Thread: Helpppp meeeee, I am stuckkkk and probably going to get violent any minute!!!!

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this java method combines the two equations and gives me a logic error and ultimately the wrong answer... so it's combining the two equations into "7x + 9y -14 = 0" , which evaluates to x=2 and y=0. This does not hold true for each individual equation

I don't think I could offer much help at the moment but first of all, your title kind of turns people off from helping you. We understand that you're confused about your problem but it doesn't make you different from anyone else. Anyways, to begin addressing your problem, you're confused about how to create a java program that will solve a two-variable equation. From a quick glance at your code, I'm not sure if you understand what you programmed.

From what I see, you're printing an initial statement with the two equations but your loops are a bit confusing. Do you know what a for loop or while does?

I suggest reading The for Statement (The Java™ Tutorials > Learning the Java Language > Language Basics) and the while loop one in the same page.

Instructor says that I will need to use string methods to extract the coefficients so that I can solve the equation.

I think you should take your instructors advice and think about what string methods you've learned that will enable you to do this program.

I haven't reached to the loop section yet, well, we did a little bit of "for" and "while"... Here's what I got so far. But I am wondering if java will allow me to subtract two equations with two variables. Ex "5x+6y =9" - "6x+10y=3". However, I multiplied one of my equations so that when you subtract, it cancels out the "y" variable and you can solve for x.. then as you know, you would plug that into "Y' to solve for both equations... here is a copy of the code... it might not make sense, as I am in the middle of it:

<import java.util.Scanner;
 
 public class TwoVariable
	{
		public static void main(String[] args)
		 {
			double a,b,c,d,e,f,g,h,i,j,k,l,m
			Scanner ps = new Scanner(System.in);
 
			System.out.println("Enter an \'X\' and a \'Y\' integer for the following equations: \n5x+3y=9 2x+6y=5");
			System.out.println();
 
			a = 5x+3y=9;
			b = 2x+6y=5;
 
			c = a.indexOf(5);
			d = a.indexOf(3);
			e = a.indexOf(9):
			f = b.indexOf(2);
			g = b.indexOf(6);
 
			h = c*2;
			i = d*2;
			j = e*2;
 
			k = a.replace(c,h);
			l = a.replace(d,i);
			m = a.replace(e,j);
 
 
 
 
		 }
 
	}>

Well, looking at your current code, I think you might have some issues. Logically, I can understand what you're trying to do. You want a and b to be the variables holding the equation. C through g will hold the coefficients and h through j will multiply them by two. K through l will replace the coefficients, allowing you to cancel out the y variable. Now what I think you would have a problem with is using the a.indexOf(int) method.

I'd like you to try the following and see what happens:

c = a.indexOf(5);
System.out.println(c);
d = a.indexOf(3);
System.out.println(d);
e = a.indexOf(9):
f = b.indexOf(2);
g = b.indexOf(6);

and etc. for the variables that you're using. Are they doing what they want you to? If not, do you understand why they aren't?

so how do i take the index of an integer expression then? use parseInt, then covert it back to integer????? i do not want to use the substring method...yes, i guess my variables are doing what i want them to do. But I am stuck here... ok so after i multiply the coefficients, and replace them in the original "a" integer expression, I am getting ready to subtract expression "b" from expression "a"... how do I do that? and after that (when the "y"'s get subtracted out), how do I divide the right side of the equation by the coefficient of x, so that I can get the x value? Once I know how to do this, I can mimic this to get the "y" value. then solve this equation.

Well, I tried to compile the code you have now and it won't compile. There quite a bit of errors but I'd like to point out that you won't get the values that you're expecting. If you followed what I suggested, you'd find that your variables will return -1. Also, you declared all your variables as type double. I honestly think you should restart this and go step by step with a pencil and paper. I'd like to help you more but I'm about to go to bed, so I'd have to help you tomorrow. Hopefully someone else will take over and give you some support.

What I'll leave off with is this:

When you declared
a = 5x+3y=9;
b = 2x+6y=5;

This is not a working code. To make it how you want it to be, I'd suggest making a String, so that you'd be able to use String methods, *possibly* the charAt() method *wink* *wink*

Example: String a = "Expression here";
*note* The index starts from left to right, 0 to whatever size it is. Like "5x+3y=9" will have 5 at index 0, 3 at index 3, and 9 at index 6.

