How To: Transfer elements between vectors in C++

Sometimes I end up needing to do this, so I re-implement the functionality… And it segfaults or behaves strangely. This time I’ll write it down somewhere I know where I can find it!

  1. #include <iostream>
  2. #include <string>
  3. #include <vector>
  5. using namespace std;
  7. int main()
  8. {
  9. 	// Create an empty vector of type string for our shopping list
  10. 	vector<string> shoppingList;
  12. 	// Add a few items to it
  13. 	shoppingList.push_back("Milk");
  14. 	shoppingList.push_back("Bread");
  15. 	shoppingList.push_back("Eggs");
  17. 	// Create an empty vector of type string for our purchases
  18. 	vector<string> purchases;
  20. 	// Create a string to hold the user's answer
  21. 	string answer;
  23. 	// Transfer items bought from the shopping list to the purchases list (includes removal from the shoppingList vector)
  24. 	vector<string>::iterator i = shoppingList.begin();
  25. 	while (i != shoppingList.end() )
  26. 	{
  27. 		cout << "Item: " << *i << endl;
  28. 		cout << "Purchase item? (y/n)" << endl;
  30. 		cin >> answer;
  32. 		// If we said yes...
  33. 		if (answer == "y")
  34. 		{
  35. 			// ...add the item to the purchases vector and...
  36. 			purchases.push_back(*i);
  38. 			// ...remove the item from the shoppingList vector. Erase returns an
  39. 			// iterator which points at the next element in the vector, so we need to
  40. 			// skip incrementing the iterator in this case or we might go out of bounds!
  41. 			i = shoppingList.erase(i);
  42. 		}
  43. 		else // Otherwise just move on to the next item!
  44. 		{
  45. 			i++;
  46. 		}
  47. 	}
  49. 	cout << endl;
  51. 	// Display both lists
  52. 	cout << "----- Shopping List ----" << endl;
  53. 	for (i = shoppingList.begin(); i != shoppingList.end(); i++)
  54. 	{
  55. 		cout << *i << endl;
  56. 	}
  58. 	cout << endl;
  60. 	cout << "----- Purchases ----" << endl;
  61. 	for (i = purchases.begin(); i != purchases.end(); i++)
  62. 	{
  63. 		cout << *i << endl;
  64. 	}
  66. 	cout << endl;
  68. 	return 0;
  69. }

TLTB ;-)

How-To: Create A Simple OpenGL 2D Particle Fountain in C++

I was recently asked about some particle systems I’d put together, so in response, this isn’t really a “look what I’ve coded” post – instead, it’s more of a “this is an easy way to set up a particle system framework” post.

To demonstrate the setup of a particle system using OpenGL, I’ve put together some simple starter code which displays a 2D particle fountain. I was going to just dump this code as a zipped project into a reply, but then (as my wife pointed out) it would be pretty much buried in the comments, so if other people starting OpenGL coding wanted to learn the basics of particle systems my code wouldn’t be as visible. As such, I’ve made this into a separate post – and all our example code does is this:

It’s a dirt-simple effect, but what it does isn’t so important right now – it’s deliberately simple. How it does what it does is what we’ll talk about.

Looking at particle systems from a high level – you generally have to choose between two different approaches to the system as a whole:

  1. You have a fixed number of particles (i.e. you have a fixed size array of however many particles) and you reset each particle to “recycle” it, or
  2. You have a dynamic number of particles (i.e. you have a dynamically resizable array) and you destroy each particle and create a new one as required.

I’ve gone with the latter approach for this code using a vector of particles (nothing to do with Vec2/Vec3 math – just the name of resizable array kinda construct). Admittedly, there’s more overhead in the creation and destruction of particles, but on the upside you don’t get that initial rush of particles you get when using fixed size arrays and as soon as you start the program BLAM! All your particles going at once.

To demonstrate what I mean, in the video below I’ve modified the code to instantiate ALL particles up to the particle limit at once, and then as soon as a particle goes off the bottom of the screen it gets destroyed and a new particle is created. The effect of which is that you get a single big burst of particles, and then as they all get destroyed and recreated at different times they turn into a smooth flow within a couple of seconds:

If you’re using a fixed size particle array, you’ll need to implement some mechanism to delay the instantiation or “launch” of the particles – for example, you might give each particle a random framesUntilLaunch value and decrease it by 1 each frame until it gets to 0 and you can let the particle do its thing. I wrote such a delay system for some fireworks code I did a while back if you’d like to see a concrete example.

