How to reverse an expression?

[lalili]

Hello,
I'm using Symja library to evaluate expressions as below.
For example, my formula is F3=(F1+F2)*0.2
I need to reverse it to calculate F1 (assuming F2 and F3 are given). So as a result I need to get to formula (F3/0.2)-F2
Would Symja be able to help to get to the formula of F1?

        ExprEvaluator util = new ExprEvaluator();
        util.defineVariable("F1", 10);
        util.defineVariable("F2", 20);
        IExpr exp = util.evaluate("(F1+F2)*0.2");

[axkr]

Don't know, if I understand exactly, but the Solve function can probably be used:

import org.matheclipse.core.eval.ExprEvaluator;
import org.matheclipse.core.interfaces.IExpr;
import org.matheclipse.parser.client.SyntaxError;
import org.matheclipse.parser.client.math.MathException;

public class GithubIssue294 {
  public static void main(String[] args) {
    try {
      ExprEvaluator util = new ExprEvaluator();
      IExpr f1 = util.eval("f1=10");
      // print: 10
      System.out.println(f1.toString());
      IExpr f2 = util.eval("f2=20");
      // print: 20
      System.out.println(f2.toString());
      IExpr result = util.eval("(f1+f2)*0.2");
      // print: 6.0
      System.out.println(result.toString());

      IExpr exp = util.eval("Solve(f3==(x+f2)*0.2,x)");
      // print: {{x->-20.0+5.0*f3}}
      System.out.println(exp.toString());

    } catch (SyntaxError e) {
      System.out.println(e.getMessage());
    } catch (MathException me) {
      System.out.println(me.getMessage());
    } catch (Exception e) {
      e.printStackTrace();
    } catch (final StackOverflowError soe) {
      System.out.println(soe.getMessage());
    } catch (final OutOfMemoryError oome) {
      System.out.println(oome.getMessage());
    }
  }
}

[lalili]

Thank you so much, that would work.
But is there a way to get a numeric value of eval? I get "x -> ..." format, and I would need only the value of X.
I would expect exp.evalDouble() to work, but it fails (the last line)

    ExprEvaluator util = new ExprEvaluator();
     IExpr f1 = util.eval("f1=10");
     // print: 10
     IExpr f2 = util.eval("f2=20");
     // print: 20
     IExpr result = util.eval("(f1+f2)*0.2");
     // print: 6.0
     IExpr exp = util.eval("Solve(f3==(x+f2)*0.2,x)");
     exp.evalDouble();

[lalili]

Is there an elegant way to get the value of X from the Solve expression? Lets say I do it like this:

		ExprEvaluator util = new ExprEvaluator();
		util.defineVariable("F3", 200);
		util.defineVariable("F1", 300);
		util.defineVariable("F4", 300);

		IExpr exp2 = util.evaluate("Solve(F1==(X+F3+F4)/2, X)");
		System.out.println(exp2.toString());//{{X->100.0}}

[TiagoCavalcante]

@lalili you could do this instead:

IExpr exp = util.eval("x /. Solve(f3==(x+f2)*0.2,x)[[1]]");
exp.evalDouble();

The [[1]] gets the 1st element of the list (it's a list because the variable can have multiple values).
The x /. part replaces x with its value from the rule ({x -> number}).

The solution is a rule because you can solve for multiple variables, resulting in a rule like {x -> 2, y -> 4}.

[lalili]

Thanks, I've got this working:
util.evaluate("Solve(F3==(X+F2)*0.2,X)[[1]]").getAt(1).getAt(2).evalDouble()

the x /. part failed with evalDouble. Maybe because I'm using an old version of the library..