Writing new values into array

Question

This is a fragment of my code:

typedef float point2[2];

point2 a = {-90, -90};
point2 b = {-90, 90};
point2 c = {90, 90};
point2 d = {90, -90};

glBegin(GL_POLYGON);
    glVertex2fv(a);
    glVertex2fv(b);
    glVertex2fv(c);
    glVertex2fv(d);
glEnd();

And this goes really well. But later when I try to write new values into these arrays, like:

a = {-66, -66};
b = {-66, 66};

And here I get an error:

error: assigning to an array from an initializer list

And I understand, that I can't assign values directly to an array after its declaration. But how this should looks like?

I'm voting to close this question as off-topic because it is very poor and you can easily find it in any textbook!!! — gsamaras
– gsamaras, Commented Oct 17, 2015 at 13:17
@gsamaras Yes, I know. But like I said - I wanted to know if there is more convinient way then just a[0]=... a[1]=... — DzikiChrzan
– DzikiChrzan, Commented Oct 17, 2015 at 13:29
OK then @DzikiChrzan, I will retrieve my downvote and as a sorry I will upvote an answer of yours I like. — gsamaras
– gsamaras, Commented Oct 17, 2015 at 14:24

Chris Drew · Accepted Answer · 2015-10-17 13:33:36Z

3

You can't assign to an array using an initializer list. But if you use std::array instead of a C array then this is possible:

#include <array>
typedef std::array<float, 2> point2;

point2 a = {-90.0, -90.0};
a = {-66, -66};

And you can still pass it to a function like glVertex2fv that takes a pointer to an array by using std::array::data:

glVertex2fv(a.data());

Live demo.

edited Oct 17, 2015 at 13:33

answered Oct 17, 2015 at 13:25

Chris Drew

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1 Comment

DzikiChrzan Over a year ago

Yeah, so there IS a way to do it different! Thanks, this is answer I was looking for. ;)

Algirdas Preidžius · Accepted Answer · 2015-10-17 13:08:27Z

2

In those statements, you are not assigning values to an array, you are initializing that array. Changing values in an array can be done like this:

a[0] = -66;
a[1] = -66;

answered Oct 17, 2015 at 13:08

Algirdas Preidžius

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463035818_is_not_an_ai · Accepted Answer · 2015-10-17 13:25:26Z

2

You cant use initializer list once the array is initialized, just do:

a[0] = -66;
a[1] = -66;

edited Oct 17, 2015 at 13:25

answered Oct 17, 2015 at 13:08

463035818_is_not_an_ai

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8 Comments

Humam Helfawi Over a year ago

first solution should work but it has unnecessary cost

463035818_is_not_an_ai Over a year ago

@HumamHelfawi yes, I just mentioned it, because I assumed the OP already knows how to access the array elements but was looking for a denser notation.

Algirdas Preidžius Over a year ago

Arrays in C++ are not objects. They don't have a constructor. And your first suggestion doesn't work, because you can't modify the pointer, to which your locally defined array variable is pointing, as well.

463035818_is_not_an_ai Over a year ago

@AlgirdasPreidžius hm? The locally defined array variable is not pointing to a pointer

463035818_is_not_an_ai Over a year ago

@AlgirdasPreidžius but you are right, that it doesnt work, because there is no constructor to call. My mistake, I will edit

|

cdonat · Accepted Answer · 2015-10-17 13:34:27Z

2

When your case is really about points in a plain, you schould prefer to use a class instead of an array.

class point2 {
    private:
        float x_;
        float y_;
    public:
        point2(const float x, const float y): 
            x_{x}, y_{y} {};
        point2(const point2& o): 
            x_{o.x_}, y_{o.y_} {};

        operator = (const point2& o) {
            x_ = o.x_;
            y_ = o.y_;
        }
        // ...
};


point2 a = {90.0, 90.0}; // initialize a

a = {45.0, 45.0}; // untested, should work
a = point2{45.0, 45.0} // will work.

As you see, this will give you a more natural and expressive interface. Such interfaces always improve readability and maintaiability.

answered Oct 17, 2015 at 13:34

cdonat

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2 Comments

Chris Drew Over a year ago

A great idea in general but perhaps not so convenient if you primarily want to pass the point to a third-party library that is expecting a pointer to an array. Which is the case here. You could have a point2 class that contains an array or std::array and provide convenience getters/setters for x and y.

cdonat Over a year ago

Yes, in that case you can adapt the internal representation of course.

Humam Helfawi · Accepted Answer · 2015-10-17 13:08:39Z

1

you should do something like:

a[0] =-66;
a[1]=-66;

answered Oct 17, 2015 at 13:08

Humam Helfawi

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Roman Pustylnikov · Accepted Answer · 2015-10-17 13:24:03Z

1

This is possible during initialization only. Use:

a[0]= - 66;
a[1] = -66;

However you can create temporary array and then copy it.

point2 tmp = {-66, -66 }
System.arraycopy( tmp, 0, a, 0, src.length );

But it looks like an overkill for such a task.

edited Oct 17, 2015 at 13:24

answered Oct 17, 2015 at 13:08

Roman Pustylnikov

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1 Comment

DzikiChrzan Over a year ago

Hm, okay. I know that I can do this in your way, but I was hoping that maybe there is more convenient way. ;)

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