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1) a Vec4i is a struct holding 4 integers, you can access them using the [] operator, like in:

Vec4i l(1,2,3,4);   l[2] == 3;

2) a std::vector is a (template)container for various things.

in the houghlines example, each found line has 2 endpoints, they are kept in a Vec4i like (x1,y1, x2,y2). since the algorithm will find many lines, those are stored in a vector<Vec4i> lines, and you would access it with

lines[which_line][which_coordinate]