Question

I need a variable which will hold 22 pairs of ints (positions in a grid) so I was thinking of having a matrix array. So in my header file is:

int points[22][2];

but when I put the following in the constructor of the object:

this->points = {{1,2},{2,3},...};

It says “must be a expression must be a modifiable lvalue” I’ve tried using the const keyword and making it a pointer in the header file as described here Expression must be a modifiable L-value

I’ve also tried creating a separate 2d array and then assigning it but this doesn’t work either.

int points2 = {{1,2},{2,3},...};
this->points = points2;

I’m used to Java and I’m not too experienced with C++. There is a default constructor that will assign the values as above and a constructor which will have the matrix as parameter.

The following does work:

this->point[1][1] = 4;

But this means I can’t pass another value as a parameter and I end up with 44 lines of messy code in the default constructor! And I was going to use a struct with 2 ints and put them in a vector put that seems like a bit of an overkill and would mean I need 22 vector inserts before I even called the constructor with the manual values and I just thought there must be a better way 🙂

Thanks

Answer 1

Since you’re using C++, a much better choice would be to use a vector of pairs of ints.

Declare it like this:

std::vector<std::pair<int, int> > points;

In your constructor you can specify size at initialization

: points(22), 

or set it at any point like this:

points.resize(22);

You can access individual coordinates with

points[1].first = 1;
points[1].second = 44;

or with

points[1] = make_pair(1, 44);

or you can build it up without having to worry about exceeding its allocated size with

points.push_back(make_pair(1, 44));

etc