Go Premium for a chance to win a PS4. Enter to Win

x
?
Solved

Need help with source code; Euler's method

Posted on 1997-04-04
3
Medium Priority
?
498 Views
Last Modified: 2010-04-10
The following is the beginnings of a program which I am currently attempting to write.  Its purpose is to do an numerical approzimation for a linear first order differential equation using euler's method.  It needs a lot of work.  The problem is mainly that it only goes through one step of the analysis.  Of course there may be several more problems.  If anyone has an idea on how to get it to work, I would appreciate a response.






// Program -- Euleran's
// Uses Euler's meathod to approximate the solution of the differential
// equation y'=f(x,y) for a given interval.  

//header files

#include <math.h>                  
#include <iostream.h>            
#include <iomanip.h>            
float euler (float);

float x,y,f,equation,h;
int n;



void main()
{

cout.setf(ios::fixed, ios::floatfield);            //set up floating point
cout.setf(ios::showpoint);                              //  output format


           
cout << "Enter the initial value for x: " << endl;
cin >> x;
cout << "Enter the initial value for y: " << endl;
cin >> y;
cout << "Enter the final value for x: " << endl;
cin >> f;                  

cout << "Enter the number of steps for which you wish the " << endl
        <<      "function to be analyzed: " << endl;
cin >> n;
cout << "Enter the first order differential equation which you " << endl;
cout << "wish to analyze:" << endl;
cin >> equation;
h=(f-x)/n;

euler (equation);
cout << x << "      " << y << endl;
}


float euler (float equation)
{                
for(int i=1;i<=n;i++);
      {          

      y=y+h*equation;
      x=x+h;
      return x,y;
      }
}



                        Thank you,
                        Ben Key
0
Comment
Question by:Ben Key
  • 2
3 Comments
 

Author Comment

by:Ben Key
ID: 1162565
I beleive the problem may be as simple as incorrect syntax in
my for loop.  It may also be improper calling of the function
euler.
0
 

Accepted Solution

by:
aleroy earned 150 total points
ID: 1162566
First of all, entering an equation in a float is not right way
to evaluate this equation.
For that, you must use (or program) a parser, wich is certainly
not a piece of cake.
To debug your program, I've inserted the equation (y'=x+y) in
the right place directly. If you don't intend to get a expression parse working, you can do in this way (but you will have to recompile for each different equation you plan to analyse.

All the others problems where syntaxical.
Here is the revised code :
#include <math.h>
#include <iostream.h>
#include <iomanip.h>

void euler ();
float x,y,f,equation,h;
int n;

void main()
{
cout.setf(ios::fixed, ios::floatfield); //set up floating point
cout.setf(ios::showpoint); // output format

cout << "Enter the initial value for x: " << endl;
cin >> x;
cout << "Enter the initial value for y: " << endl;
cin >> y;
cout << "Enter the final value for x: " << endl;
cin >> f;
cout << "Enter the number of steps for which you wish the " << endl<< "function to be analyzed: " << endl;
cin >> n;
h=(f-x)/n;
euler();
cout << x << " " << y << endl;
}
void euler ()
{
for(int i=0;i<n;i++) //there was a ; here, wich is bad ...
{
y+=h*(x+y);
x+=h;
}
}

That's about it.

You should avoid global variable and use parameters of function
correctly (I haven't done it because there was no real need, it's
just for the beauty of the code)






0
 

Expert Comment

by:aleroy
ID: 1162567
What is a parser ?

Suppose your equation is in a character string.
For your specific problem, a parser is a piece of program
that will analyze the string, recognize the variables and the
operator (and parenthesis and so on).
It will so create a data structure (generally a tree) that
will allow your equation to be evaluated for given value of variables.

As most language like C and C++ are compiled, once they run,
their variables are nothing more than memory adresses without name. This is the thing that prevent from doing like you've done
in the question.
If you want easy evaluations of runtime entered equations, you
will have to learn a language such as miranda or LISP.

A good book to read about data structures (not specially trees or parsing) :
E. Horowitz, S. Sahni, and D. Mehta, Fundamentals of Data Structures in C++, W.H.
       Freeman & Co, 1995.



0

Featured Post

New feature and membership benefit!

New feature! Upgrade and increase expert visibility of your issues with Priority Questions.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Written by John Humphreys C++ Threading and the POSIX Library This article will cover the basic information that you need to know in order to make use of the POSIX threading library available for C and C++ on UNIX and most Linux systems.   [s…
Introduction This article is the first in a series of articles about the C/C++ Visual Studio Express debugger.  It provides a quick start guide in using the debugger. Part 2 focuses on additional topics in breakpoints.  Lastly, Part 3 focuses on th…
The viewer will learn how to use the return statement in functions in C++. The video will also teach the user how to pass data to a function and have the function return data back for further processing.
The viewer will learn additional member functions of the vector class. Specifically, the capacity and swap member functions will be introduced.
Suggested Courses

824 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question