Why this code is outputting 8 instead of 5

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I am trying to beat a HackerRank problem where am required to concatenate the date elements into a single integer and check whether the number formed by the concatenation is divisible by either 4 or 7. I am using a while loop to iterate through the dates between the given dates but my code is printing 8 instead of 5 to the terminal for the test case with start date as "02-08-2025" and end date"04-09-2025". Both dates are inclusive

What I have tried:

C#

public class Solution{
static void Main(string args[]){
     //read date from the user
     var input = Console.ReadLine();
     var date1 = input.Split(new char[]{' '})[0];
     var date2 = input.Split(new char[]{' '})[1];
     //Parse date objects out of the strings
     DateTime A = DateTime.Parse(date1);
     DateTime B = DateTime.Parse(date2);
     int lucky = 0;
     while(A<= B){
           //Get a string representation of the date and    //replace the hyphens
           var str = A.ToString("dd-MM-yyyy").Replace("-","");
            var i = Int32.Parse(str);
            if(i%4==0 || i%7 ==0)
                lucky ++;
            
         //Update the day A
         A = A.AddDays(1);
        }
        Console.WriteLine(lucky);
    }

}

Posted 26-Feb-24 6:21am

Tim the Gamer

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Comments

Richard MacCutchan 26-Feb-24 12:47pm

I just ran that code and it gave 5 as the answer.

Add some print statements so you can see the values being used as the code runs. Or just run it in the debugger.

Tim the Gamer 26-Feb-24 12:56pm

When I ran it on HackerRank web portal, their output shows my code giving 8 as a response. Does their site have a bug?

0x01AA 26-Feb-24 13:04pm

I would guess it is maybe something with 'dd-mm-yyyy' vs. 'mm-dd-yyyy'
I'm pretty sure your code will print 5 when you enter "08-02-2025 09-04-2025" ;)

Tim the Gamer 26-Feb-24 13:16pm

No Hacker Rank explicitly say that the format should be dd-MM-yyyy

0x01AA 26-Feb-24 13:22pm

I mean it will show the correct result on your system. On what OS HackerRank runs I dont' know.

To be on the safe side then use explicit format definition for DateTime.Parse(date);

Something like this:
DateTime A = DateTime.ParseExact(date1, "dd-MM-yyyy", null);

Richard MacCutchan 26-Feb-24 15:00pm

How would I know. I have no idea what HackerRank is or what service it provides. But since both GNU and Microsoft provide their compilers free, I would go for a product that has millions of users.

0x01AA 26-Feb-24 13:43pm

I assume in UK it is also dd-MM-yyyy and I assume HackerRank runs on a US system. In case all my assumtptions are correct, then one needs to use DateTime.ParseExact instead of DateTime.Parse.
Now, because of my limited English, I'm not able to answer this in an understandable way :-)

Tim the Gamer 26-Feb-24 14:06pm

Thanks for the tip will sire try that method

2 solutions

Solution 2

I would suggest doing something along these lines.

C#

string[] input = "02-08-2025 04-09-2025".Split(' ');
DateTime startDate = DateTime.Parse(input[0]);
DateTime endDate = DateTime.Parse(input[1]);
int luckyNumberCount = 0;
for (DateTime date = startDate; date <= endDate; date = date.AddDays(1))
{
    var n = int.Parse(date.ToString("dd-MM-yyyy").Replace("-", ""));
    if (n % 4 == 0 || n % 7 == 0) luckyNumberCount++;
}
Console.WriteLine(luckyNumberCount);
Console.ReadLine();

Posted 26-Feb-24 9:24am

George Swan

Updated 26-Feb-24 9:25am

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0x01AA · Accepted Answer · 2024-02-26T08:06:00

I don't know Hacker Rank, that why a way too much assumptions in my answer, sorry for that. Btw. see also the comments to the question.

Assumption: Hacker Rank runs on a US OS and therefore will take e.g. for date parsing the US conventions for date which is 'mm/dd/yyyy' and by chance this works with your test interval without exception. In case you would e.g. enter '29-12-2025 31-12-2025' your current implementation in the question will throw an exception.

Hacker Rank does explicitely request to supply the date as 'dd/mm/yyyy'

Conclusion:
Instead of simply use DateTime.Parse which uses the OS settings you need to use a more specific method like DateTime.ParseExact

Finally using ...
DateTime A = DateTime.ParseExact(date1, "dd-MM-yyyy", null);
DateTime B = DateTime.ParseExact(date2, "dd-MM-yyyy", null);
... should solve your issue.

Note:
With DateTime.TryParseExact you can parse a Date without throwing an exception in case an invalid date has been typed in. It is then your job to handle this situation in code.

I hope it helps and sorry for my English ;)