Anyways, back at this code, our main is using three main classes to encapsulate data and provide methods to manipulate it:

  • Colour4f.hpp – A class to store a colour as red/green/blue/alpha floating point values and manipulate ’em (including interpolation of colours),
  • Vec2.hpp – A templatised class to store two values as a vector (Not a resizable array this time! An actually “euclidian vector” – i.e. two values which represent a direction and magnitude). It also includes lots of overloaded operators so you can add two vectors together (“up” + “right” gives you “up-right” etc.), multiply a vector by a scalar (i.e. moving “up-right” multiplied by 10 means you’re now moving up-right ten times as fast). By templatised, I mean that you can create a Vec2 of ints, or floats, or doubles, or shorts, or whatever numeric type makes sense for your application – take a look at the source below if you want examples, and finally
  • Particle2D.hpp – A particle class which uses the above Colour4f and Vec2 classes to provide powerful movement and colour modification options in a small & easy to use package.

You can download the complete source code as a Code::Blocks project here, if you’d like:

Note: As my OS of choice is GNU/Linux (LMDE, to be specific), the source files provided will have Linux line-endings – so you’ll need to open them with a good text editor like Notepad++ if you’re in in Windows, otherwise each file will look like a single massive block of noise!

Or, if you’d prefer to just browse the source code so you can copy and paste sections, you’ll find it all laid out below.

I love particle systems – you can do some visually stunning things with really simple code, or you can do amazing things if you take it further. Either way, I hope you have a lot of fun with them (there’s lots more particle stuff on this site if you’re looking for inspiration – try Actionscript tag for a start!) – and if you make anything cool or pretty using and you think I’ve helped – please do show me or let me know – I’ve love to see or hear about what you’ve done! =D

Also – if you make this – show me how, okay?


Continue reading How-To: Create A Simple OpenGL 2D Particle Fountain in C++

Vec3: A Simple Vector Class in C++

This is really the second post of three updating an earlier post on how to write some simple FPS-type controls for OpenGL – last post we looked at a FpsManager class so we could get frame timings to implement framerate independent movement. This time, we’re looking at a simple Vec3 class which stores 3D coordinates and helps to perform some operations on them. This is going to form part of the Camera class in the final part, but I thought I’d put it here on its own so it can be re-used.

There’s probably about ten million different vector classes out there, but I really wanted to write my own so I understood exactly how it worked, and I’ve commented it pretty heavily in the process – so hopefully if you’re looking for a vector class you might be able to look at this one, see how it works and what it does, and take and modify it for your own work.

Before skipping to the code, let’s just take a quick look at what it does – it packages up 3 values called x, y and z, and allows us to easily manipulate them. Quite what you store in these values is up to you, it could be a vertex position, or a direction vector, or a RGB colour – anything that has three values, really.

If we were dealing with two vertexes, we can do stuff like this:

