Problem with program logic

[Dukelord123]

I am trying to write a program to solve a quadratic equation.If the value of (B**B-4*A*C) is 0 or negative, it should immediately write that "The roots of the equation is complex", but if positive, it should evaluate. It seems my logic is faulty cos no matter what values I give for A,B & C, I keep getting "The roots of the equation are complex". Please see code and results below. Thanks.



PROGRAM QUADEQN
INTEGER A,B,C
REAL D,X,Y,Q
D=(B**2-4*A*C)
Q=SQRT(D)
READ(*,5)A
READ(*,6)B
READ(*,7)C
IF(B**2-4*A*C)10,15,20
X=(-B+Q)/(2*A)
Y=(-B-Q)/(2*A)
20 WRITE(*,25)X,Y
5 FORMAT(I2)
6 FORMAT(I2)
7 FORMAT(I2)
10 WRITE(*,*)'THE ROOTS OF THE QUADRATIC EQUATION IS COMPLEX'
15 WRITE(*,*)'THE ROOTS OF THE QUADRATIC EQUATION IS COMPLEX'
25 FORMAT(/,'THE ROOTS OF THE EQN ARE',1X,F8.4,'AND',1X,F8.4)
STOP
END

**RESULT**

D:\Postgraduate\Programming\FORTRAN>gfortran quad.f

D:\Postgraduate\Programming\FORTRAN>a.exe
8
3
2
THE ROOTS OF THE QUADRATIC EQUATION IS COMPLEX
THE ROOTS OF THE QUADRATIC EQUATION IS COMPLEX

D:\Postgraduate\Programming\FORTRAN>

 

[gullipe]

Hi Dukelord123

Well, it is better to read A, B and C before you calculate D=(B**2-4*A*C). Also to calculate X and Y before you jump in all cases over it as you do. The program below should work better.

      PROGRAM QUADEQN

      INTEGER A,B,C
      REAL D,X,Y,Q

      READ(*,5) A
      READ(*,5) B
      READ(*,5) C
5     FORMAT(I10)

      D = (B**2-4*A*C)
      IF(D.LT.0) GOTO 10
      
      Q = SQRT(D)
      X = (-B+Q)/(2*A)
      Y = (-B-Q)/(2*A)

      WRITE(*,25) X,Y
25    FORMAT(/,'THE ROOTS OF THE EQN ARE',1X,F8.4,'AND',1X,F8.4)
      GOTO 90
	
10    WRITE(*,*) 'THE ROOTS OF THE QUADRATIC EQUATION IS COMPLEX'

90    CONTINUE
      END

 

[Dukelord123]

It works well now. Thanks a lot dude.