# interp1 monotonic increasing error

3 views (last 30 days)
Brasco , D. on 16 Jul 2014
Edited: Roger Parkyn on 8 Oct 2014
hi guys i got two arrays to interpolate (interp1). but i got an error :
>>Ct_related=interp1(J_00_B,J_00_Ct,10)
Error using griddedInterpolant
The grid vectors are not strictly
monotonic increasing.
Error in interp1 (line 191)
F =
griddedInterpolant(X,V,method);
how can i solve this ? any help?

Star Strider on 16 Jul 2014
If you run this code:
difB = find([NaN; diff(J_00_B)] <= 0);
Q = [[difB-2:difB+2]' J_00_B(difB-2:difB+2)]
you will see that the values of J_00_B for indices 138 and 139 are the same. The independent variable has to be strictly monotonically increasing, so consecutive duplicate values are not allowed. See ‘Strictly Monotonic’ in the ‘More About’ section of the interp1 documentation. (The NaN in the diff call is to make the index references correct.)
Correcting that by adding a small value to the second duplicate:
J_00_B(difB) = J_00_B(difB)+1E-10;
Ct_related=interp1(J_00_B,J_00_Ct,10)
produces:
Ct_related =
96.1895e-003
Roger Parkyn on 8 Oct 2014
Edited: Roger Parkyn on 8 Oct 2014
I have used that same trick (i.e. using diff to find duplicated x-data and then adding a very small increment to the duplicates) but yesterday I found a weakness in this approach: if you have more than two identical x-values then adding the increment to all the duplicates (i.e. two or more duplicates) still leaves some of them the same. With my data it was OK just to eliminate duplicates so that is what I did (may not suit all purposes of course), e.g.:
idx = diff([-9.99; x_data]) == 0.0; % Find duplicate x-values
zz = interp1(x_data(~idx), y_data(~idx), x_i); % Only use non-duplicated data

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