QT and C++

[Kaiserklein]

For only one fucking exercise, I'm supposed to use a code and a software I never used before... And I can't get it to work. There's a QT project, and I'm supposed to code a few functions in C++, about the Lagrange interpolation. So here's the QT project:

pack of shit.zip

(20.06 KiB) Downloaded 15 times

(the main file to work on is Ex1_LagrangePolynomial/Ex1_LagrangePolynomial.cpp)

Here's the link to the exercise sheet: http://gfx.uni-kl.de/~alggeom/exercise/WS1718/Ex1.pdf (it's the exercise 4)

And here's the code I did for one of the functions (which doesn't work because for the life of me I couldn't get variables from a scope to another in fucking C++):

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std::vector<float> LagrangePolynomial::computeLagrangeBasisDenominators() {
    float xi, xj;
    std::vector<float> d = std::vector<float>(); // Array containing all the denominators to return
    for (int i=0; i<3; i++) {
       float di = 1; // Denominator number i
        xi = p[i][0]; // Find a way to bring fucking p in here
        for (int j = 0; j < 3; j++) {
            if(j != i) {
                xj = p[j][0];
                di *= (xi - xj);
            }
        }
        d.push_back(di);
    }
    return d;
}

The code itself should be easy, the algorithmic is basic. But I just can't seem to use this software and I have no idea about C++. I don't even understand how to call a function, which I need to do if I want to test my code.


So plz halp anyone, wud be veri much appreciated :'(

[Dolan]

I can't take a serious look at this right now, because I'm in the middle of an emergency, but from what I remember, you can define a variable in a class with public access and then use that scope to call it in another class.

For example:

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class A {
  public: static int x;   // declare variable x
};

class B {
  public:
    void func() {
      A::x = 2;   // use variable x declared in another scope
    }
};

Then you could instantiate these classes in objects. Ie:

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A *obj = new A(); // on the heap
A obj; // on the stack

Well, these are some generic examples to give you an idea on how to use a variable declared in another scope.

[deleted_user]

I could do it but I don't feel like it.

All that's needed is a simple conversion into Java.

[Dankl]

I think you want to pass p in as a parameter to the function computeLagrangeBasisDenominators. So, the function definition would look like,
computeLagrangeBasisDenominators (float[3][3] p) {...}.
Then in your main when you want to call the function you would say,
Vector<float> ans = computeLagrangeBasisDenominators(p);
Assuming you have some 3x3 float array called p.

Sorry for formatting. I'm on mobile.

[QueenOfdestiny]

Nerd thread

[Kaiserklein]

@Dolan the problem is that I need to keep the projects the teachers gave me, and just code 3 functions. Everything else should be left untouched, I think. So I can't change the way they declared their variables.


@Dankl I thought about that, it makes most sense imo. However, in the C++ header file (which I guess kind of declares the functions and global variables used in the C++ source file..?), I have:

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private:
   std::vector<float> computeLagrangeBasisDenominators();

Which means that the computeLagrangeBasisDenominators function doesn't take any parameter, right? And since I'm not supposed to touch the framework I was given, but only the bodies of 3 functions, I'm afraid I can't do that :S


But thanks a lot guys for helping anyway, it's much appreciated

[Goodspeed]

(which doesn't work because for the life of me I couldn't get variables from a scope to another in fucking C++)

Clarify? Are you unable to call the function or unable to access variables that you need inside the function?
I'm guessing the latter because in your code it says "find a way to get fucking p in here". Is p a field in the same class your function is in? Then you can use it by saying this->p

[Kaiserklein]

(which doesn't work because for the life of me I couldn't get variables from a scope to another in fucking C++)

Clarify? Are you unable to call the function or unable to access variables that you need inside the function?
I'm guessing the latter because in your code it says "find a way to get fucking p in here". Is p a field in the same class your function is in? Then you can use it by saying this->p

I am unable to access variables from inside the function. Plus, I am also unable to call my function to test it, because I can't get to run my code in QT and get output in a console.

p is a variable that was declared in, I think, another function of the same class:

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LagrangePolynomial::LagrangePolynomial() : OpenGLWindow()

{
   std::vector<Vector> p = std::vector<Vector>();
   Vector v = Vector(4);
   v[3] = 1.0;
   v[0] = -1.0;
   v[1] = v[2] = 0.0;
   p.push_back(Vector(v));
   v[0] = 0.0;
   v[1] = 1.0;
   p.push_back(Vector(v));
   v[0] = 1.0;
   v[1] = -1.0;
   p.push_back(Vector(v));
}

According to what the exercise says, p is indeed the variable I need to use. But it wasn't declared in the header file (which is where some global variables were declared I think), instead it was declared in this weird function. And I need to use it in another function of the same class.

[AraGun]

Pls report kaiser for cheating on his homework.

[Dankl]

That weird function is the constructor (because it has the same name as the class). It creates a temporary variable p and stores it in the curve supportPoints. To access it you need the line,

std::vector<Vector> p = supportPoints.getPositions();

inside your function.

[Ashvin]

Passing p as an argument it is kinda hard to see how that would work, are you sure you are not allowed to do that?

[Kaiserklein]

That weird function is the constructor (because it has the same name as the class). It creates a temporary variable p and stores it in the curve supportPoints. To access it you need the line,

std::vector<Vector> p = supportPoints.getPositions();

inside your function.

Wow, I don't get any error anymore when I declare p the way you said. Sounds good, thanks a lot!
Do you know how I can test it now, though? When I run the project, it just launches an empty command prompt, in which I can't type anything, and nothing happens. I guess I need to call my function somehow?

Passing p as an argument it is kinda hard to see how that would work, are you sure you are not allowed to do that?

I'm pretty sure I'm not, though I could ask. But with Dankl's solution, I guess I don't need to.

[deleted_user0]

Code: Select all

class A {Somppukunkku, Class D: Princeofkabul
  public: static int x;   // declare variable x = Somppukunkku god. Somppukunkku > Princeofkabul)
};

class B {GAY SECTION: Princeofkabul]
  public:
    void func(Somppukunkku > Princeofkabul) {
      A::x = 2;   // use Taunt 11 to show how bad Princeofkabul is
    }

Here are the solutions for your homeworks....

[Kaiserklein]

Taunt 12