Code for identifying negative and positive even/odd number not working

Question

I'm trying to write a code that tells me whether a number is negative/positive, even/odd. For example:

given -2 -> e -1,
given -3 -> o -1

and so on. The thing is it's working for everything expect negative odd numbers.

Here's the part of the code that's responsible for deciding:

if (number % 2 == 0 && number < 0)
{
    Console.WriteLine("e" + " " + "-1");
}
else if (number % 2 == 0 && number > 0)
{
    Console.WriteLine("e" + " " + "1");
}
else if (number % 2 == 1 && number > 0)
{
    Console.WriteLine("o" + " " + "1");
}
else if (number % 2 == 1 && number < 0)
{
    Console.WriteLine("o" + " " + "-1");
}
else if (number == 0)
{
    Console.WriteLine("0");
}

Does this answer your question? [How do I check if a number is positive or negative in C#?](https://stackoverflow.com/questions/4099366/how-do-i-check-if-a-number-is-positive-or-negative-in-c) and https://stackoverflow.com/questions/18818680/testing-if-a-list-of-integer-is-odd-or-even both have information pertaining to your issue. — Trevor, Nov 14 '20 at 18:56
`if (number == 0) Console.WriteLine("0") else Console.WriteLine($"{(number % 2 == 0 ? 'e' : 'o')} {(number < 0 ? -1 : 1)}")` — Dmitry Bychenko, Nov 14 '20 at 20:07

Codeek · Answer 1 · 2020-11-14T18:58:59.710

0

if it is a negative no. it's remainder is going to be negative as well when divided by 2 so :

else if (number % 2 == -1 && number < 0)
{
    Console.WriteLine("o" + " " + "-1");
}

or directly

else if (number % 2 == -1)

edited Nov 14 '20 at 18:58

answered Nov 14 '20 at 18:51

Codeek

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Dmitry Bychenko · Answer 2 · 2020-11-14T20:22:47.467

Remainder computation can vary. In math when we compute

  r = a mod b

we, usually, assume that r in [0 .. |b|) range, i.e. r is non negative. However, in case of modulo arithmetics we can add k * |b| (where k is arbitrary integer) term. For instance

 -5 mod 3 == -8, -5, -2, 1, 4, 7 etc.
                      ^  ^
              C# choice  |
                         Typical math choice: r in [0 .. 3) range

In case of c# implementation we have -2, not 1. So the safe method to test for being odd / even is to compare with 0, not -1, 1:

 if (number == 0) 
   Console.WriteLine("0"); // 0 - special case 
 else 
   Console.WriteLine($"{(number % 2 == 0 ? 'e' : 'o')} {(number < 0 ? -1 : 1)}");

Please, note number % 2 == 0 test bor being even / odd

Code for identifying negative and positive even/odd number not working

2 Answers2