Question on using interpolate function

This is a follow-up to my previous post here. I want to find the x value at which data has 5.0 (first occurence).

I tried using the find function suggested in the comments.

l = load('var.mat'); % file attached
l = l.unnamed;
x = l(:,1);
data = l(:,2);
tsinterp = getinterpmethod(x)
xmatch = x(find(data == 5.0,1))

The answer that I obtain is 1.71. However, the first occurrence of 5.0 is at 0.20 (x value). I'm not sure what's going wrong.

Any suggestions on how to proceed will be really helpful

Answer 1

[Sindar]

Check your data again. "data" is slightly less than 5 until x=0.485, and the first point exactly equal to 5 is x=1.71. You could add a tolerance like so:

tol = 1e-4;
xmatch = x(find(data >= 5.0-tol,1,'first'))

[Deepa Maheshvare]

Hi, Many thanks for the response. Could you please explain why 1,'first') is required? I understand first picks the first occurrence. Could you please explain why 1 is required?

[Sindar]

"1" tells the number of occurences you want, in case there are multiple. 'first' is the direction; it isn't required, since it is the default, but I like to include it out of habit

Answer 2

[Walter Roberson]

See ismembertol()

The data that is stored corresponding to time 0.20, is not bit-for-bit identical to the representation for double precision 5.0. If you were to subtract 5 from the content at that location you would get something not exactly 0.