I hope that you'll be able to straighten this out and don't worry, coding is not easy and it takes time. It's good to make these mistakes early and to learn from them.

hello, I just bumped to your thread, and did a fast read... I dont know if i could give a help and I'm not sure if this would be a good suggestion. but you could check with converting infix-to-postfix notation, to evaluate any expression easily, Its what im doing right now, and I found that instead of evaluating infix expression directly, its better to convert it to postfix first then later evaluate it...

ok guys, i got a little bit further. First here is the code:

< import java.util.Scanner;
 
 public class TwoVariable
	{
		public static void main(String[] args)
		 {
			double v,w,x,y,z,z1,z2,z3;
			int c,e,g,j,l,n,p,q,r,s,t,u;
			String a,b,d,f,h,i,k,m,o;
			Scanner ps = new Scanner(System.in);
 
 
			// Two equations in strigns
			a = "5x+3y+9 ==0";
			b = "2x+6y+5==0";
 
			//Take index of each coefficients 
			c = a.indexOf("5");
			d = a.substring(c, c+1);
 
			e = a.indexOf("3");
			f = a.substring(e, e+1);
 
			g = a.indexOf("9");
			i = a.substring(g, g+1);
 
			j = b.indexOf("2");
			k = b.substring(j, j+1);
 
			l = b.indexOf("6");
			m = b.substring(l, l+1);
 
			n = b.indexOf("5");
			o = b.substring(n, n+1);
 
			// Parse the integer so I can apply math
			p = Integer.parseInt(d);
			q = Integer.parseInt(f);
			r = Integer.parseInt(i);
			s = Integer.parseInt(k);
			t = Integer.parseInt(m);
			u = Integer.parseInt(o);
 
			//Multiply the coefficients in the first equation
			v = p*2;
			w = q*2;
			x = r*2;
 
			//Subtract second coefficient from first coefficient
			y = v-s;  //8
			z = w-t; //0
			z1 = x-u; //13
 
			//Evaluate x
			z2 = y/z1; 
 
			//Evaluate y
			z3 = (-5 - (2*z2))/6; // y
 
			//Print out solutions
			System.out.println("The solution for \'X\' and \'Y\' are: " +z2 + " " + z3);
 
 
		 }
 
	}>

This code only codes one set of equations. I know I have to repeat it for another set of equations to fulfill my assignment... But here is my assignment

 
<Write a java program that receives two Strings as arguments (like "2x+3y=8" and "3x+4y=11"), and solve the two-variable equation and prints the solution of the equation on the screen. Name your class TwoVariable. You will need to use String methods in order to extract the equations coefficients in order to solve the equations
 
    Variable names always will be "x" and "y".
    Coefficients can be any number from the range 1 to 99 (when the coefficent is 1 we will write it and won't omit it).
    There will be no extra space inside the equations, and two equations are separated using one space character.
 
Sample input:
 
    java TwoVariable 2x+3y=8 3x+4y=11
    java TwoVariable 22x+20y=-2 1x+2y=1
 
Sample output:
 
    x=1, y=2
    x=-1, y=1
 
    Submit your java source to the submit server.
    Assignment tag for this assignment is ass02.
    Do not forget to add the proper header (as instructed at the bottom of this page) at the beginning of your program.
>

Basically I have to enter two sets of equations at the command prompt (yea it's stupid). But for example, after I compile it, I would type "java TwoVariables n5x+3y=9 2x+6y=5" . And when I click enter, the command prompt will display the solution. Then I will have to make another equation with another set of solutions. So this program will have two different equations, but whatever one I type, will display the solution (at least that's how I interpret this f**ker's instructions). One person told me that I will need to use the args[] but I don't know how to do that...

zaphod, i just read what you typed. but is my way similar to yours? and after I type it after it compiles, say, "java TwoVariable 5x+3y=9 2x+6y=5", how do I get it to print out the solution? I can change the class to "TwoVariable 5x+3y=0 2x+6y=5", but this doesn't work, because I'm supposed to have to different sets of equations in the program... and if I type "java TwoVariables 5x+3y=9 2x+6y=5" it prints one results, and if I type "java TwoVariables "8x+7y=6 3x+8y=9", it prints another result. At least that's how I interpret the instructions above. Some guy was mentioning using args[] , and reading the input at the command prompt...

I didn't show how to extract coefficients from strings. but I gave some hints, as have other helpers.

