# Random Pick with Weight in C++

C++Server Side ProgrammingProgramming

Suppose we have an array w of positive integers, were w[i] describes the weight of index i, we have to define a function pickIndex() which randomly picks an index in proportion to its weight.

So if the input is like [1,3], call pickIndex() five times, then the answer may come as − 0, 1, 1, 1, 0.

To solve this, we will follow these steps −

• Define an array v,
• Through the initializer, initialize as
• n := w[0]
• for i in range 1 to size of w
• w[i] := w[i] + w[i – 1]
• n := w[i]
• v = w
• The pickIndex() will work as follows −
• Take a random number r, perform r mod last element of v
• take the smallest number, not smaller than r from v, then subtract first number of v from the element and return.

Let us see the following implementation to get better understanding −

## Example

Live Demo

#include <bits/stdc++.h>
using namespace std;
class Solution {
public:
int n;
vector <int> v;
Solution(vector<int>& w) {
srand(time(NULL));
n = w[0];
for(int i = 1; i < w.size(); i++){
w[i] += w[i - 1];
n = w[i];
}
v = w;
}
int pickIndex() {
return upper_bound(v.begin(), v.end(), rand() % v.back()) - v.begin();
}
};
main(){
vector<int> v = {1,3};
Solution ob(v);
cout << (ob.pickIndex()) << endl;
cout << (ob.pickIndex()) << endl;
cout << (ob.pickIndex()) << endl;
cout << (ob.pickIndex()) << endl;
cout << (ob.pickIndex()) << endl;
}

## Input

Initialize with [1, 3] and call pickIndex five times.

## Output

1
1
1
1
0
Published on 02-May-2020 13:40:45