How to use inerpolation option with end values? If I use interp1 command it's giving NaN as the output if input is out of the range. I want to use end values if input is out of the range?

1  120
2  5000
3  12000

My input is something like this. If i give 4 as my input currently it's giving NaN as the output but I want output to be 12000(i.e end value)

Answer 1

[Stephen23]

>> X = [1,2,3];
>> Y = [12,5000,12000];
>> interp1(X,Y,4,'nearest','extrap')
ans =  12000
>> interp1(X,Y,4,'linear','extrap')
ans = 19000

But note that you cannot pick a method like linear, spline, etc, and also limit the extrapolation to the end values:

>> interp1(X,Y,[0,1.5,4],'linear','extrap')  % linear -> not end values
ans =
   -4976    2506   19000
>> interp1(X,Y,[0,1.5,4],'nearest','extrap') % nearest -> not linear interpolated
ans =
      12    5000   12000

But you can easily select the interpolation method and limit to the end values, by simply using max and min:

>> max(Y(1),min(Y(end),interp1(X,Y,[0,1.5,4],'linear','extrap')))
ans =
      12    2506   12000

If you only need to extrapolate in one direction then you could set the extrapolation value:

>> interp1(X,Y,[1.5,4],'linear',Y(end))
ans =
    2506   12000

[Parthiban C]

Thank you for your response. This one worked for my problem. max(Y(1),min(Y(end),interp1(X,Y,[0,1.5,4],'linear','extrap'))) ans = 12 2506 12000

[Christian]

This min/max stuff does not work that way, if Y is not monotonously increasing, let's say

>> Y = [12,5000,4000];

Then I get this:

>> max(Y(1),min(Y(end),interp1(X,Y,[0,1.5,4],'linear','extrap')))
ans =
          12        2506        3000

And I guess for X=4 we want to see 4000 instead of 3000.

In general monotony assumptions should only be made towards X, not towards Y.