program for real roots of quadratic equation

Hi

I'm trying to write a simple console program to find the real roots of a quadratic equation. It's part of self-study. I have a rough idea which you can have a look on below. Please help me to finish it and correct the errors. I'm highly grateful for your guidance and help. Thanks a lot.

I hope this general pseudocode is correct:

Code:

Read the coefficients “a” and “b”, and constant “c” of ax^2 + bx + c = 0
Let Discriminant D = b^2 - 4ac
   IF D >= 0 THEN
        Real root R1 = (-b + sqrt(D))/(2a)
        Real root R2 = (-b - sqrt(D))/(2a)
   ELSE
        Print “no real solutions”
   ENDIF
   Print R1 and R2
Stop

Here is my 'rough' and full of errors incomplete implementation:

Code:

#include <iostream>
using namespace std;
int main ()
{
        float a, b, c, R1, R2 //variables and constants used;
        cout<< "Enter a, b, c of the equation ax^2+bx+c==0=";
        cout<< "Enter a=";
        cin>> a;
        cout<< "Enter b=";
        cin>> b;
        cout<< "Enter c="
        cin>> c;
        D = b^2 - 4ac;
        if (D>=0)
           {
           R1 = (-b + sqrt(D))/(2a)
           R2 = (-b - sqrt(D))/(2a)
              else
                no real roots exist
            }

instead of

Code:

R1 = (-b + sqrt(D))/(2a)

try

Code:

R1 = (-b + sqrt(D))/(2 * a)

Edit: D = b^2 - 4ac; is wrong look up the pow/power function from math.h
http://www.cplusplus.com/reference/clibrary/cmath/

Originally Posted by stahta01

instead of

Code:

R1 = (-b + sqrt(D))/(2a)

try

Code:

R1 = (-b + sqrt(D))/(2 * a)

Edit: D = b^2 - 4ac; is wrong look up the pow/power function from math.h
cmath (math.h) - C++ Reference

Hi stahta01

Thanks for the reply. Do you mean I have to include header file "#include math.h"? I was told the header files with ".h" extensions are outdated.

On the webpage you referred me to I found this:

Code:

/* pow example */
#include <stdio.h>
#include <math.h>

int main ()
{
  printf ("7 ^ 3 = %lf\n", pow (7,3));
  printf ("4.73 ^ 12 = %lf\n", pow (4.73,12));
  printf ("32.01 ^ 1.54 = %lf\n", pow (32.01,1.54));
  return 0;
}

I don't understand the bold parts. And could you please complete that code in my first post?

Regards
Jackson

Why the heck would you use pow() to implement the squaring of a number? Just multiply it with itself and be done with it.

Originally Posted by jackson6612

Hi stahta01

Thanks for the reply. Do you mean I have to include header file "#include math.h"? I was told the header files with ".h" extensions are outdated.

On the webpage you referred me to I found this:

Code:

/* pow example */
#include <stdio.h>
#include <math.h>

int main ()
{
  printf ("7 ^ 3 = %lf\n", pow (7,3));
  printf ("4.73 ^ 12 = %lf\n", pow (4.73,12));
  printf ("32.01 ^ 1.54 = %lf\n", pow (32.01,1.54));
  return 0;
}

I don't understand the bold parts. And could you please complete that code in my first post?

Regards
Jackson

Never mind the printf nonsense, just use std::cout like is in the code you posted before (although printf is good background knowledge, you should avoid it in C++).

For standard headers, instead of #include <abc.h>, in C++, you write #include <cabc>. So use #include <cmath> to use std::sqrt.

Also, Brewbuck is right about using pow being silly here, to do b^2, just write b * b.

Aside from that you're missing several semicolons, "else" should occur after the closing brace for the "if", and while you're at it you should fix up your indentation a bit so it looks more like this (I've deliberately only adjusted indentation without fixing other problems here):

Code:

#include <iostream>
using namespace std;
int main ()
{
    float a, b, c, R1, R2 //variables and constants used;
    cout<< "Enter a, b, c of the equation ax^2+bx+c==0=";
    cout<< "Enter a=";
    cin>> a;
    cout<< "Enter b=";
    cin>> b;
    cout<< "Enter c="
    cin>> c;
    D = b^2 - 4ac;
    if (D>=0)
    {
        R1 = (-b + sqrt(D))/(2a)
        R2 = (-b - sqrt(D))/(2a)
        else
            no real roots exist
    }

D is declared no where.

Thanks everyone. Below is the improved code. I humbly request you to correct mistakes and let me have a running program ans ask some questions related to it. Thanks.

Code:

#include <iostream>
# include <cmath>
using namespace std;
int main ()

{
    clrscr()
    float a, b, c, R1, R2 //variables and constants used;
    cout<< "Enter a, b, c of the equation ax^2+bx+c==0=";
    cout<< "Enter a=";
    cin>> a;
    cout<< "Enter b=";
    cin>> b;
    cout<< "Enter c=";
    cin>> c;
    float D = b*b - 4ac;
    if (D>=0)
    {
        cout<< "R1 is="  (-b + sqrt(D))/(2*a);
        cout<< "R2 = (-b - sqrt(D))/(2*a);
     }
      else
      cout<< "no real roots exist"<<endl;
      getch()
}

clrscr() is not standard, it should be avoided.

