ran2.f90

FORTRANvisualfortran常用数值算法集及源码

FUNCTION ran2(idum)
INTEGER idum,IM1,IM2,IMM1,IA1,IA2,IQ1,IQ2,&
        IR1,IR2,NTAB,NDIV
REAL ran2,AM,EPS,RNMX
PARAMETER (IM1=2147483563,IM2=2147483399,AM=1./IM1,&
           IMM1=IM1-1,IA1=40014,IA2=40692,IQ1=53668,&
		   IQ2=52774,IR1=12211,IR2=3791,NTAB=32,&
		   NDIV=1+IMM1/NTAB,EPS=1.2e-7,RNMX=1.-EPS)
INTEGER idum2,j,k,iv(NTAB),iy
SAVE iv,iy,idum2
DATA idum2/123456789/, iv/NTAB*0/, iy/0/
if (idum<=0) then
  idum=max(-idum,1)
  idum2=idum
  do j=NTAB+8,1,-1
    k=idum/IQ1
    idum=IA1*(idum-k*IQ1)-k*IR1
    if (idum<0) idum=idum+IM1
    if (j<=NTAB) iv(j)=idum
  end do
  iy=iv(1)
endif
k=idum/IQ1
idum=IA1*(idum-k*IQ1)-k*IR1
if (idum<0) idum=idum+IM1
k=idum2/IQ2
idum2=IA2*(idum2-k*IQ2)-k*IR2
if (idum2<0) idum2=idum2+IM2
j=1+iy/NDIV
iy=iv(j)-idum2
iv(j)=idum
if(iy<1) iy=iy+IMM1
ran2=min(AM*iy,RNMX)
END FUNCTION ran2