Quadratic Equation problems

This is a discussion on Quadratic Equation problems within the C++ Programming forums, part of the General Programming Boards category; I have to write a programme in C++ Builder 4 that will calculate all roots of a quadratic equation, and ...

  1. #1
    Registered User
    Join Date
    Mar 2006
    Posts
    1

    Quadratic Equation problems

    I have to write a programme in C++ Builder 4 that will calculate all roots of a quadratic equation, and am struggling to produce complex roots, it gives a message of
    the first complex root = -1.5+ i+.AN
    the second complex root = -1.5- i+.AN

    it also gives the worng answer when the roots are equal, giving one root as 0.

    this is the code

    #include <stdio.h>
    #include <iostream.h>
    #include <conio.h>
    #include <math.h>
    #include <complex.h>

    int main ()
    {

    float a,b,c,d,root1, root2, real1, real2,imag1,imag2; //define variables



    Code:
    cout<<"p(x)= ax^2 + bx + c = 0 " << endl;          //define the equation to be solved
    
    cout<<"    " << endl;
    
    cout<<"enter the 3 parameters a, b, c: " << endl;            //user enters 3 parameters to be calculated
    cin >>a>>b>>c;
    
    {
    if (a==0)                                  //if a is 0 then this is not a quadratic equation form
    cout << "a cannot be 0 in a quadratic equation" << endl;
             }
    
    
    if(a==0){
    if (b==0)                  //if a and b are 0 this is not a quadratic equation
    cout<< "both a and b cannot be 0 in ax^2 + bx + c =0" << endl;
    
     }
    
    
    
    
    {
    d = b*b-4*a*c;
    {if (d>0)                        // if d>0 this will give real roots
             {
    root1= (-b +(sqrt(b*b-4*a*c)))/(2*a);
    root2= (-b-(sqrt(b*b-4*a*c))) / (2*a);
    
    cout <<"the first root = " << root1 << endl;       //outputs the first root
    cout <<"the second root = " << root2 << endl;         //outputs the second root
       }
              }
       {
    if(d<0)                          // if d<0 this will give complex roots
    
    real1= ((-b)/(2*a)) ;
    real2= ((-b)/(2*a)) ;
    imag1= (((sqrt(b*b-4*a*c))/(2*a))) ;
    imag2= (((sqrt(b*b-4*a*c))/(2*a)))  ;
    
    cout <<"the first complex root = " << real1 <<"+ i" << imag1 << endl;       //outputs the first root
    cout <<"the second complex root = " << real2 << "- i" << imag2 << endl;         //outputs the second root
    
    
    }
     }
    
    getch();                             // keeps the window open until a key is pressed
    return 0;
    }

    could anyone help me with this, i'm not very good with c++. thanks

  2. #2
    Registered User
    Join Date
    Feb 2006
    Posts
    65
    > imag1= (((sqrt(b*b-4*a*c))/(2*a))) ;

    Here bē-4ac is negative, so you are calculating the square root of a negative number. You need:

    imag1= sqrt(-d)/(2*a) ;

    (You had already calculated d = b*b-4*a*c, there is no need to repeat that calculation.)

    You don't seem to handle the situation when d is equal to 0, that is, when there is only one root, so I don't see how the code could produce bad output (or any output at all).

    If you are interested in producing more accurate results please read this:
    http://en.wikipedia.org/wiki/Loss_of_significance

  3. #3
    Algorithm Dissector iMalc's Avatar
    Join Date
    Dec 2005
    Location
    New Zealand
    Posts
    6,288
    Most excellent advice!

    Do what he says, and read the link too!

Popular pages Recent additions subscribe to a feed

Similar Threads

  1. quadratic equation
    By nynicue in forum C Programming
    Replies: 5
    Last Post: 12-26-2008, 04:57 PM
  2. quadratic equation cpp program problems
    By faluiretoread in forum C++ Programming
    Replies: 2
    Last Post: 11-09-2008, 02:22 PM
  3. Quadratic Equation
    By Lucid15 in forum C Programming
    Replies: 20
    Last Post: 09-16-2008, 05:59 PM
  4. Replies: 8
    Last Post: 03-10-2008, 07:15 PM
  5. Quadratic Equation Program
    By Ambizzy in forum C Programming
    Replies: 4
    Last Post: 02-19-2002, 08:21 PM

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21