## Get row vector of 2d vector in C++

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I have a vector of vector in C++ defined with: `vector < vector<double> > A;`

Let's suppose that A has been filled with some values. Is there a quick way to extract a row vector from A ?

For instance, `A[0]`

will give me the first column vector, but how can I get quicky the first row vector?

**2D vector in C++ with user defined size,** Like 2D arrays, we can declare and assign values to 2D matrix Like Java's jagged arrays, each row of 2D vector can contain different Get hold of all the important DSA concepts with the DSA Self Paced Course at a� Like Java’s jagged arrays, each row of 2D vector can contain different number of columns. filter_none. edit close. play_arrow. link brightness_4 code

There is no really fast way to do that. Also as pointed out, I would say that the convention is the other way around, meaning that `A[0]`

is actually the first row, rather than the first column. However even *trying* to get a column is not really trivial, since

{0, 1, 2, 3, 4} {0} {0, 1, 2}

is a very possible `vector<vector<double>> A`

, but there is no real column `1`

, `2`

, `3`

or `4`

. If you wish to enforce behavior like same length columns, creating a `Matrix`

class may be a good idea (or using a library).

You could write a function that would return a `vector<double>`

by iterating over the rows, storing the appropriate column value. But you would have to be careful about whether you want to copy or point to the matrix values (`vector<double>`

/ `vector<double *>`

). This is not very fast as the values are not next to each other in memory.

**Vector of Vectors in C++ STL with Examples,** The code creates a 2D vector by using the push_back() function and then displays the matrix. This function removes element 9 from the last row vector. Get hold of all the important DSA concepts with the DSA Self Paced� Case 2 : To sort the entire 2D vector on basis of a particular column. In this type of sorting 2D vector is entirely sorted on basis of a chosen column. For example if the chosen column is second, the row with smallest value in second column becomes first row, second smallest value in second column becomes second row, and so on. {3, 5, 1}, {4

The answer is: in your case there is no corresponding simple option as for columns. And one of the reasons is that vector> is a particular poorly suited container for multi-dimensional data.

In multi-dimension it is one of the important design decisions: which dimension you want to access most efficiently, and based on the answer you can define your container (which might be very specialized and complex).

For example in your case: it is entirely up to you to call A[0] a 'column' or a 'row'. You only need to do it consistently (best define a small interface around which makes this explicit). But STOP, don't do that:

This brings you to the next level: for multi-dimensional data you would typically not use vector> (but this is a different issue). Look at smart and efficient solutions already existing e.g. in ublas https://www.boost.org/doc/libs/1_65_1/libs/numeric/ublas/doc/index.html or eigen3 https://eigen.tuxfamily.org/dox/

You will never be able to beat these highly optimized libraries.

**Defining a 2D vector in C++ - OpenGenus IQ,** In this article, we have explored the different ways of defining a 2D vector in C++. Before getting in the nitty-gritty of 2-D vectors, let us first define some working #include <iostream> #include <vector> int main(){ //inputting the no. of rows in� 'vector<vector<int>> v(num_row, row) – In this statement, we create our complete two-dimensional vector, by defining every value of the 2-D vector as the 'row' created in the last statement. After understanding the above procedure, we can improve our initialization of 2D vectors in C++ by:

**Column size and row size of a 2D vector in C++,** The following code can be used to iterate through varying two dimensional vector. vector_name. size() gives you the numbers of column in a 2D vector and vector_name[0]. size() gives you the numbers of rows in a 2D vector in C++. I have an 2D vector array, but a function that can calculate only with 1D arrays. Now I need to pass every row of the 2D array to that function. So how can I pass a part (row or column) of a 2D vector array to a function as a pointer - the result shall be in the 2D array automatically. With C-Arrays A[N][N] I know the solution by passing

**2d vector c++ Code Example,** Get code examples like "2d vector c++" instantly right from your google For the amount of rows (int vectors) in the 2d vector, you can just use myVector.size(). 9. Now to create a vector of 5 vectors in which each vector is initialized as above, we will use following syntax, std::vector <std::vector > vec2D(5, std::vector(4, 1)); Let’s see the code to initialize and print 2D vector as follows,

**2D vector in C++ STL with user defined size,** C++ STL 2D Vector: Here, we are going to learn about the vector of 3) C++ STL code to declare and print a 2D Vector (Numbers of rows,� Similarly, a row vector or row matrix is a 1 × m matrix, that is, a matrix consisting of a single row of m elements = […]. Throughout, boldface is used for the row and column vectors. The transpose (indicated by T) of a row vector is a column vector

##### Comments

- strictly speaking in your
`vector<vector>>`

there is no row-vector. If you want one you need to construct it. Depending on what you want you should consider to have a view rather than creating a new container - I guess I'm misunderstanding things but wouldn't A[0] be the first row?
- @c_student its a matter of convention, swap "row" with "column" and the question is still the same
- If this is supposed to be a matrix, you shouldn't use
`vector<vector<double>>`

(memory locality, allocations). Look at Eigen, it has all you need! - what did you try? why is it too slow? what to you need the row vector for? I am afraid without more context you cannot get a better answer than "create a new vector and fill it via looping over your matrix"
- I wouldn't say there is any kind of "convention" over rows and columns. That's completely arbitrary decision which is which. One way is probably more
*common*, but it's not a convention to follow. - Good point. I guess I would just like to believe that my way is the way everyone does it :D. If there is no performance aspect involved, because when I write it as a literal using
`{ {} {} {} }`

over multiple lines, you get that nice row-wise schema.