# what is the expected number of collisions?

Suppose we use a hash function $H$ to hash $N$ distinct balls into $M$ distinct bins. Assuming simple uniform hashing, what is the expected number of collisions?

Note that a collision is defined by adding a ball to an already occupied bin. If the already occupied bin has $k$ balls in it, then the number of collisions upon adding a new ball is $k.$

By using expectation, I tried as :

=> 1 × Probability of collision in first insertion +

2 × Probability of collision in second insertion + .......... +

n × Probability of collision in nth insertion

=> $(1 ∗ 0) + (2 ∗ 1/m) + (3 ∗ 2/m) + (4 ∗ 3/m) + … + (n ∗ n−1/m)$

Actually, The answer is $(n^2 - n)/2m$

But, I am not getting the answer. Where am I wrong here ?

• note that it should be $n\times P(\textrm{n collisions}),$ not collision on the $n$th insertion – cats Dec 9 '16 at 15:09
• @cats Sorry, didn't get it. Could you please explain it as answer ? – Jon Garrick Dec 9 '16 at 15:10
• I'm just saying that your definition of expectation is incorrect. You need $1\times P(\textrm{1 collision}) + 2\times P(\textrm{2 collisions}) + \ldots ,$ which is not the same as what you've written – cats Dec 9 '16 at 15:12
• @cats okk !! When I write "2 × Probability of collision in second insertion" then it means that for 2 collisions to happen, what is the probability ? Similarly, for 3 collisions to happen what is the probability? But instead, If I write "1 × Probability of collision in second insertion" then it means probability of having one collision in second attempt. Similarly, what is the probability of having one collision in 3rd attempt ? Am I right now ? – Jon Garrick Dec 9 '16 at 15:26
• Collisions probably means the number of times you put a ball into an already occupied bin – cats Dec 9 '16 at 15:30

The expected number of new collisions caused at the time of inserting the $k$-th ball is $\frac{k-1}{M}$ since it has a $\frac1M$ collision probability with each ball already placed.

Thus the expected number of collisions is

$\frac0M+\frac1M+\frac2M+\cdots+\frac{N-1}{M}=\frac{N(N-1)}{2}\cdot\frac1M$

• This is wrong. If there was already collisions then there are less spots of having collisions with. You need a recurrent definition and maybe have to do this with recursion – Charlie Tian Jan 30 '18 at 8:45
• The recursive relationship I derived was that E(k, M) = (M-1/M)E(k-1) + (k-1)/M – Charlie Tian Jan 30 '18 at 8:59
• My bad, I had read the question wrong. My way is the recursive DP way to do it when collision is just 1. This is for general k – Charlie Tian Jan 30 '18 at 9:07

Note that if you end with $k$ occupied bins, then there were $N-k$ collisions. In other words, we want $N$ minus the expected number of occupied bins. This is easy - the probability a bin is unoccupied at the end is $\left(\frac{M-1}{M}\right)^N,$ so the expected number of unoccupied bins is this times $M$ and the expected number of occupied bins is $M\left(1-\left(\frac{M-1}{M}\right)^N\right),$ so our answer is $N$ minus this.

edit: this answers a different version of the problem. see paw's answer for the updated :)

• I enjoy this succinct explanation for the more common problem. Ironically, I could not find as good an explanation for the more common problem. I think it is good to have answers to both questions on the same one – Charlie Tian Jan 30 '18 at 9:08

you are claiming that $P(A \cap B)=P(A)+P(B)$, which is true iff the events are independent and they are not. Consider that when you insert 3rd element "Probability of collision in this insertion" = $1 \over m$ if there was a collision on 2nd insertion (2nd end up in the same bin as 1st) but it's $2 \over m$ is there was not, so $P_n$ depends on $P_1, \ldots ,P_{n-1}$

assuming @cats statment that "Collisions probably means the number of times you put a ball into an already occupied bin"

you can approach this by describing steps of the process and thinking about graph of states.

like: if while counting $c$ collisions so far and having $i$ balls left there are $k$ empty bins left then with probability $p={1 \over k}$ you move to the state "$i-1$ balls ; $k-1$ empty bins; $c$ collisions" and with probability $1-p$ you move to state "$i-1$ balls; $k$ empty bins; $c+1$ collisions".

In short:

Let $S(c,i,k)$ be state described above, the state graph looks like this:

$S(0,N,M)$ - start state

$S(c,0,k)$ - end stats (dla dowolnych $c,k$)

and the edges are:

$S(c,i,k)\begin{matrix}\overset{1 \over k}{\rightarrow} S(c,i-1,k-1)\\ \overset{k-1 \over k}{\rightarrow}S(c+1,i-1,k)\end{matrix} for\, i>0$

now Lets define $p_{cik}$ as probability of reaching state $S(c,i,k)$ and variable $\Bbb X$ which will be a number of collisions.

If we ask "from what state can i move into state $S(c,i-i,k-1)$", the answer will be "We where in $S(c,i,k)$ and we hit an empty bin or we where in $S(c-1,i,k-1)$ and we made a collision"

That gives us a recursive function: $$p_{c\,i-1\,k-1}={1 \over k}p_{c-1\,i\,k}+{k-1 \over k}p_{c-1\,i\,k-1}$$ And additionally we can say: $$p_{cik}=0\,for\,\begin{matrix}i \notin \{0,\dots,N\}\\ k \notin \{0,\dots,M\}\end{matrix}$$ since those are not possible and that $$p_{0NM}=1$$

that is enough to calculate $p_{cik}$ for $(i,k) \in \{0,\dots,N\} \times \{0,\dots,M\}$

Now we can say $$P(\Bbb X=x)=\sum_{c_n,i,k} p_{c_nik}$$ where $0\le i \le N$ , $0\le k \le M$ , $\sum_nc_n=x$

Such model could be implemented in any programing language.

• see my answer for a quick way to solve this version of the problem – cats Dec 9 '16 at 20:05