# Computation Contest #3 Results and Solution

in #programming3 years ago

## Solution

The problem of this contest was to solve the differential equation for the pendulum numerically:

I implemented an example solution in java which also plots a graph of the angle of the pendulum for the both solutions:

``````import javax.swing.JFrame;
import java.awt.Color;
import java.awt.Graphics;
import java.util.ArrayList;

public class main extends JFrame {
ArrayList<Double> valuesToGraph = new ArrayList<>();
public static int num = 0;
public main(double c, boolean visible) {
this.c = c;
num = 0;
setSize(1600, 800);
setUndecorated(true);
setVisible(visible);

long t0 = System.currentTimeMillis() + 20;
while(running) {
try{Thread.sleep(t0 - System.currentTimeMillis());} catch(Exception e) {e.printStackTrace();}
t0 += 20;
update();
}
// Make sure not to waste resources on not visible frames.
if(!visible)
dispose();
else {
for(int i = valuesToGraph.size(); i < 1600; i++) {
update(); // Make sure to draw the full image.
}
repaint();
}
}
double dt = 0.0001;
double phi = 0;
double omega = 2*Math.PI;
double c;
double g = 9.80665;
double l = 1;
boolean running = true;
public void update() {
for(int i = 0; i < 100; i++) {
// Perform one iteration of the Differential Equation
double alpha = -c*omega - g/l*Math.sin(phi);
omega += alpha*dt;
phi += omega*dt;
// Make sure phi stays within ±π to make the graph look better:
if(phi > Math.PI) {
phi -= Math.PI*2;
num++;
}
if(phi < -Math.PI) {
phi += Math.PI*2;
num++;
}

// Stop the program when the pendulum changes direction:
if(omega < 0) {
running = false;
}
}
}
public void paint(Graphics gr) {
// Paint it:
gr.fillRect(0, 0, 1600, 800);
gr.setColor(Color.WHITE);
for(int i = 1; i < valuesToGraph.size(); i++) {
gr.drawLine(i-1, 400-(int)(valuesToGraph.get(i-1)*400/Math.PI), i, 400-(int)(valuesToGraph.get(i)*400/Math.PI));
}
}

public static void main(String[] args) {
double c0 = 0.01;
new main(c0, true);
System.out.println("Number of swings around = " + num);
if(num == 3)
System.out.println("c₁ = " + c0); // A solution is already found.
if(num < 3) { // Slowly reduce c:
while(num != 3) {
c0 *= 0.9;
new main(c0, false);
}
new main(c0, true);
System.out.println("c₁ = " + c0);
}
if(num > 3) { // Slowly increase c:
while(num != 3) {
c0 *= 1.1;
new main(c0, false);
}
new main(c0, true);
System.out.println("c₁ = " + c0);
}
}
}
``````

This prints out:

``````Number of swings around = 3
c₁ = 0.0018530201888518425
``````

And plots the graph of the angle mod 2π, so a straight line means it swings once around the top(top → c = c₀, bottom → c = c₁):

This code prints the result 14.1332 which is not perfectly accurate even though I used 1000000 iterations in the final run.

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### List of participants with their entries:

NameSolution found
@kaeserotor0 and 0.00146484375Both are correct assuming g = 9.81 ms⁻² instead of the value I used above.

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## Winners:

Congratulations @kaeserotor. You won 6 SBI due to lack of competition!

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