VECTOR2.f95

	PROGRAM VECTOR2
!
! Purpose: Reads in two vectors and calculates their dot product and
!          their angle.
!
	IMPLICIT NONE
	REAL:: R1(3),R2(3)	
	REAL:: DOT_PRODUCT,RLEN1,RLEN2,COSINE,ANGLE,PI
	INTEGER:: I		
!
!  Read in the components of vector 1, then the components of vector 2.
	DO I=1,3
	   WRITE(*,11,ADVANCE='NO')I
	   READ(*,*)R1(I)
	END DO
	DO I=1,3
	   WRITE(*,12,ADVANCE='NO')I
	   READ(*,*)R2(I)
	END DO
!
!  Dot product and lengths of the two vectors
	DOT_PRODUCT=R1(1)*R2(1)+R1(2)*R2(2)+R1(3)*R2(3)
	WRITE(*,20)DOT_PRODUCT
!
!  Calculate the angle. First calculate the length of the two vectors
!  and check that neither R1 nor R2 has zero length
	RLEN1=SQRT(R1(1)**2+R1(2)**2+R1(3)**2)
	RLEN2=SQRT(R2(1)**2+R2(2)**2+R2(3)**2)
	IF(RLEN1*RLEN2 == 0.0)THEN
	   WRITE(*,*)'R1 or R2 has zero length.'
	ELSE
!  Cosine of the angle between the two vectors.
	   COSINE=DOT_PRODUCT/(RLEN1*RLEN2)
!  Angle (in radians) between the two vectors.
	   ANGLE=ACOS(COSINE)
!  Convert to degrees and print the result.
	   PI=4.0*ATAN(1.0)
	   ANGLE=ANGLE*180.0/PI
	   WRITE(*,30)ANGLE
	END IF
!
  11	FORMAT('Enter component  ',I2,' of vector 1 : ')
  12	FORMAT('Enter component  ',I2,' of vector 2 : ')
  20	FORMAT('The dot product of the two vectors is ',E15.7)
  30	FORMAT('The angle between the two vectors is ',E15.7, ' deg.')
	STOP
	END PROGRAM VECTOR2