polint.f90

FORTRANvisualfortran常用数值算法集及源码

SUBROUTINE polint(xa,ya,n,x,y,dy)
INTEGER n,NMAX
REAL dy,x,y,xa(n),ya(n)
PARAMETER (NMAX=10)
INTEGER i,m,ns
REAL den,dif,dift,ho,hp,w,c(NMAX),d(NMAX)
ns=1
dif=abs(x-xa(1))
do i=1,n
  dift=abs(x-xa(i))
  if (dift<dif) then
    ns=i
    dif=dift
  endif
  c(i)=ya(i)
  d(i)=ya(i)
end do
y=ya(ns)
ns=ns-1
do m=1,n-1
  do i=1,n-m
    ho=xa(i)-x
    hp=xa(i+m)-x
    w=c(i+1)-d(i)
    den=ho-hp
    if(den==0.) pause 'failure in polint'
    den=w/den
    d(i)=hp*den
    c(i)=ho*den
  end do
  if (2*ns<n-m) then
    dy=c(ns+1)
  else
    dy=d(ns)
    ns=ns-1
  endif
  y=y+dy
end do
END SUBROUTINE polint