## Tuesday, July 22, 2014

### Problem

Write a method to randomly generate a set of m integers from an array of size n Each element must have equal probability of being chosen.

### Solution

This question depends on 2 things:
Now that we have fisher yates algorithm in our quiver, we can solve this problem :
```let m be the number of elements to select
for i = 1; i <= m; i++
pick a random number from 1 to n, call it j
swap array[j] and array [n] (assuming 1 indexed arrays)
n--
```

```public static int[] chooseNFromM(int[] array, int m) {
if (m > array.length)
return null;
int[] results = new int[m];
for (int i = 0; i < m; ++i) {
int index, temp;
do {
index = new Random().nextInt(array.length);
} while (index < i);
temp = array[index];
array[index] = array[i];
array[i] = temp;
}
for (int i = 0; i < m; ++i) {
results[i] = array[i];
}
return results;
}
```

We don't need to keep generating random numbers. We just need to label which ones are 'dead' (in this case, 0 to i) and generate random index in the valid range (i+1 to n).
We should first make an copy of the original array as we don't want to modify it.

References