#---------------------------------------------------------------------------
.help ab_index May94 source
.ih
NAME
ab_index -- Compute effective index of data value in array.
.endhelp
#---------------------------------------------------------------------------
procedure ab_index (xa, n, x, index, rindex)

double	xa[n]			# I:  Vector to be searched
int	n			# I:  Number of points in vector
double	x			# I:  Value to search for
int	index			# O:  Index position of X
double	rindex			# O:  Fractional index position of X.

# Declartions
int	hi, low

begin
        # Initialize hi and low to end points
        low=1
        hi=n

        # Bisect array and reset hi or low depending on which side of their
        # center x lies.
        # Stop when hi=low+1
        while ((hi-low) > 1) {
                index=(hi+low)/2
                if (xa[index] > x) 
                        hi=index
                   else
                        low=index
        }

        # Compute fractional index using interpolation
        index = low
        rindex = index + (x-xa[index])/(xa[index+1]-xa[index])
end
#---------------------------------------------------------------------------
# End of ab_index
#---------------------------------------------------------------------------