Menu
I am going to initialize a 2D vector in a member function, where the input arguments are planned to be fed into determined indexes within the function argument. Furthermore, I'm not gonna use [] operator to member access due to safety stuffs. How can I use at() function in order to access to the index of a 2D vector, as below?
vector<vector<double>> weight;
void Connection::setWeight(const double& value, const double& i, const double& j)
{
// The other scheme except: weight[i][j] = value;
}
214
The most idiomatic way is to make your function take iterators: template<typename It> It::value_type maxsubarray(It begin, It end) { ... } and then use it like this: std::vector<int> nums(...); auto max = maxsubarray(begin(nums) + 2, end(nums));
Element access: reference operator [g] – Returns a reference to the element at position 'g' in the vector. at(g) – Returns a reference to the element at position 'g' in the vector. front() – Returns a reference to the first element in the vector. back() – Returns a reference to the last element in the vector.
Access an element in vector using vector::at() reference at(size_type n); reference at(size_type n); It returns the reference of element at index n in vector. If index n is out of range i.e. greater then size of vector then it will throw out_of_range exception.
In a 2D vector, every element is a vector. Another approach to access the vector elements: Like Java’s jagged arrays, each element of a 2D vector can contain a different number of values. Exercise Problem: Define the 2D vector with different sizes of columns. 2D vectors are often treated as a matrix with “rows” and “columns” inside it.
How to find index of a given element in a Vector in C++. find (): Used to find the position of element in the vector. Subtract from the iterator returned from the find function, the base iterator of the vector . Finally return the index returned by the subtraction.
Each elements of a vector can be accessed by using its index. It works similar to array, i.e. the index of the first element is 0, index of the second element is 1 etc. By using at () method. Let’s check how to do that with example for both. It returns a reference to the element at position n in the vector.
Each index of vector stores a vector which can be traversed and accessed using iterators. It is similar to an Array of Vectors but with dynamic properties. vector<vector<int>> vec { { 1, 2, 3 }, { 4, 5, 6 }, { 7, 8, 9, 4 } }; where vec is the vector of vectors with different number of elements in different rows
You can use at twice. Also note, if i and j are indexes, they really shouldn't be double.
void Connection::setWeight(const double value, const size_t i, const size_t j)
{
weight.at(i).at(j) = value;
}
Risking to be downvoted for premature optimization, and because it doesn't really answers the question asked, still I would like to remind about a serious problem in such approach.
Like any multidimensional array, vector of vector is not contiguous, which makes its performance characteristics terribly bad because of lacking of spatial locality and associated cache misses.
You can trivially solve the problem by wrapping a single-dimensional array with a convenient interface of two-dimensional array.
template <class T>
class MyArray2D
{
std::vector<T> data;
size_t sizeX, sizeY;
public:
const T& at(int x, int y) const { return data.at(y + x * sizeY); }
T& at(int x, int y) { return data.at(y + x * sizeY); }
// wrap other methods you need of std::vector here
};
Note that code above written on the fly and wasn't compiled. Still, I believe you've got the idea.
See also:
21
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
