How to find the indices that a point lays between?

I have a vector X and a vector Y with 112 points. See picture above, the points are depected with a cross symbol.

There is a point A in the position where the arrow points out that I know its value (xA,yA).

How can I find the points that this point A lays between? For example here the answer would be 83 and 84.

[ARGY B]

It was too simple:

point2 = find(X>xA,1);

point1 = point2-1;

[Bruno Luong]

Simple but your method assumes X is increasing.

[ARGY B]

You are right.

Do you suggest another way? In case X is not increasing.

[Bruno Luong]

If your array are 2-columns: X is n x 2 and Xa is 1 x 2, I propose

d2 = sum((X-Xa).^2,2)
[~,i] = min(d2);
if i==length(d2) || (i > 1 && d2(i-1) < d2(i+1))
    i = i-1;
end
point1 = X(i,:);
point2 = X(i+1,:);

Answer 1

[Matt J]

Do you suggest another way? In case X is not increasing.

If X,Y are non-monotonic, I would do

[~,points] = pdist2([X(:),Y(:)],[xA,yA] ,'Smallest',2)

Answer 2

[Matt J]

find is inefficient. You should instead do,

 point1=discretize(xA,X);
 point2=point1+1;

[ARGY B]

I get this:

Error using discretize

Expected input number 2, edges, to be non-decreasing valued.

Error in discretize (line 61)

validateattributes(edges, {'numeric','logical'},{'vector', 'real', ...

[Matt J]

Is your X monotonically decreasing? If so, just reverse the order to make it increasing.

point1 = numel(X) + 1 - discretize(xA,flip(X));

[Bruno Luong]

This should also work for decreasing, no?

discretize(-xA,-X)

[Matt J]

I think maybe you still need

numel(X)+1 - discretize(-xA,-X)

[Bruno Luong]

>> X=10:-1:1

X =

    10     9     8     7     6     5     4     3     2     1

>> xA=2.5 % data fall between 3 and 2, meaning indexes 8 and 9 of X

xA =

    2.5000

>> numel(X) + 1 - discretize(xA,flip(X)) % this should be index of Point 2

ans =

     9

>> discretize(-xA,-X) % this should be index of Point 1

ans =

     8

[Matt J]

Ah. Yes, you are right.