Where is the problem with these statements related to function 'interp1' with option 'pchip'?

Question

Ricardo Boza Villar 2016-5-26

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回答： Ricardo Boza Villar 2016-5-27

When I write this on the workspace I get an error:

x=[0 2 8 5 6 8 9 11 12 14 15];
y=[10*ones(1,6) 10.5 15 50 60 85];
s=x(1):0.01:x(end);
ys=interp1(x,y,s,'pchip');

It indicates that the grid vectors are not strictly monotonic increasing. I don't know what the 'grid vectors' refer to, but I suppose it refers to the vector of nodes, 'x', the vector of y values belonging to those nodes, 'y' and the vector of x values (longer than the vector of nodes) where I want to find the values that the interpolant has. 'x' and 's' are strictly monotonic increasing; 'y' isn't. As 'y' could be the reason why the statements don't work as planned, I tried y=[10.001 10.002 10.003 10.004 10.005 10.006 10.5 15 50 60 85], but that hasn't worked either and I keep getting the same error. Changing 'pchip' for 'linear' doesn't keep the same error message from appearing.

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Answer 1

Walter Roberson 2016-5-26

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You have

x=[0 2 8 5 6 8 9 11 12 14 15];

That is not monotonically increasing: it has an 8 between the 2 and the 5, and then it has another 8 between the 6 and the 9. Perhaps the first 8 is intended to be a 3.