ppval and polyval giving different results

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I created a piecewise polynomial using mkpp. But when I evaluate a set of points I get incorrect results using ppval, although polyval() gives correct result. Looks like I'm missing something. Any idea what that is?
Thanks!
>> ppm
ppm =
struct with fields:
form: 'pp'
breaks: [749.5 16747 32768 48771 64784]
coefs: [4×2 double]
pieces: 4
order: 2
dim: 1
K>> ppm.coefs
ans =
0.00031269 -10.246
0.00031268 -10.246
0.00031274 -10.247
0.00031265 -10.245
>> ppval(ppm,ppm.breaks(1))
ans =
-10.246
>> polyval(ppm.coefs(1,:),ppm.breaks(1))
ans =
-10.012
>> polyval(ppm.coefs(1,:),8750)
ans =
-7.51
>> ppval(ppm,8750)
ans =
-7.7444

Accepted Answer

Bruno Luong
Bruno Luong on 19 Apr 2022
Edited: Bruno Luong on 19 Apr 2022
Here is a correct way to know how ppval works
pp=spline(cumsum(rand(1,10)),rand(1,10));
x=3;
ppval(pp,x)
ans = 2.1433
%
i=discretize(x,pp.breaks);
polyval(pp.coefs(i,:),x-pp.breaks(i))
ans = 2.1433
Your evaluation
polyval(ppm.coefs(1,:),8750)
is wrong unless pp.breaks(1) is 0.

More Answers (1)

Torsten
Torsten on 19 Apr 2022
I think you fed "polyval" with the coefficients cut to a certain number of digits.
You must use the coefficients in full precision to get equal results from ppval and polyval.
  6 Comments
Torsten
Torsten on 19 Apr 2022
You are right. This also explains the deviations in the breakpoints themselves.

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