Combining "Product" and "If"

Question

ClearAll["Global`*"];
x1 = {1, 2, 3, 4};
y1 = {3, 4, 2, 5};
f = Factor[\!\(
\*UnderoverscriptBox[\(\[Sum]\), \(i = 1\), \(Length[x1]\)]\(\((
\*UnderoverscriptBox[\(\[Product]\), \(j = 1\), \(Length[x1]\)]\((If[
        j != i, 
\*FractionBox[\(x - x1[\([j]\)]\), \(x1[\([i]\)] - 
          x1[\([j]\)]\)]])\))\)*y1[\([i]\)]\)\)]

I don't have idea why that "Null" appear so, anyone can help me? (I'm an amateur)

Answer 1

It seems that you are trying to implement Lagrange interpolation. You can use Delete:

Clear[f]
f[pts_List, x_] := Module[{l, n = Length[pts], xys = pts\[Transpose]},
  (*define the interpolation function*)
  l[i_] := Product[(x - xys[[1, j]])/(xys[[1, i]] - xys[[1, j]]), {j, Delete[Range[n], i]}];
  (l /@ Range[n]).xys[[2]]
  ]

where the outer summation is implemented via Dot (.) in the last line. The code can be tested as

points = {{x0, y0}, {a, ya}, {b, yb}, {x1, y1}};
f[points, x]

In terms of your data, points = Transpose[{x1, y1}];. Actually, there is a built-in function doing the very same job: InterpolatingPolynomial.

Answer 2

The Null appears because two-argument If[] produces Null if the condition in the first argument is not satisfied. Αλέξανδρος shows one possibility, but you can fix your original code by recalling that $1$ is the identity element for multiplication; thus, you can implement Lagrangian interpolation like so:

With[{x1 = {1, 2, 3, 4}, y1 = {3, 4, 2, 5}}, 
     p1 = Sum[Indexed[y1, i]
              Product[If[i != j, (x - Indexed[x1, j])/(Indexed[x1, i] - Indexed[x1, j]), 1],
              {j, 1, Length[x1]}], {i, 1, Length[x1]}]];

Check:

Simplify[p1 == InterpolatingPolynomial[Transpose[{{1, 2, 3, 4}, {3, 4, 2, 5}}], x]]
   True

---

That being said, the classical version is not the best way to implement Lagrangian interpolation; in particular, one might consider instead using the so-called barycentric form:

With[{x1 = {1, 2, 3, 4}, y1 = {3, 4, 2, 5}},
     w = Table[1/Apply[Times, x1[[j]] - Delete[x1, j]], {j, 1, Length[x1]}];
     p2 = (w/(x - x1)).y1/Total[w/(x - x1)]];

Check:

Simplify[p2 == InterpolatingPolynomial[Transpose[{{1, 2, 3, 4}, {3, 4, 2, 5}}], x]]
   True

See the linked paper for more details.