    // Define two initial vectors
    Vec3<double> vector1(1, 2, 3); // A coordinate +1 on the X axis (horiz), +2 on the Y axis (vert), and +3 on the Z axis (depth)
    Vec3<double> vector2(4, 5, 6); // A coordinate +4 on the X axis (horiz), +5 on the Y axis (vert), and +6 on the Z axis (depth)
    // Create a new vector which is the sum of these two vectors added together
    Vec3<double>result = vector1 + vector2;
    result.display();           // (5, 7, 9)
    // Add the first vector to our result vector
    result += vector1;
    result.display();           // 6, 9, 12
    // Subtract the second vector from our result vector
    result -= vector2;
    result.display();           // 2, 4, 6
    // Divide our result vector by the scalar value 2
    result /= 2;
    result.display();           // 1, 2, 3
    // Multiply both vectors together and assign to our result vector (this is a dot-product
    // operation, but we have specific dot product functions we can use, too)
    result = vector1 * vector2;
    result.display();           // 4, 10, 18
    // Multiply our result vector by 2
    result *= 2;
    result.display();           // 8, 20, 36
    // Normalise our result vector so that all values fall within the range -1 to +1
    result.display();           // 0.190693, 0.476731, 0.858116
     // Calculate the distance between two points in 3D space
    double distance = Vec3<double>::getDistance(vector1, vector2);
    cout << "Distance between points: " << distance << endl; // 5.19615
    // Dot products only work on normalised values (i.e. each x/y/z value in the vector must be in the range -1 to +1)
    // So remember to normalise your vectors before computing the dot product!
    double dotProduct = Vec3<double>::dotProduct(vector1, vector2);
    cout << "Dot product: " << dotProduct << endl;           // 0.974632
    // Define some vectors pointing up, down, left, and right
    Vec3<double> up(   0,  1, 0);
    Vec3<double> down( 0, -1, 0);
    Vec3<double> left(-1,  0, 0);
    Vec3<double> right(1,  0, 0);
    // ------------ Dot Product Tests ------------
    // The dot product of two vectors pointing the same direction is 1
    dotProduct = Vec3<double>::dotProduct(up, up);
    cout << "Dot product of up and up: " << dotProduct << endl;
    // The dot product of two vectors which are perpendicular to each other is is 0
    dotProduct = Vec3<double>::dotProduct(up, right);
    cout << "Dot product of up and right: " << dotProduct << endl;
    // The dot product of two vectors pointing in opposite directions is -1
    dotProduct = Vec3<double>::dotProduct(up, down);
    cout << "Dot product of up and down: " << dotProduct << endl;
    // ------------ Cross Product Tests ------------
    // The cross product of a vector is the vector which is perpendicular to the plane made
    // by the two vectors specified. Whether it points "up" or "down" depends on the
    // handedness of the coordinate system and/or the order of vectors provided.
    // Test 1
    Vec3<double> crossProduct = Vec3<double>::crossProduct(up, right);
    // x = 0, y = 0, z = -1 (i.e. the vector perpendicular to up and right points INTO the screen)
    // Test 2
    crossProduct = Vec3<double>::crossProduct(right, up);
    // x = 0, y = 0, z = 1 (i.e. the vector perpendicular to right and up points OUT from screen)

Seems pretty easy to work with to me…

Here’s the code for the Vec3 class itself as a templatised, header-only C++ .hpp file – just include the Vec3.hpp class and you’re good to go:

