theonlygusti - 6 months ago 26

C# Question

I am trying to write a function which generates the lucky numbers,

`static IEnumerable<int> LuckyNumbers()`

{

IEnumerable<int> luckyNumbers = Enumerable.Range(1, int.MaxValue);

int counter = 1;

while (true)

{

int number = luckyNumbers.ElementAt(counter++);

yield return number;

luckyNumbers = luckyNumbers.Where((_, index) => (index + 1) % number != 0);

}

}

but this generates:

`2,5,7,11,13,17,21,...`

which are not the lucky numbers.

Why doesn't my code work? I am trying to:

- start with all the natural numbers:

`IEnumerable<int> luckyNumbers = Enumerable.Range(1, int.MaxValue);`

int counter = 1;

- iterate through them and return the next lucky number:

`while (true)`

{

int number = luckyNumbers.ElementAt(counter++);

yield return number;

- remove all th numbers from the sequence:
`n`

`luckyNumbers = luckyNumbers.Where((_, index) => (index + 1) % number != 0);`

I don't see why this doesn't work as I intend.

Answer

There are a couple of reasons why your code does not work:

- Collections in programming are zero-indexed. This is why the first number you generate is 2, because its the number with index 1. You should rather initialize
`counter`

to 0. - I think you initialized counter to
`1`

to avoid canceling the 1-th number out of the sequence (which will effectively kill all numbers and therefore taking the element at a given position is doomed to fail). The problem lies in the definition of the lucky numbers here: While the first lucky number is 1, the first iteration is to strike every*second*number. So you have to take`Math.Min(number, 2)`

.

Finally, you arrive at the following:

```
static IEnumerable<int> LuckyNumbers()
{
IEnumerable<int> luckyNumbers = Enumerable.Range(1, int.MaxValue);
int counter = 0;
while (true)
{
int number = luckyNumbers.ElementAt(counter++);
yield return number;
int moduloCheck = Math.Max(number, 2);
luckyNumbers = luckyNumbers.Where((_, index) => (index + 1) % moduloCheck != 0);
}
}
```

From a performance perspective, though, I think the solution is horrible for large numbers as you will repeatedly check the first numbers forever at the `ElementAt`

. Because a where-expression is not indexable, this will always start checking each number for several where conditions. The good thing though is that you can simply use it as `LuckyNumbers().Take(n)`

to get the first `n`

lucky numbers.