PPVAL , a short question

If I use 2 piecewise polynoms with both y=x and breaks at 1 2 3 and input is 1 2 3

I would expect y = 1 2 3

But I get y = 0 0 1.

Why ?

example :

x = linspace(1,3,3)

breaks=[1 2 3];

coefs=[1 0; 1 0];

pp = mkpp(breaks,coefs);

y=ppval(pp,x);

Answer 1

[Wayne King]

Hi Stephan, the polynomials are not y=x.

Your polynomials are

f(x) = x-1   % x-break(1)
g(x) = x-2   % x-break(2)

So that is why you get y = [0 0 1].

From the command line help:

The matrix COEFS must be L-by-K, with the i-th row, COEFS(i,:), representing the local coefficients of the order K polynomial on the interval [BREAKS(i) ... BREAKS(i+1)], i.e., the polynomial COEFS(i,1)*(X-BREAKS(i))^(K-1) + COEFS(i,2)*(X-BREAKS(i))^(K-2) + ... COEFS(i,K-1)*(X-BREAKS(i)) + COEFS(i,K)

Hope that helps,

Wayne

Answer 2

[Andrei Bobrov]

x = linspace(1,3,3)
breaks=[1 2 3];
coefs=[1 1; 1 2];
pp = mkpp(breaks,coefs)
y=ppval(pp,[1 2 3])

Answer 3

[Stephan Moeller]

Thanks for the answer, - now I understand !