Polynomial interpolation works fundamentally by determining the value of an interpolating polynomial at a number of desired points. To obtain this polynomial, first we must determine its coefficients by computing divided differences.
The interpol operation begins by allocating
space for the divided differences as well as for the coefficients to
be determined (see Example
13-1). Note that since the entries in each row in a
divided-difference table depend only on the entries computed in the
row before it (see Figure 13.2 and
Figure 13.3), we do not have to keep
all of the table around at once. Thus, we allocate space only for the
largest row, which has
n entries. Next, we
initialize the first row in the table with the values in
fx. This is so that we are ready to compute
what equates to the third row of the divided-difference table.
(Nothing needs to be done for the first two rows because these entries
are already stored in
fx.) The final initialization step is to
store the value of
coeff since this is the first
coefficient of the interpolating polynomial.
The process of computing divided differences revolves around a single nested loop, which uses the
formula for divided differences discussed earlier in the chapter. In
terms of Figure 13.2 and Figure 13.3, the outer loop,
k, counts the number of rows for which
entries must be computed (excluding the rows for
i, controls which entry ...