从向量中擦除元素（hackerrank）

Question

I'm a beginner in C++. I have tried to solve this problem but couldn't, thus I checked the solution afterwards but there are some lines I don't understand:

v.erase(v.begin() + x - 1); // i want to know why the "-1" is put here
v.erase(v.begin() + b - 1, v.begin() + c - 1);

Here is the code:

int main()
{
  int N, a, x, b, c;
  vector <int> v;
  cin >> N;
  for (int i = 0; i<N; i++)
  {
    cin >> a;
    v.push_back(a);
  }
  cin >> x >> b >> c;
  v.erase(v.begin() + x - 1);
  v.erase(v.begin() + b - 1, v.begin() + c - 1);
  cout << v.size() << endl;
  for (int i = 0; i < v.size(); i++)
  {
    cout << v[i] << " ";
  }
}

Answer 1

The question aims to make you familiar with 2 common syntax of vector erase method. For deleting single element use,

v.erase( pass iterator pointing to the element that you want to erase )

For example, v.erase(v.begin()) will erase first element of the vector or in other words will erase element at position 0 of the vector.

Since here v.begin() is iterator to the first element of the vector, provided vector isn't empty.

Similarly,

v.erase(v.begin() + x -1);

erases element at position x of the vector.

Now to erase a range in the vector, overloaded method of erase is used. It is used as follows,

v.erase(iter1,iter2)

It will erase all the elements in the range of iter1 to iter2 but not including iter2, that is elements in the range [iter2, iter2) will be erased. Remember iter2 won't be erased. Thus this code,

v.erase(v.begin() + b - 1, v.begin() + c - 1);

will erase all the elements from index b to index c, but not including index c.

Answer 2

#include <cmath>           //Header section
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;


int main() {  
    vector<int> v;            //initializing vector v
    int n,i,b,c,d,size,val;
    cin>>n;                   //no. of integers in vector v
    for(i=0;i<n;i++)          
    {
        cin>>val;
        v.push_back(val);     /*syntax for input elements in vector, "val" is the 
                                    element*/
    }
    cin>>b;   // input position of element to be erased

    cin>>c>>d;  //input range of elements to be erased

    v.erase(v.begin()+b-1); // func to be used to erase an element

    v.erase(v.begin()+c-1,v.begin()+d-1) // func used to erase a range

    size=v.size();
    cout<<size<<endl;
    for(i=0;i<size;i++)
    {
        cout<<v[i]<<" ";  //leftover elements output
    }
    return 0;
}