Problem with answer of interp1

Hi

I am trying to use interp1 and i keep getting a WRONG answer i was hoping you could help me find the bug

P_vect = 6.1201  6.12  6.232  6.28   4.12   3.32
P = 4.747
Y_vect = 3000    3500       4500       5000       5500       5800

What is Y?

Y=interp1(P_vect, Y_vect, P)

matlab gives me 4873 when the answer should be 5354

thanks

[Azzi Abdelmalek]

Why it should be 5354?

[noa]

well if P is between the 4th and the 5th values in P_vect

then Y should also be in those locations

[Walter Roberson]

It is not between the 4th and 5th values once you have sorted Y_vect according to P_vect

Answer 1

[Walter Roberson]

interp1() does not appear to define the behavior when the x coefficients are not monotonic.

[Jan]

[EDITED] The fact that INTERP1 calls HISTC is interesting, but does not matter the problem. But the call of SORT for the x-values clears, that not monotonic inputs are handled correctly.

Therefore I think Matlab's interpolation is correct, and assumptions of the OP contain a mistake.

Answer 2

[Azzi Abdelmalek]

Let us sort your vector

[P1,idx]=sort(P_vect )
Y1=Y_vect(idx)
Y=interp1(P1, Y1, P)

% in this case P is between the second(4.12) and the third (6.12) which correspond to value between 5500 and 3500

[Jan]

After sorting the calculated result still equals the results of the OP.

I think your first comment hits the point: Why is the result 5354 assumed?

Answer 3

[Jan]

I get 4873 also, when I calculate this manually. So I assume Matlab is correct and your assumption is not.

The relevant data are:

x = [4.12, 6.12]
y = [5500, 3500]
delta_x = 2.0
delta_y = -2000
z = 4.747;
result = y(1) + (delta_y / delta_x) * (z - x(1))
       = 5500 - 1000 * 0.637 
       = 4873