Method to "convert" odd numbers to even numbers using a specific sequence

I’m recreating an application that has been done in Visual Basic 6.0 in C#, the problem is that I don’t have the source code, and there’s a method that reads a text file that could contain even numbers or odd numbers.

If the input value is an even number, does nothing, but if the input value is an odd number, it does something that I can’t understand, doing some tests I could find a sequence:

What formula should I use to do this?

>Solution :

This produces the desired output:

for (int i = 1; i < 50; i += 2) {
    Console.WriteLine($"{i} -> {(i + 1) / 4 * 4}");
    if (i % 10 == 9) {
        Console.WriteLine();
    }
}

Explanation: We are using integer arithmetic, i.e., the result of the division is truncated. For i = 5 we get (5 + 1) / 4 = 1, i.e., trunc(1.5) = 1 then 1 * 4 = 4.

For i = 7: (7 + 1) / 4 = 2, then 2 * 4 = 8.

Whenever the last digit of i is 9 we append an empty line to create the groups of 5 lines. % is the modulo operator. It is complementary to the integer division and yields the remainder of the division, e.g., 6 % 4 = 2. We can reverse the integer division like this: 6 / 4 * 4 + 6 % 4 = 6.