If you had given the exact input specification at first, my previous post could have been a little less wordy. (That would have been a blessing, right?)

Anyhow...

Here's the assignment in a nutshell:

You are going to solve a system of two equations and two unknowns.
The user enters two strings. Each string represents one of the equations. The form of the strings is in your assignment.

Get it? The program solves one problem. The user gives the equation coefficients and other values in a particular form. It takes two strings to specify values for the two equations. The program solves this system for for x and y. That's one solution. A value of x and a value of y.


To solve the equations, you must first extract the four left-hand side coefficients and the two right-hand side values from the two user input strings.

I called the variables a, b, c, d, e, f. (Which conveniently corresponds to the variables in the Wikipedia article that I linked to.)

Now, once you get the six numeric values for those variables, you solve the system for x and y. I indicated one way. The complete Cramer's rule formulas for two equations and two unknowns are in the Wikipedia reference that I gave you. If you don't want to use those expressions, do it any way that you want to. Well, do it any way that is mathematically correct (and, in particular that gives the correct answers that we so carefully worked out ahead of time).

Bottom line: There are lots of ways to write a program that solves two equations and two unknowns, but if your program reads input values correctly and gives incorrect answers, then it the algorithm is wrong. That's why I always (yes always) start with a problem for which I know the answer. (But I said that already.) Period. Full stop.

Some variation of Gauss reduction and back substitution using matrix variables is the most common way that "real" numerical analysts do it, especially for systems with more than a couple of equations. You can look them up.

Anyhow...

Here is my vision of a run. User input is black, program output is blue.


Enter the first equation in the form ax+by=e: 3x+3y=9
Enter the second equation in the form cx+dy=f: 4x+6y=11
Here are the equations to be solved:
3 * x + 3 * y = 9
4 * x + 6 * y = 11

Here is the solution:
x = 3.500000, y = -0.500000


The program skeleton looks like this:

public class Z {
    public static void main(String [] args)
    {
        int a, b, c, d, e, f;
        double x, y;
        Scanner keyboard = new Scanner(System.in);
        System.out.printf("Enter the first equation in the form ax+by=e: ");
        String first = keyboard.nextLine();
        System.out.printf("Enter the second equation in the form cx+dy=f: ");
        String first = keyboard.nextLine();
        //
        // Your code goes here:
        //   Extract values of a, b, e from the first  String
        //   Extract values of c, d, f from the second String
        //
        //
 
        // Print out the equations so that we know the values
        // were read in correctly.
        //
        System.out.println("Here are the equations to be solved:");
        System.out.printf("%2d * x + %2d * y = %2d\n", a, b, e);
        System.out.printf("%2d * x + %2d * y = %2d\n", c, d, f);
        //
        // Your code goes here:
        //   Calculate x and y from the values of a, b, c, d, e, f
        //
        //
 
 
        //
        // Now print out the answer.
        System.out.println("Here is the solution:");
        System.out.printf("x = %f, y = %f\n", x, y);
    }
}

While debugging, you can just set the values of the Strings internally instead of reading from user input, so that you won't have to type the blasted stuff each and every stinking time that you run it while correcting bugs.

public class Z {
    public static void main(String [] args)
    {
        int a, b, c, d, e, f;
        double x, y;
        // Comment out user input while debugging
        //Scanner keyboard = new Scanner(System.in);
        //System.out.printf("Enter the first equation in the form ax+by=e: ");
        //String first = keyboard.nextLine();
        //System.out.printf("Enter the second equation in the form cx+dy=f: ");
        //String first = keyboard.nextLine();
 
 
        // While debugging, just use fixed Strings instead of user input
        //
        String first = "3x+3y=9";
        String second = "4x+6y=11";
        //
        // Your code goes here:
        //   Extract values of a, b, e from the first  String
        //   Extract values of c, d, f from the second String
        //
        //
.
.
.

Cheers!

