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# Count number of ways to partition a set into k subsets in C++

Given two numbers e and p. The goal is to count the number of ways in which we can divide e elements of a set into p partitions/subsets.

**For Example**

## Input

e=4 p=2

## Output

Count of number of ways to partition a set into k subsets are: 7

## Explanation

If elements are: a b c d then ways to divide them into 2 partitions are: (a,b,c)−(d), (a,b)−(c,d), (a,b,c)−(d), (a)−(b,c,d), (a,c)−(b,d), (a,c,d)−(b), (a,b,d)−(c). Total 7 ways.

## Input

e=2 p=2

## Output

Count of number of ways to partition a set into k subsets are: 1

## Explanation

If elements are: a b then ways to divide them into 2 partitions are: (a)− (b). Total 1 way only.

**Approach used in the below program is as follows** −

In this approach we will use a dynamic programming approach. The calculations used in the solution would always be recursive. If we divide elements into p partistions then −

If e−1 elements can be divided into p partitions in ways(e-1,p). Then we can put current element into one of these p partitions in p*ways(e-1,p).

If e−1elements are divided into p−1 partitions in ways(e−1,p−1) then putting 1 element in that separate 1 partition will have 1*ways(e−1,p−1). Total ways will be p*ways(e−1,p)+ways(e−1,p−1). This method would become recursive −

As shown above duplicate computations will be done. To avoid this we will use a dynamic programming approach.

Take variables, elements and partitions as input.

Function partition_k(int elements, int partition) takes both variables and returns the count of the number of ways to partition a set into k subsets.

Take 2D array arr[elements + 1][partition + 1] to store values of ways(e,p) in arr[e][p].

Using a for loop from i=0 to i=elements, set arr[i][0] = 0 as the number of partitions is 0 then ways(i,0)=0.

Again using a for loop from j=0 to i=partitions, set arr[0][j] = 0 as number of elements is 0 then ways(0,i)=0.

Now traverse arr[][] using two for loops from i=1 to i<=elements and j=1 to j<=i. and fill rest values.

For a single element, ways=1 or for dividing x elements into x partitions there is only 1 way. So set arr[i][j] = 1 in case i==j or j==1.

Otherwise set temp_1 = arr[i−1][j−1] and temp_2 = arr[i−1][j] and update arr[i][j] = j * temp_2 + temp_1.

At the end of all loops we will have arr[elements][partition] as total ways.

Return arr[elements][partition] as result.

## Example

#include<iostream> using namespace std; int partition_k(int elements, int partition){ int arr[elements + 1][partition + 1]; for(int i = 0; i <= elements; i++){ arr[i][0] = 0; } for(int j = 0; j <= partition; j++){ arr[0][partition] = 0; } for(int i = 1; i <= elements; i++){ for (int j = 1; j <= i; j++){ if (j == 1 || i == j) { arr[i][j] = 1; } else{ int temp_1 = arr[i−1][j−1]; int temp_2 = arr[i−1][j]; arr[i][j] = j * temp_2 + temp_1; } } } return arr[elements][partition]; } int main(){ int elements = 4; int partition = 2; cout<<"Count of number of ways to partition a set into k subsets are: "<<partition_k(elements, partition); return 0; }

## Output

If we run the above code it will generate the following output −

Count of number of ways to partition a set into k subsets are: 7

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