Why can't we input a vector as we input an array in C++?

Question

The wiki says:
The elements of a vector are stored contiguously. AND
Vectors allow random access; that is, an element of a vector may be referenced in the same manner as elements of arrays (by array indices).

So why can't we input the elements of a vector as:

vector<int> v;
for(int i=0;i<3;i++)
{
    cin>>v[i];
}

If you give the vector a size, yes. Otherwise it starts with 0 elements. If you have a fixed size, you can use std::array instead. But what is wrong with push_back? — crashmstr
– crashmstr, Commented Dec 10, 2014 at 14:46
@crashmstr given the number of questions on here about it I would add the caveat that if you have a fixed size and that size can fit on the stack you can use std::array instead. — sjdowling
– sjdowling, Commented Dec 10, 2014 at 14:53

Wojtek Surowka · Accepted Answer · 2014-12-10 15:07:01Z

4

Either you need to resize the vector upfront - as other answers say - or you can use C++ standard library. Then the equivalent of your for loop is the following one line:

copy_n(istream_iterator<int>(cin), 3, back_inserter(v));

and it takes care of allocation/resizing.

edited Dec 10, 2014 at 15:07

answered Dec 10, 2014 at 14:59

Wojtek Surowka

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h8pathak · Accepted Answer · 2014-12-10 14:58:55Z

3

Your vector has zero elements right now. Try allocating it some space as:

vector<int> v(5);

Then your method would work.

answered Dec 10, 2014 at 14:58

h8pathak

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Notinlist · Accepted Answer · 2014-12-10 14:47:20Z

3

The problem is that you need to allocate the elements of the vector first. So try vector<int> v(4);, so it will start with 4 elements. Then you can load values into them.

answered Dec 10, 2014 at 14:47

Notinlist

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