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Given a string S of length n and a positive integer k <= n, we want to randomly, and with equal probability, choose a string from the set of all strings of length k that may be formed with a subset of the letters of S. For example, if S = daddy and k = 2, we want to choose each of the strings da, dd, dy, ad, ay, yd and ya with equal probability 1/7.

Obviously, one could do this by calculating all possible permutations. However, is there a more efficient way?

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  • $\begingroup$ Nice question. The best algorithm might depend on which parameters we expect to be large. Are $n, k$ large and the number of possible letters is small (like $26$)? Or might all three be large? $\endgroup$ – 6005 2 days ago
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Let $l$ be the number of distinct characters in the input string. So our input size is summarized by $n, k, l$, with $k \le n$ and $l \le n$.

Here is one efficient way to do it. In the first step, since the order of characters in the string doesn't matter, let's turn it into a map: $$ M = \{a \mapsto 1, d \mapsto 3, y \mapsto 1\}. $$ The idea of our algorithm is to first calculate the number of possible permutations of $k$ elements from $M$, and then to use this to generate one randomly.

Step 1: calculating the number of possible permutations

We use dynamic programming. We actually calculate, more generally, the quantity $$ p[i,j] \qquad \text{ for } 0 \le i \le k \text{ and } 0 \le j \le l, $$ which represent the *number of permutations of length $i$, which use only the first $j$ elements of the map. For example, in the example problem (input "daddy", $k = 2$), we have \begin{align*} p[0, 0] &= p[0, 1] = p[0, 2] = p[0, 3] = 1 \quad \text{(empty string)} \\ p[1, 1] &= 1 \quad \text{(strings of length 1 using only a -- so just "a")} \\ p[1, 2] &= 2 \quad \text{(strings of length 1 using a and d -- so "a" and "d")} \\ p[1, 3] &= 3 \quad \text{("a", "d", and "y")} \\ p[2, 1] &= 0 \quad \text{(no strings of length two using only a since only one a is allowed)} \\ p[2, 2] &= 3 \quad \text{("ad", "da", and "dd")} \\ p[2, 3] &= 7 \quad \text{("ad", "da", "dd", "ay", "ya", "dy", "yd")} \end{align*}

Now how do we calculate $p$? We can use a recursive algorithm. Let $M[j]$ denote the value of $M$ on the $j$th character (so in our example, $M[1] = 1, M[2] = 3, M[3] = 1$). Then $$ p[i, j] = \sum_{i' = 0}^{M[j]} \binom{i}{i'} p[i - i', j-1]. $$

What this formula says is: pick $i'$ places to put character $j$. For example, in our running example for $p[2, 3]$, it says pick $i'$ places to put the character $y$ in a string of length $2$. Then for the remaining $i- i'$ places, we fill them in with a string only using the first $j-1$ characters (which can be done in $p[i-i', j-1]$ ways). The variable $i'$ ranges from $0$ to $M[j]$ because we can only use character $j$ at most $M[j]$ times.

This recurrence allows us to calculate the entire table of $p[i,j]$ in approximately $O(nk)$ time.

Step 2: Picking a permutation at random

The total number of permutations is $p[k, l]$. So pick a random number $r$ from $1$ to $p[k, l]$.

Now recall the recurrence relation $$ p[i, j] = \sum_{i' = 0}^{M[j]} \binom{i}{i'} p[i - i', j-1]. $$

Very roughly, we can use this recurrence relation to enumerate the $r$th permutation. To evaluate the $r$th permutation of $p[i,j]$ total, evaluate the above sum and locate the term where the $r$th permutation would fall. For example if $r = 4$ and there are $7$ total permutations, and the sum gives you $3 + 4$, then $4$ falls fourth out of $7$, so it falls in the second group. Then, we are looking for the first out of $4$ permutations in the second group, which we can recursively find by picking a random permutation from $p[i - i', j-1]$.

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