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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

Community
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Connel
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  • `int points[22][2];` is not a matrix, it's a C-array, which does not support the initialization semantics you want with that compiler. Use [a _proper_ matrix library](http://www.boost.org/libs/numeric/ublas/) and you won't have this problem. – ildjarn Apr 13 '12 at 00:01

2 Answers2

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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

George Skoptsov
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  • I'm getting the following error at "this->points": a pointer to a bound function may only be used to call the function – Connel Apr 13 '12 at 00:24
  • I don't understand where you're getting this error and what you're trying to do. I suggest you follow directions in my answer. – George Skoptsov Apr 13 '12 at 00:26
  • In the header I have: "vector > places(22);" and in the constructor for the object I have "this->points[0].first = 1;" as far as I can see its the same. The "->" next to this and points is whats underlined in Visual Studio. Thanks for your help – Connel Apr 13 '12 at 00:30
  • Clearly you should decide whether you want your vector to be called `places` or `points`. – George Skoptsov Apr 13 '12 at 00:31
  • OK when I remove the "(22)" from the initialisation it works! – Connel Apr 13 '12 at 00:31
  • And sorry that was a typo, they are both called the same thing. – Connel Apr 13 '12 at 00:32
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    Don't forget to call `points.resize(22)` in the constructor then. – George Skoptsov Apr 13 '12 at 00:35
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The fundamental cause of your problem is that arrays do not count as real values in C++. They are substandard in many ways- one of which you have just encountered. Any normal type would work as you expect. Unfortunately, to those of us who are not language experts, the error Visual Studio throws is incredibly unhelpful.

You must create an array on the stack and then manually loop through and assign all the values.

Puppy
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  • I'm not sure what you mean by "real values". Yes, they are "real" and their behavior is well defined. The cannot be assign to of course. The OP would be fine with declaring the array as he as done and initializing it in a loop, or maintaining a pointer (of course, a `vector` would be better). You can't assign to the literal `1` either, is it not a "real value"? rvalues and lvalues are perfectly well defined. – Ed S. Apr 13 '12 at 00:15
  • @EdS. Except all those other types are not rvalues when used in lvalue contexts. Arrays do not conform to the same rules as every other type. – Puppy Apr 13 '12 at 18:49