and then added `31` at the end of that. Therefore, only the 2001st element (or 2000th element using 0-origin) of the vector is `31` and the 1000th element is `1`. There are no "1000th element which is 31" in your code. — MikeCAT, Sep 25 '20 at 13:45
**Recommended reading:** [Why should I not #include ?](https://stackoverflow.com/q/31816095/560648) — Asteroids With Wings, Sep 25 '20 at 13:45
_"1000th element which is 31"_ No it isn't. Why do you think that? — Asteroids With Wings, Sep 25 '20 at 13:46
You probably meant to write `vec.resize(2000);` instead of `vec.reserve(2000);`. — πάντα ῥεῖ, Sep 25 '20 at 13:49
@πάνταῥεῖ Ah, but `vec.resize(1000);` will invalidate `vec.at(1543)`... — MikeCAT, Sep 25 '20 at 13:52
Yeah, it'll throw a `std::this_question_makes_zero_sense` exception. — Asteroids With Wings, Sep 25 '20 at 13:54
It's very odd to count to `vec.size()` with a loop. Do you think that the loop is any different from just writing `count = vec.size();`, or was it simplified from some other code that did something meaningful with the loop? — molbdnilo, Sep 25 '20 at 14:00

score 3 · Accepted Answer · answered Sep 25 '20 at 13:46

The 1000th element is actually just 1 due to this constructor.

vector <long long> vec(2000,1);  // fill with 2000 elements, each set to 1

Then after this line

vec.push_back(31);  // element [2000] is now 31

Therefore your final vector is

{1, 1, 1, ..., 1, 1, 31}     // length 2001 elements

Can't push_back() after 1000th element in a vector in c++11

1 Answers1