vector initialization using eigen library

Question

I am going through the documentation of Eigen library at http://eigen.tuxfamily.org/dox/classEigen_1_1Matrix.html

and I found the example given below

• Eigen::VectorXd v(10);
• v[0] = 0.1;
• v[1] = 0.2;
• v(0) = 0.3;
• v(1) = 0.4;

I could not understand the difference between v(1) and v[1] usage as one uses square bracket and other uses a different one.

What difference does it make ?

Thank you.

Answer 1

[1] is the same as (1) when it's supported, but parens support multiple dimensions like (1,2) and brackets are only available for one-dimensional Vector objects.

For uniformity, it's usually better just to stick with () parens.

These are for general access. For initialization in particular, see also the << … , operator.