Identifying position in array nearest to a value

Hello,

First time on MATLAB Answers so apologies if I miss anything in my question.

I am carrying out peak fitting on a set of data and would like to identify the left and right points nearest to a line as shown in the figure.

The line is a yline positioned halfway between the height limits of the data set (0.5*(maxy+miny)). I need to extract the data set peak width at this height but none of the array values are equal to this height value so I can't use the idx = X==height; function to find the left and right values of x.

[a1a,b1a] = max(y);

d1a = min(y);

q1a = 0.5*(a1a+d1a);

idx = y==q1a;

Instead, I was hoping to find the index position of the values nearest to the line but I am unsure how to go about doing this.

Any advice would be appreciated!

Thanks

Answer 1

[Eva-Maria Weiss]

Hi

I hope I have here some ideas that might help you:

You could try something like zerocrossing, but instead of zero, you look for crossing the y-value;

make a logical condition like

logic =  x >= y;

you get a logical vector with length of x, each position thats corresponding value is under y is 0 and above is 1;

Now you could use the diff function,

crossing =  diff(logic)

The position where x crosses y then indicated as a 1, the position when the data x cross y line again is -1; all other positions are then 0;

Now you can get the index with the find function:

x1 = find(crossing == 1)
x2 = find(crossing == -1)

Good luck!

Answer 2

[KSSV]

You can get the intersection point of the line and curve, you can use this

https://in.mathworks.com/matlabcentral/fileexchange/22441-curve-intersections?focused=5165138&tab=function

to find the intersection between two cruves.

You can find the nesrest points to a give point using knnsearch.