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Showing content from http://www.cplusplus.com/reference/vector/vector/at/ below:

public member function

<vector>

std::vector::at 
      reference at (size_type n);const_reference at (size_type n) const;

Access element

Returns a reference to the element at position n in the vector.

The function automatically checks whether n is within the bounds of valid elements in the vector, throwing an out_of_range exception if it is not (i.e., if n is greater than, or equal to, its size). This is in contrast with member operator[], that does not check against bounds.



Parameters
n
Position of an element in the container.
If this is greater than, or equal to, the vector size, an exception of type out_of_range is thrown.
Notice that the first element has a position of 0 (not 1).
Member type size_type is an unsigned integral type.

Return value The element at the specified position in the container.

If the vector object is const-qualified, the function returns a const_reference. Otherwise, it returns a reference.

Member types reference and const_reference are the reference types to the elements of the container (see vector member types).



Example 
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// vector::at
#include <iostream>
#include <vector>

int main ()
{
  std::vector<int> myvector (10);   // 10 zero-initialized ints

  // assign some values:
  for (unsigned i=0; i<myvector.size(); i++)
    myvector.at(i)=i;

  std::cout << "myvector contains:";
  for (unsigned i=0; i<myvector.size(); i++)
    std::cout << ' ' << myvector.at(i);
  std::cout << '\n';

  return 0;
}

Output: 
myvector contains: 0 1 2 3 4 5 6 7 8 9


Complexity Constant.

Iterator validity No changes.

Data races The container is accessed (neither the const nor the non-const versions modify the container).
The reference returned can be used to access or modify elements. Concurrently accessing or modifying different elements is safe.

Exception safetyStrong guarantee: if an exception is thrown, there are no changes in the container.
It throws out_of_range if n is out of bounds.

See also
vector::operator[]
Access element (public member function)
vector::front
Access first element (public member function)
vector::back
Access last element (public member function)

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