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Adding Forward Euler in Java #387

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Added Forward Euler in Java
Wesley-Arrington Sep 20, 2018
a1ec91f
Merge branch 'master' into forwardEulerJava
leios Oct 4, 2018
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52 changes: 52 additions & 0 deletions contents/forward_euler_method/code/java/Euler.java
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Original file line number Diff line number Diff line change
@@ -0,0 +1,52 @@
public class ForwardEuler {

public static double[] append(double[] oldArray, double newItem) {
double[] newArray = new double[oldArray.length + 1];

for(int i = 0; i < oldArray.length; i++) {
newArray[i] = oldArray[i];
}
newArray[newArray.length - 1] = newItem;

return newArray;
}

public static double[] solveEuler(double timeStep, int n) {
double[] result = {1};

for(int i = 0; i < n; i++) {
result = append(result, result[i] - 3 * result[i] * timeStep);
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You're thinking way too complicated here. You could just make result an array with a length of n (double[] result = new double[n]), add the initial value of 1 (result[0] = 1) and then fill the array like this:

result[i] = result[i - 1] - 3 * result[i - 1] * timeStep;

The loop would of course have to start from 1 to skip the first element.

This way, you could drop the append method above. In addition to that, if I'd need an append method for an array, I'd use an ArrayList instead.

}

return result;
}

public static boolean checkResult(double[] result, double threshold, double timeStep) {
boolean isApprox = true;

for(int i = 0; i < result.length; i++) {
double solution = Math.pow(Math.E, -3 * i * timeStep);
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@Butt4cak3 Butt4cak3 Oct 6, 2018 •
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There's a method Math.exp(double a) which does exactly this.

double solution = Math.exp(-3 * i * timeStep);


if(Math.abs(result[i] - solution) > threshold) {
isApprox = false;
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You're missing a println before inside this block. If you look at the reference implementation (Julia), you'll see this:

if (abs(euler_result[i] - solution) > threshold)
    println(euler_result[i], solution)
    is_approx = false
end

}
}

return isApprox;
}

public static void main(String[] args) {
double timeStep = 0.01;
int n = 100;
double threshold = 0.01;
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You can make these variables as memebers of the class so that all methods can access them without needing to pass them as arguments

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@Butt4cak3 Butt4cak3 Oct 6, 2018 •
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I disagree. These should be passed as arguments.


double[] result = solveEuler(timeStep, n);
boolean isApprox = checkResult(result, threshold, timeStep);

if(isApprox) {
System.out.print("All values within threshold");
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Please use System.out.println here or otherwise add a newline to the output. I assume you're using Windows, so everything works as expected, but when I run this on Linux, my shell does this:

user@hostname:aaa> java ForwardEuler
All values within thresholduser@hostname:aaa>

instead of this:

user@hostname:aaa> java ForwardEuler
All values within threshold
user@hostname:aaa>

} else {
System.out.print("Value(s) not in threshold");
}
}
}
2 changes: 2 additions & 0 deletions contents/forward_euler_method/forward_euler_method.md
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Expand Up @@ -121,6 +121,8 @@ Full code for the visualization follows:
[import, lang:"matlab"](code/matlab/euler.m)
{% sample lang="swift" %}
[import, lang:"swift"](code/swift/euler.swift)
{% sample lang="java" %}
[import, lang:"java"](code/java/Euler.java)
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Don't forget to update the file name in case you rename the source file.

{% sample lang="f90" %}
[import, lang:"fortran"](code/fortran/euler.f90)
{% endmethod %}
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