# Counting arrays with Euclidean distance at most 2 from a given binary array

I have a binary array like this:

$$A = [0,1,0,0,1,0]\,.$$

I'm trying to find a way to calculate how many arrays of the same length exist that have a Euclidean distance of 2 or less from this array.

So, how many arrays of length 6 exist where

$$\sqrt{\Sigma(A_{_i} - B_{_i})^2}\leq 2\,?$$

I'm trying to find or create a formula that takes an array like above and outputs a count of how many possible binary arrays exist that fit the conditions of the above formula.

I've looked online for a formula without success.

• If the arrays are both binary, then your problem is equivalent to counting the number of arrays $B$ with Hamming distance at most $2^2 = 4$ from $A$, which is the same as the number of arrays $B$ with Hamming distance at most 4 from any specific array -- such as the array $[0, 0, 0, 0, 0, 0]$. This should give you a hint. – j_random_hacker Apr 10 '18 at 14:55

Since your arrays are binary, $$(A_i-B_i)^2 = \begin{cases}0 &\text{if }A_i=B_i\\ 1&\text{if }A_i\neq B_i\,.\end{cases}$$
So $\sqrt{\sum(A_i-B_i)^2}\leq 2$ if, and only if there are at most four values of $i$ such that $A_i\neq B_i$. So you just need to compute the number of ways that $B$ could have zero, one, two, three or four different entries from $A$ (or, more simply, the number of ways that it could have five or six different entries, and subtract that from the total possible values of $B$). This calculation is high-school combinatorics. Note that, because the arrays are binary, there's only one value of $B_i$ that's the same as $A_i$ and only one that's different.