#ifndef VEC3_HPP
#define VEC3_HPP
#include <iostream>
template <class T> class Vec3
        // A Vec3 simply has three properties called x, y and z
        T x, y, z;
        // ------------ Constructors ------------
        // Default constructor
        Vec3() { x = y = z = 0; };
        // Three parameter constructor
        Vec3(T xValue, T yValue, T zValue)
            x = xValue;
            y = yValue;
            z = zValue;
        // ------------ Getters and setters ------------
        void set(const T &xValue, const T &yValue, const T &zValue)
            x = xValue;
            y = yValue;
            z = zValue;
        T getX() const { return x; }
        T getY() const { return y; }
        T getZ() const { return z; }
        void setX(const T &xValue) { x = xValue; }
        void setY(const T &yValue) { y = yValue; }
        void setZ(const T &zValue) { z = zValue; }
        // ------------ Helper methods ------------
        // Method to reset a vector to zero
        void zero()
            x = y = z = 0;
        // Method to normalise a vector
        void normalise()
            // Calculate the magnitude of our vector
            T magnitude = sqrt((x * x) + (y * y) + (z * z));
            // As long as the magnitude isn't zero, divide each element by the magnitude
            // to get the normalised value between -1 and +1
            if (magnitude != 0)
                x /= magnitude;
                y /= magnitude;
                z /= magnitude;
        // Static method to calculate and return the scalar dot product of two vectors
        // Note: The dot product of two vectors tell us things about the angle between
        // the vectors. That is, it tells us if they are pointing in the same direction
        // (i.e. are they parallel? If so, the dot product will be 1), or if they're
        // perpendicular (i.e. at 90 degrees to each other) the dot product will be 0,
        // or if they're pointing in opposite directions then the dot product will be -1.
        // Usage example: double foo = Vec3<double>::dotProduct(vectorA, vectorB);
        static T dotProduct(const Vec3 &vec1, const Vec3 &vec2)
            return vec1.x * vec2.x + vec1.y * vec2.y + vec1.z * vec2.z;
        // Non-static method to calculate and return the scalar dot product of this vector and another vector
        // Usage example: double foo = vectorA.dotProduct(vectorB);
        T dotProduct(const Vec3 &vec) const
            return x * vec.x + y * vec.y + z * vec.z;
        // Static method to calculate and return a vector which is the cross product of two vectors
        // Note: The cross product is simply a vector which is perpendicular to the plane formed by
        // the first two vectors. Think of a desk like the one your laptop or keyboard is sitting on.
        // If you put one pencil pointing directly away from you, and then another pencil pointing to the
        // right so they form a "L" shape, the vector perpendicular to the plane made by these two pencils
        // points directly upwards.
        // Whether the vector is perpendicularly pointing "up" or "down" depends on the "handedness" of the
        // coordinate system that you're using.
        // Further reading:
        // Usage example: Vec3<double> crossVect = Vec3<double>::crossProduct(vectorA, vectorB);
        static Vec3 crossProduct(const Vec3 &vec1, const Vec3 &vec2)
            return Vec3(vec1.y * vec2.z - vec1.z * vec2.y, vec1.z * vec2.x - vec1.x * vec2.z, vec1.x * vec2.y - vec1.y * vec2.x);
        // Easy adders
        void addX(T value) { x += value; }
        void addY(T value) { y += value; }
        void addZ(T value) { z += value; }
        // Method to return the distance between two vectors in 3D space
        // Note: This is accurate, but not especially fast - depending on your needs you might
        // like to use the Manhattan Distance instead:
        // There's a good discussion of it here:
        // The gist is, to find if we're within a given distance between two vectors you can use:
        // bool within3DManhattanDistance(Vec3 c1, Vec3 c2, float distance)
        // {
        //      float dx = abs(c2.x - c1.x);
        //      if (dx > distance) return false; // too far in x direction
        //      float dy = abs(c2.y - c1.y);
        //      if (dy > distance) return false; // too far in y direction
        //      float dz = abs(c2.z - c1.z);
        //      if (dz > distance) return false; // too far in z direction
        //      return true; // we're within the cube
        // }
        // Or to just calculate the straight Manhattan distance you could use:
        // float getManhattanDistance(Vec3 c1, Vec3 c2)
        // {
        //      float dx = abs(c2.x - c1.x);
        //      float dy = abs(c2.y - c1.y);
        //      float dz = abs(c2.z - c1.z);
        //      return dx+dy+dz;
        // }
        static T getDistance(const Vec3 &v1, const Vec3 &v2)
            T dx = v2.x - v1.x;
            T dy = v2.y - v1.y;
            T dz = v2.z - v1.z;
            return sqrt(dx * dx + dy * dy + dz * dz);
        // Method to display the vector so you can easily check the values
        void display()
            std::cout << "X: " << x << "\t Y: " << y << "\t Z: " << z << std::endl;
        // ------------ Overloaded operators ------------
        // Overloaded addition operator to add Vec3s together
        Vec3 operator+(const Vec3 &vector) const
            return Vec3<T>(x + vector.x, y + vector.y, z + vector.z);
        // Overloaded add and asssign operator to add Vec3s together
        void operator+=(const Vec3 &vector)
            x += vector.x;
            y += vector.y;
            z += vector.z;
        // Overloaded subtraction operator to subtract a Vec3 from another Vec3
        Vec3 operator-(const Vec3 &vector) const
            return Vec3<T>(x - vector.x, y - vector.y, z - vector.z);
        // Overloaded subtract and asssign operator to subtract a Vec3 from another Vec3
        void operator-=(const Vec3 &vector)
            x -= vector.x;
            y -= vector.y;
            z -= vector.z;
        // Overloaded multiplication operator to multiply two Vec3s together
        Vec3 operator*(const Vec3 &vector) const
            return Vec3<T>(x * vector.x, y * vector.y, z * vector.z);
        // Overloaded multiply operator to multiply a vector by a scalar
        Vec3 operator*(const T &value) const
            return Vec3<T>(x * value, y * value, z * value);
        // Overloaded multiply and assign operator to multiply a vector by a scalar
        void operator*=(const T &value)
            x *= value;
            y *= value;
            z *= value;
        // Overloaded multiply operator to multiply a vector by a scalar
        Vec3 operator/(const T &value) const
            return Vec3<T>(x / value, y / value, z / value);
        // Overloaded multiply and assign operator to multiply a vector by a scalar
        void operator/=(const T &value)
            x /= value;
            y /= value;
            z /= value;

Next post, we’re going to be implementing a FPS-style camera class using this Vec3 class to move around a 3D scene – and because of all our hard work in getting this vector class together, it’s going to make moving the camera around a doddle =D