Find the maximum of the object array

Question

So I want to find the oldest person in a school class. The age is calculated according to his personal code. For example, 120499-12345, where the first part is the date and the first number after the "-" can be 1 or 2,depending on when did person born(before 2000 - "1", after 2000 - "2"). Personal code is a string type and I used substrings to get the year from code and the 1 or 2 to calculate the age. And the problem is that I don't really understand how to find the oldest person.

        public void Oldest()
        {

            int age = 0;

                foreach (Student b in students)// students is an array of Student class
                {
                    string sub1 = b.pers_code.Substring(4, 2);
                    string sub2 = b.pers_code.Substring(7, 1);
                    int type = 0;
                    type = Convert.ToInt32(sub2);
                    int year = 0;
                    year = Convert.ToInt32(sub1);




                    if (type == 2)
                    {
                        age = 18 - year;


                    }

                    else
                    {
                        age = 2018 - (year + 1900);

                    }


                }    
        }

So the 120499-12345 is meaning that the person was born at 04 December 1999? — Samvel Petrosov
– Samvel Petrosov, Commented Apr 2, 2018 at 11:19
year is always zero in your code! Explain by a complete example how to get the age for a string code! — Bassem
– Bassem, Commented Apr 2, 2018 at 11:22
I assume 'gads' means 'age' in some langugae and we see an incomplete translation. — Henk Holterman
– Henk Holterman, Commented Apr 2, 2018 at 11:28
120499 is the date. What is the significance of this date? Without a clear description of how you go from 120499 to an age, nobody can help you. — John Alexiou
– John Alexiou, Commented Apr 2, 2018 at 14:50

score 3 · Accepted Answer · 2018-04-02 12:33:27Z

I don't really understand how to find the oldest person

Sure, there is too much going on in your method. Instead of trying to fix it, you can take a different approach and improve the general design first.

You can split your i'm-responsible-for-everything method into single-focused functions:

DateTime GetBirthDate(string personCode) { ... }

TimeSpan GetAge(DateTime birthDate) { ... }

Now you can test this logic separately.

Come to think of it, it actually seems the Student's responsibility to know his or her age.

class Student
{
    ...
    // instead of computing, you can set these once in the constructor
    public DateTime BirthDay => GetBirthDay(this.PersonalCode);
    public TimeSpan Age => GetAge(this.BirthDay);
    ...
    private TimeSpan GetAge(DateTime birthDate) { ... }
}

And then you can assemble your simple and testable building blocks into a larger solution

var oldest = students.OrderByDescending(s => s.Age).FirstOrDefault();
var maxAge = students.Max(s => s.Age);

It all got much clearer, not to mention that we're now able to easily find other stats -- average age, top-10 youngest students etc.

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