Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I have an n-digit number, say X. Now, what-is-the/how-can-I-come-up-with-an equation [or function] that I should use to get the first n/2-digit number and second n/2-digit numbers from the n-digit number?

For example:

X = 1234 ; n = 4
a = 12 (first n/2 digits)
b = 34 (second n/2 digits)

X = 56789 ; n = 5
a = 567 (first n/2 digits - ceiling)
b = 89 (second n/2 digits)
share|cite|improve this question
You should post it on (if you are looking for an algorithm that helps in writing a program) – Kirthi Raman Mar 15 '12 at 11:26
Well, I can write one in C++ as this was one of my very first programming exercises, before I really gave up. It is not particularly hard to put this into a program if you know what the syntax is. But, yes I think this site is not well-suited for more technical questions about coding and algorithms. – user21436 Mar 15 '12 at 11:28
up vote 4 down vote accepted

when $n=1234$, for instance $x = \lceil \log_{10}(n) \rceil = 4$, also what if the number were $4321$ instead of $1234$ ? How would you use this $x$ to get the first portion you are interested in?

$ \displaystyle{\left\lceil \frac{n}{10^{x/2}} \right\rceil} $ gives you the first portion if $x$ here were even, and if $x$ were odd then $ \displaystyle{\left\lceil \frac{n}{10^{(x+1)/2}} \right\rceil} $ gives you the first portion

Use similar logic to explore the second half of what you want.

(Check Wolfram Alpha to get to understand floor and ceil functions better)

share|cite|improve this answer
It looks like an obvious one now! .. duh!! Thanks! :) – Sangeeth Saravanaraj Mar 15 '12 at 11:58
Good, I know with that hint you can get it – Kirthi Raman Mar 15 '12 at 12:03
It would help to be explicit that $x$ counts the number of digits in a given number. – user21436 Mar 15 '12 at 12:13
Well, this was not going to reveal much. It would have been more helpful to the OP if he knew why this works. On a completely unrelated note, you could use the @ before user's name if you're replying to them. – user21436 Mar 15 '12 at 12:21
@KVRaman Why do you delete a message that was here? Yes you have the right to, but not when it will outrightly make the next comment pointless! – user21436 Mar 30 '12 at 12:41

It would be the quotient and remainder when divided by $k$ where

$$k=\begin{cases} \dfrac n 2, \mbox{$n$ is even} \\\\ \dfrac{n+1}{2}, \mbox{$n$ is odd}\end{cases}$$

If you're writing a computer program, you may also want to determine $n$ by having a loop tell you when the remainder by $10^n$ is no more an integer.

share|cite|improve this answer
@Downvoter Care to explain. – user21436 Mar 30 '12 at 12:37

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.