Z

problem here though.. the coefficients can be a one or two digit number. So the substring method will not catch the two digit coefficients since I am programming it for one. How do I get it to read one or two digits? use the .length method?

ok I got my code working, but it has many logic errors... the coefficients can only be one number. how can i get it to read if it's one of two numbers. The substring is only coded for one digit coefficients. Here is the code

<import java.util.Scanner;
 
 public class TwoVariable1
	{
		public static void main(String[] args)
		 {
			double g,i,j,k,l,m,n,o,p,z1,z2;
			int t,u,v,w,x,y;
			String a,b,c,d,e,f,a1,a2;
			Scanner ps = new Scanner(System.in);
 
			System.out.println("Enter two equations in the \'ax+by=c\' form, separating by a single space: ");
			a1 = ps.next();
			a2 = ps.nextLine();
 
 
			a = a1.substring(0,1);
			b = a1.substring(3,4);
			e = a1.substring(6,7);
			c = a2.substring(1,2);
			d = a2.substring(4,5);
			f = a2.substring(7,8);
 
			t = Integer.parseInt(a);
			u = Integer.parseInt(b);
			v = Integer.parseInt(e);
			w = Integer.parseInt(c);
			x = Integer.parseInt(d);
			y = Integer.parseInt(f);
 
			// Second coefficent multiply by all coefficents first equation
			g = x*t; //a
			i = x*u; //b
			j = x*v; //e
 
			// First coefficent multiply by all coeffiecents second equation
			k = u*w; //c
			l = u*x; //d
			m = u*y; //f
 
			n = g-k; //a-c
			o = j-m; //e-f
			p = o/n; //value of x
 
			z1 = m - (g*p);
			z2 = z1/i; //value of y
 
 
			//Print out solutions
			System.out.println("The solution for \'X\' and \'Y\' are: " +z1 + " " + z2);
 
 
		 }
 
	}>

I AMMMMMMMMMMM A GENIUSSSSSSSSSSSSSSSSS MUAHAHHA BUHAHAHA BUHAHA MUHAHAHHA.... I AM DA BEST PROGRAMMER EVER.

real talk, thanks for your help me...

unfortunately I have a lot of unfinished questions that I need to ask;
1. what is "printf" , and why not use "print" or "println"
2.

<
        int indx1 = s1.indexOf('x');
        System.out.printf("indx1 = %d\n", indx1);
        int indx2 = s2.indexOf('x');
        System.out.printf("indx2 = %d\n", indx2);>

What is the %d? i understand % is modula, and is used to find the remainder.

And thanks for the hints. Here's my code:

<import java.util.Scanner;
 
 public class TwoVariable
	{
		public static void main(String[] args)
		 {
			//Declare integers, doubles, strings
			double g,h,t,u,v,w,x,y;
			int a3,a4,a5,a6,a7,a8,a9,a10,a11,a12;
			String a,b,c,d,e,f,a1,a2;
			Scanner ps = new Scanner(System.in);
 
			//Take the user input
			System.out.println("Enter two equations in the \'ax+by=c\' form, separating by a single space: ");
			a1 = ps.next();
			a2 = ps.nextLine();
 
			//Get coefficients in 1st string
			a3 = a1.indexOf("x");
			a4 = a1.indexOf("+");
			a5 = a1.indexOf("y");
			a6 = a1.indexOf("=");
			a7 = a1.length();
 
			//Get coefficients in 2nd string
			a8  = a2.indexOf("x");
			a9  = a2.indexOf("+");
			a10 = a2.indexOf("y");
			a11 = a2.indexOf("=");
			a12 = a2.length();
 
 
			//Take substring of all coefficients
			a = a1.substring(0,a3); //a (ax+by=e cx+dy=f)
			b = a1.substring(a4+1,a5); //b
			e = a1.substring(a6+1,a7); //e
 
			c = a2.substring(1,a8); //c
			d = a2.substring(a9+1,a10); //d
			f = a2.substring(a11+1,a12); //f
 
			//Parse all integers
			t = Double.parseDouble(a);
			u = Double.parseDouble(b);
			v = Double.parseDouble(e);
			w = Double.parseDouble(c);
			x = Double.parseDouble(d);
			y = Double.parseDouble(f);
 
			// Equations for y and x
			g = ((t*y)-(w*v))/((t*x)-(u*w)); //y     y=(af-ce)/(ad-bc)
		    h = ((v - (u*g)))/t; //x                 x=(e-by)/a
 
			//Print out solutions
			System.out.println("The solution for \'X\' and \'Y\' are: " + h + " " + g);
 
		}
	}
		>

Does that look solid to you? And is there an easier way to do this? I barely beat my deadline of 11:59, so any changes that you recommend will probably be for my personal learning

also, if you look at string 'a1", i used ps.next() instead of ps.nextLine()... I thought nextLine was unnecessary. Why did you use nextLine for both of yours in your example?