How do I express the concept of the highest "place" of a number so that I could use it in further calculations as part of an expression

Question

So if I had the number "123" (which would be the result of an expression of terms), I would get 100 since the highest place digit used is the hundreds place. I would also get 100 for 285 and for 999, etc. But for the number 20 I would get 10, or for the number 37423 I would get 10000.

What I'm looking for is the correct mathematical notation to express this concept so that it can be used in expressing a function.

e.g. f(x,y) = x + y + (x-y) * highestdigitplace(x+y) * 10

What I'm wondering is if there is a mathematical notation that could be used in place of "highestdigitplace".

Answer 1

$$ 10^{\lfloor \log_{10}(x) \rfloor}$$