d1/d32/genf_8f_source.html

       REAL FUNCTION genf(dfn,dfd)

 C**********************************************************************

 C

 C     REAL FUNCTION GENF( DFN, DFD )

 C                GENerate random deviate from the F distribution

 C

 C

 C                              Function

 C

 C

 C     Generates a random deviate from the F (variance ratio)

 C     distribution with DFN degrees of freedom in the numerator

 C     and DFD degrees of freedom in the denominator.

 C

 C

 C                              Arguments

 C

 C

 C     DFN --> Numerator degrees of freedom

 C             (Must be positive)

 C                              REAL DFN

 C      DFD --> Denominator degrees of freedom

 C             (Must be positive)

 C                              REAL DFD

 C

 C

 C                              Method

 C

 C

 C     Directly generates ratio of chisquare variates

 C

 C**********************************************************************

 C     .. Scalar Arguments ..

       REAL dfd,dfn

 C     ..

 C     .. Local Scalars ..

       REAL xden,xnum

 C     ..

 C     JJV changed this code to call sgamma directly

 C     .. External Functions ..

 C      REAL genchi

 C      EXTERNAL genchi

       REAL sgamma

       EXTERNAL sgamma

 C     ..

 C     .. Executable Statements ..

       IF (.NOT. (dfn.LE.0.0.OR.dfd.LE.0.0)) go to 10

       WRITE (*,*) 'Degrees of freedom nonpositive in GENF - abort!'

       WRITE (*,*) 'DFN value: ',dfn,'DFD value: ',dfd

       CALL xstopx('Degrees of freedom nonpositive in GENF - abort!')


  10   xnum = 2.0*sgamma(dfn/2.0)/dfn


 C      GENF = ( GENCHI( DFN ) / DFN ) / ( GENCHI( DFD ) / DFD )

       xden = 2.0*sgamma(dfd/2.0)/dfd

 C     JJV changed constant so that it will not underflow at compile time

 C     JJV while not slowing generator by using double precision or logs.

 C      IF (.NOT. (xden.LE. (1.0E-38*xnum))) GO TO 20

       IF (.NOT. (xden.LE. (1.0e-37*xnum))) go to 20

       WRITE (*,*) ' GENF - generated numbers would cause overflow'

       WRITE (*,*) ' Numerator ',xnum,' Denominator ',xden

 C     JJV next 2 lines changed to maintain truncation of large deviates.

 C      WRITE (*,*) ' GENF returning 1.0E38'

 C      genf = 1.0E38

       WRITE (*,*) ' GENF returning 1.0E37'

       genf = 1.0e37

       go to 30


    20 genf = xnum/xden

    30 RETURN


       END

genf
real function genf(dfn, dfd)
Definition: genf.f:1