Help with Java

[mittacc]

Hey!
I'm trying to create a program that calculates the vertex coordinates of the maximi/minimivalue in a quadratic equation with the "pq-formula" and I've run into a problem.

Here comes my code!

//Det här programmet ska räkna ut koordinaterna för vertex i en andragradsekvation
//(där det är givet att formeln står i "pq-kompatibelt läge")
import java.util.*;
import static java.lang.Math.*;
public class Vertex

{ public static void main(String [] args)
{
Scanner Skanna = new Scanner(System.in);
double p = Skanna.nextDouble();
double q = Skanna.nextDouble();

// x1=(-p/2)+sqrt((Math.pow((p/2),2) - q))
System.out.println("x1 = "+ (-p/2)+sqrt((Math.pow((p/2),2))-q));
System.out.println("x2 = "+ (-p/2)-sqrt((Math.pow((p/2),2))-q));
}

}

-------------------------------------------------------------------------------------------------------------
Error message:

D:\Dropbox\Programmering\Vertex.java:15: error: bad operand types for binary operator '-'
System.out.println("x2 = "+ (-p/2)-sqrt((Math.pow((p/2),2))-q));
^
first type: String
second type: double
1 error

Tool completed with exit code 1
--------------------------------------------------------------------------------------------------------------

So I understand that it somehow thinks that I'm trying to write a text, but why can't I subtract the sqrt and how do I correct it?

[Kaawumba]

You should try harder before posting this to a list. Anyways, break the println into two:

double x1 = (-p/2)+sqrt((Math.pow((p/2),2))-q);
System.out.println("x1 = " + x1);

[teamedward]

The error is being thrown because the program is attempting to concatenate the separate segments together, via '+'. This actually means that both of your print statements will be incorrect. In the case of the error the '-' is viewed as an attempt to concatenate the String, but is not a valid operator. To remedy this, you could surround the equation with an extra set of parenthesis. However, I would recommend doing the calculation separately and storing it as a variable (as recommended above). This will also make the code more readable.

[mittacc]

Kudos for the tips!

So, I've encountered a new problem which I've been stuck at for an hour, going back and forth. The problem is that I keep getting the wrong Y-coordinate for the maximi/minimivalue, everything else works like a charm.

------------------------------------------------------------------------------
//Det här programmet ska räkna ut koordinaterna för vertex i en andragradsekvation

import java.util.*;
import static java.lang.Math.*;
public class Vertex

{ public static void main(String [] args)
{
Scanner Skanna = new Scanner(System.in);
double a = Skanna.nextDouble();
double EntryP = Skanna.nextDouble();
double EntryQ = Skanna.nextDouble();
System.out.print (a+"x^2 + ");
System.out.print (EntryP+"x = ");
System.out.println (EntryQ);
double p = EntryP/a;
double q = (EntryQ/a);


// Räknar ut nollpunkterna
double sqrt= sqrt((Math.pow((p/2),2))-q);
double X1 = ((-p/2)+sqrt);
double X2 = ((-p/2)-sqrt);
System.out.println("X1 = "+ X1);
System.out.println("X2 = "+ X2);


//räknar ut symetrilinjens placering samt koordinaten för maximi/minimipunkten
double Xvertex = ((X1+X2)/2);
System.out.println("Symetrilinjens placering = "+ Xvertex);

double Yvertex = (a * Xvertex) + (EntryP * Xvertex) ;
if( (a>0 ) )
{
System.out.println( "Maximi-koordinat = (" + Xvertex + ";" + Yvertex + ")");
}
else
{
System.out.println( "Minimi-koordinat = (" + Xvertex + ";-" + Yvertex + ")");
}


}

}

[Kaawumba]

I'm not sure why you used this expression for Yvertex. If the expression for the parabola is y = a*x^2+b*x+c then Yvertex = a*Xvertex^2+b*Xvertex + c.

By the way, if you know calculus it can be shown that Xvertex = -b/2a. No need to mess around with the quadratic formula.

Also, an hour is not long to be working on a problem.

Kudos for the tips!


double Yvertex = (a * Xvertex) + (EntryP * Xvertex) ;
if( (a>0 ) )
{
System.out.println( "Maximi-koordinat = (" + Xvertex + ";" + Yvertex + ")");
}
else
{
System.out.println( "Minimi-koordinat = (" + Xvertex + ";-" + Yvertex + ")");
}


}

}