A semicolon is still missing here:

Code:

    float a, b, c, R1, R2; //variables and constants used;

and another one here:

Code:

      getch();

I don't know what that getch() is doing there (are you running the program by double-clicking a .exe file on windows, and want to stop the screen from closing after the calculation?), I think that's a C function, use cin.get() if you must.

You're also missing * between 4, a, c on this line:

Code:

    float D = b*b - 4*a*c;

And you're missing a "<<" on this line:

Code:

        cout<< "R1 is=" << (-b + sqrt(D))/(2*a);

Now a couple of style points:

Mainly, indentation, when using spaces instead of tabs (which I recommend), choose a set number of spaces to use for each indentation level, and stick with it (I recommend sets of 4 spaces, which it appears you are mostly using). The lines in red are indented incorrectly:

Code:

    if (D>=0)
    {
        cout<< "R1 is="  (-b + sqrt(D))/(2*a);
        cout<< "R2 = (-b - sqrt(D))/(2*a);
     }
      else
      cout<< "no real roots exist"<<endl;
      getch()

Also with this code, I don't have a problem with omitting the braces for single statements in if/else blocks, but if any block in the if/else sequence requires braces, I say they should all have braces:

Code:

    if (D>=0)
    {
        cout<< "R1 is="  (-b + sqrt(D))/(2*a);
        cout<< "R2 = (-b - sqrt(D))/(2*a);
     }
      else
      {
      cout<< "no real roots exist"<<endl;
      }
      getch()

I think this looks ugly:

Code:

    cout<< "Enter b=";
    cin>> b;

I would put spaces on both sides of "<<" and ">>":

Code:

    cout << "Enter b=";
    cin >> b;

In fact I would do that for arithmetic operators as well, but that's getting down to my personal taste:

Code:

    float D = b*b - 4*a*c; // bad
    float D = b * b - 4 * a * c; // good

Mozza314: I genuinely thank you for all you help and guidance. It is really kind of you. I would stick to your advice on the style from this point onward.

I do still have a couple of questions to ask about the above code which I would ask soon. But here is a BIG question:

When I have entered the values of "a", "b" and "c" it simply closes. It doesn't even display "R1" and "R2". What can I do to stop this and make this program functional? Please help me. I'm using Dev-C++. Thanks.

Code:

#include <iostream>
# include <cmath>
#include <stdlib.h>

using namespace std;
int main ()

{
    float a, b, c, R1, R2; //variables and constants used
    cout << "Enter a, b, c of the equation ax^2 + bx + c = 0" << endl;
    cout << "Enter a = ";
    cin >> a;
    cout << "Enter b = ";
    cin >> b;
    cout << "Enter c = ";
    cin >> c;
    float D = b*b - 4*a*c;
    if (D >= 0)
    {
        cout << "R1 is = " << ( -b + sqrt(D) )/( 2*a );
        cout << "R2 is = " << ( -b - sqrt(D) )/( 2*a );
    }
    else
    {
    cout << "no real roots exist" << endl;
    }
    return 0;
}

The concern is not that the program does not display the output. It is that the program terminates, and the operating system - in its quest to be helpful and clean things up - closes the window showing the output before you can see it.

The conventional way to stop the program from terminating immediately is to add additional code at the end of main (before the return 0) that waits for additional user input. There are many opinions on the "right" way to do this that I won't buy into, but one way is;

Code:

   std::cin.ignore ( std::numeric_limits<streamsize>::max(), '\n' );
   std::cin.get();

(assuming <limits> has been #include'd).

Originally Posted by jackson6612

Mozza314: I genuinely thank you for all you help and guidance. It is really kind of you. I would stick to your advice on the style from this point onward.

I do still have a couple of questions to ask about the above code which I would ask soon. But here is a BIG question:

When I have entered the values of "a", "b" and "c" it simply closes. It doesn't even display "R1" and "R2". What can I do to stop this and make this program functional? Please help me. I'm using Dev-C++. Thanks.

You're welcome. I agree with grumpy's answer to your question.

I used "system("PAUSE")" at the end and it doesn't close now.

Code:

#include <iostream>
# include <cmath>
#include <stdlib.h>

using namespace std;
int main ()

{
    float a, b, c, R1, R2; //variables and constants used
    cout << "Enter a, b, c of the equation ax^2 + bx + c = 0" << endl;
    cout << "Enter a = ";
    cin >> a;
    cout << "Enter b = ";
    cin >> b;
    cout << "Enter c = ";
    cin >> c;
    float D = b*b - 4*a*c;
    if (D >= 0)
    {
        cout << "R1 is = " << ( -b + sqrt(D) )/( 2*a ) << endl;
        cout << "R2 is = " << ( -b - sqrt(D) )/( 2*a ) << endl;
    }
    else
    {
    cout << "no real roots exist" << endl;
    }
    
    system("PAUSE");
}

But now your code is non-standard. system("PAUSE") is windows specific.

Seeing as the problem being addressed is Windows specific, it's probably a fair tradeoff.

Originally Posted by Clairvoyant1332

Seeing as the problem being addressed is Windows specific, it's probably a fair tradeoff.

What problem is windows specific? I don't see how finding the roots of a quadratic equation is windows specific.