# Place value function

This has probably already been done before, but I had lots of fun discovering it. :)

I wanted to create a function $f$, which, given a number $n$, would take in the place value of a digit $d$ of $n$ ($x$, such that the place value of $d$ is $10^{x}$), and output $d$ itself.

For example, if $n = 1345.829$, than $f(0) = 5$, $f(3) = 1$, and $f(-2) = 2$.

Here's what I came up with:

$f(x) = \dfrac{n \operatorname{mod} 10^{x+1} - n \operatorname{mod} 10^{x}}{10^{x}}$

And a Desmos graph.

I came up with it somewhat inductively, noting patterns while calculating the units digit, the tens digit, and so on, but I understand now why it works. The function essentially chops off everything after the desired place value, subtracts from that everything before the desired place value, and then divides by the power of ten associated with that place value. For example:

$n = 1345.829$, $x = 2$ (should retrieve the digit at $10^2$, or $3$)

$n \operatorname{mod} 10^{2+1} = 345.829$

$n \operatorname{mod} 10^{2} = 45.829$

$345.829 - 45.829 = 300.000$

$\frac{300.000}{10^{2}} = \boxed{3}$

Note by David Stiff
4 months, 3 weeks ago

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Now you can trick people by making it look a little more complicated using the method I had shown earlier using trig and inverse trig, making it non-obvious to the naked eye(the formula you have shown is what I normally use for getting digits in a computer program, but I was just reminded that I had found the modulus function in terms of trig and inverse trig)

- 4 months, 3 weeks ago

How sinister! :)

- 4 months, 3 weeks ago

It was actually in a daily challenge, in a comment as a reply to Siddarth’s comment(one or two fire), I don’t seem to remember the name so I have to go on treasure hunt too

- 4 months, 3 weeks ago

Aha found it(joking it was quite recent) First solution, first comment, first reply to that, first reply to the first reply

- 4 months, 3 weeks ago

Rats, I'm not a Premium member. It says the problem has expired. :/

- 4 months, 3 weeks ago

Oh in that case I will put down here what I did there

- 4 months, 3 weeks ago

"A better one is $\frac{10}{π}\cot^{-1}(\cot(\frac{3π}{10}x))$ "

"Got the perfect one $5+\frac{10}{π}(\tan^{-1}(\tan(\frac{3π}{10}x-\frac{π}{2})))$ "

These are both for $3x \mod {10}$, a little modification should get them to $x \mod {10^n}$

- 4 months, 3 weeks ago

Very cool! Confusing at first too. :)

- 4 months, 3 weeks ago

I tried out the graph using this function, it didn’t look good

- 4 months, 3 weeks ago

Hm. For the trig one you mean?

- 4 months, 3 weeks ago

Yeah I tried the trig, but I assume neither will look good

- 4 months, 3 weeks ago

Yeah, the simpler one looks the same I think. It only really looks nice if you keep $n$ constant and then graph $f(x)$ for all integers between like $x = -10$ and $x = 10$ (or something else depending on $n$).

- 4 months, 3 weeks ago

In fact when you add or subtract inverse trig stuff, it can be simplified to a single inverse trig expression, so after that it will look all the more confusing

- 4 months, 3 weeks ago