constantly update range of values (mouse x axis)

I would like to make a system that constantly update the two extreme points of input data (point max and point min)
Assuming I'm starting in point 200. the system should update both min and max point to 200. I then move the mouse to the right and now the mouse is at point 203. The max point should be updated to 205 while the min point should stay at 200. I continue to move the mouse and I'm at 197. The min point should be updated to 197 while the max point should be stay at 205. Any tips how can I do the following?

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another question I have is how can I output the index of an element in a list if the input data is equal to any of the element within that list

I have a list with the following numbers: 5.5 7.7 3.33 11.11

if input data is equal to one of those numbers then it should output the index of that number in that list.
so if the number I check is 3.33 the output should be 2 (3.33 is at index 2 of that list)

thanks!