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A C++ Quadratic equation solver

page last modified 2006-03-19

This program will solve quadratic equations.

You can copy and paste it into an editor, save it and compile from there.I have compiled and run this on a RedHat GNU/Linux machine using kernel 2.4.20-6 and it works. If you are using another system ( windows or mac you may need to change the coding ). To compile: name it quadsolv.cpp then type < g++ -o quadsolv quadsolv.cpp > (not the < or the >) then hit enter. When your command prompt returns type < ./quadsolv > and hit enter then follow the directions.

You may need to use math.h like this: #include <math.h> if you are using windows C++ programming software. (I tried it without the math.h and got an "undeclared identifier" error)

Thanks to "Shahi" for pointing out some errors in my coding. I have fixed them.

update 2005-03-24: added math.h. The program would not compile on Fedora core 3 without adding this.

Compiled ok on my machine after adding math.h again ( gcc -dumpversion 3.4.2 ; Fedora core 3 )

uname -a: Linux illiac.perpetualpc.net 2.6.9-1.667 #1 Tue Nov 2 14:41:25 EST 2004 i686 i686 i386 GNU/Linux

Update 2012-11-17: This program compiles and runs on Ubuntu 11.10 (oneiric):

3.0.0-12-generic #20-Ubuntu SMP Fri Oct 7 14:50:42 UTC 2011 i686 i686 i386 GNU/Linux

//copy the part below this

/*program to find solution to quadratic equation in the standard form ax^+bx+c=0. Author David Tarsi. The logic portion of this program was developed from instruction. The coding is by the author. Any questions or comments welcome at dtarsi@premier1.net*/ #include <iostream> #include <math.h> using namespace std; void one(){ float a = 0.0; //here we declare the variables and use float because we float b = 0.0; //are dealing with square roots float c = 0.0; float x1 = 0.0; float x2 = 0.0; float x3 = 0.0; float x4 = 0.0; //this section gets user input and displays message cout << "Enter the coefficients a , b , c for equation in the form ax^ + bx + c = 0:\n"; cout << "Enter value for a:\n"; cin >> a; cout << "Enter value for b:\n"; cin >> b; cout << "Enter value for c:\n"; cin >> c; //are all the coefficients 0? if so both roots are 0 if(a == 0 && b == 0 && c == 0){ x1 = 0; x2 = 0; cout << "The roots are:" "\n" << "x1 = " << x1 << " , " << "x2 = " << x2 << "\n"; } //is c the only non-zero number? if so tell the user if(a == 0 && b == 0 && c != 0){ c = c; cout << "There are no roots" "\n" << "c = " << c << "\n"; } //is a zero? if so solve the resulting linear equasion and notify user if(a == 0 && b != 0 && c !=0){ cout << "The values entered do not make a quadratic expression" "\n" << "x = " << -c/b << "\n"; } //if b is zero and c is zero tell user if(a == 0 && b != 0 && c == 0){ x1 = 0; x2 = 0; cout << "The roots are:" "\n" << "x1 = " << x1 << " , " << "x2 = " << x2 << "\n"; } //if b and c are equal to zero then ax^=0 and since a cannot be zero without x being // zero also let user know if(a != 0 && b == 0 && c == 0){ x1 = 0; x2 = 0; cout << "The values entered result in ax^= 0; so both roots are 0" "\n" << "x1 = " << x1 << " , " << "x2 = " << x2 << "\n"; } //factor out x from ax^+bx=0 and either x = 0 or ax + b =0 //then solve the linear equation if(a != 0 && b != 0 && c == 0){ x1 = 0; x2 = -b/a; cout << "The roots are:" "\n" << "x1 = " << x1 << " , " << "x2 = " << x2 << "\n"; } //now we get to use the square root function and let the user //know they have some imaginary numbers to deal with if(a < 0 && b == 0 && c < 0){ x1 = -b/(2*a); x4 = (b*b)-(4*a*c); x4 = -x4; x2 = sqrt(x4)/(2*a); x3 = -sqrt(x4)/(2*a); cout << "The roots are not real numbers:" "\n" << "x1 =" << x1 << " + " << x2 << " * i " << "\n" << "x2 =" << x1 << " + " << x3 << " * i " << "\n"; } if(a > 0 && b == 0 && c > 0){ x1 = -b/(2*a); x4 = (b*b)-(4*a*c); x4 = -x4; x2 = sqrt(x4)/(2*a); x3 = -sqrt(x4)/(2*a); cout << "The roots are not real numbers:" "\n" << "x1 =" << x1 << " + " << x2 << " * i " << "\n" << "x2 =" << x1 << " + " << x3 << " * i " << "\n"; } //now a and c are opposite signs so the answer will be real if(a > 0 && b == 0 && c < 0){ x1 = (-b + (sqrt(pow(b,2)-(4*a*c))))/(2*a); x2 = (-b - (sqrt(pow(b,2)-(4*a*c))))/(2*a); cout << "The roots are:" "\n" << "x1 = "<< x1 << " , " << "x2 = "<< x2 << "\n"; } if(a < 0 && b == 0 && c > 0){ x1 = (-b + (sqrt(pow(b,2)-(4*a*c))))/(2*a); x2 = (-b - (sqrt(pow(b,2)-(4*a*c))))/(2*a); cout << "The roots are:" "\n" << "x1 = "<< x1 << " , " << "x2 = "<< x2 << "\n"; } //ok now if we end up not having to take the square root of a neg // do the math if(a != 0 && b != 0 && c != 0 && (4*a*c) <= pow(b,2)){ x1 = (-b + (sqrt(pow(b,2)-(4*a*c))))/(2*a); x2 = (-b - (sqrt(pow(b,2)-(4*a*c))))/(2*a); cout << "The roots are:" "\n" << "x1 = "<< x1 << " , " << "x2 = " << x2 << "\n"; } //here we have to deal with non x intercepts ie: sqrt(-1) // alter the formula slightly to give correct output and // let the user know if(a != 0 && b != 0 && c != 0 && (4*a*c)> pow(b,2)){ x1 = -b/(2*a); x4 = (b*b)-(4*a*c); x4 = -x4; x2 = sqrt(x4)/(2*a); x3 = -sqrt(x4)/(2*a); cout << "The roots are not real numbers" "\n" << "x1 =" << x1 << " + " << x2 << " * i " << "\n" << "x2 =" << x1 << " + " << x3 << " * i " << "\n"; } return; } //keep output from vanishing before we can read it. void two(){ char c ; cout << "Press c and then Enter to continue...." "\n"; cin >> c; for(;;){ if ( c ){ break; } } cout << "Done" "\n"; } int main(){ one(); two(); return 0; }//copy the part above this

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Copyright (c) 2002 David Tarsi.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA

A copy of the license is included in the section entitled "GNU GPL".

A copy of the license is included in the section entitled "GNU GPL".