The first issue with what you're doing is with
x == some_non_integer_value
It will work for some values. For others, it won't, e.g:
x = 0.1 + 0.1 + 0.1 % == 0.3 right?
x == 0.3 % guess not, return false, find would return empty!
This is due to the way numbers are stored on computers. Not all decimal values can be stored exactly (e.g 0.1 can't), meaning that testing for exact decimal value is dangerous. The proper way to compare floating point values is to check that their absolute difference is smaller than a value much smaller than their magnitude:
abs(x - 0.3) <= 1e-10 %test for equality
Second issue:
y_1 = find(y == x_1)
Remember that x_1, if not empty, is an index, always integer, from 1 to 1244. That is what you're searching for in y. From your description, I don't think that's what you meant to do. The above line will also fail if the find for x_1 returned more than index (i.e. the search value is present more than once in x).
"I know I should be looping". Most likely not. It is likely that it can all be done in a few lines with no loop. E.g, all your x_i could be found in one go with
searchvalues = [799.6585468680, 3995.72148499300, 2021.00198661820];
[found, where] = ismembertol(searchvalues, x)
where would correspond to your [x_1, x_2, x_3]
A better explanation of what you're trying to do with y (provide an example with numbers) would allow me to complete this answer.
