Iterate through an array in an object

Question

I'm trying to iterate through the array of Grades which is initiated in the students object but get 'Object reference not set to an instance of an object'. I believed the following line for (int i = 0; i < (students[i].Grades.Length); i++) only would run three times (size of Grades array) then terminate, but it seems like its aiming to fulfill the iteration of students?:

If so, I'm kind of lost and wonder how to reach the field of the Subject instead of using this kind of iteration? Might add I'm a newbie so any advice is much appreciated!

// Iteration problems
        for (int i = 0; i < (students[i].Grades.Length); i++)
        {
            Console.Write(" {0}: ", students[i].Grades[i].Subject);
        }

Parts of the code in larger context:

class Program
{
    static void Main(string[] args)
    {
        public Student[] students = new Student[] { };
        students = new Student[5];

// Iteration problems

        for (int i = 0; i < (students[i].Grades.Length); i++)
        {
            Console.Write(" {0}: ", students[i].Grades[i].Subject);
        }
    } 
}
class Student
{
    public GradeSubject[] Grades { get; set; } = new GradeSubject[] 
    {
        new GradeSubject("Art"),
        new GradeSubject("Geography"),
        new GradeSubject("Math"),
    };
}
public class GradeSubject
{
    private string subject;
    private char letter;

    // Properties
    public string Subject { get { return subject; } set { subject = value; } }
    public char Letter { get { return letter; } set { letter = value; } }

    public GradeSubject(string _subject)
    {
        subject = _subject;
    }
}

This answers your question: [What is a NullReferenceException, and how do I fix it?](https://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-and-how-do-i-fix-it) — Ňɏssa Pøngjǣrdenlarp, Jul 14 '20 at 15:50
`students = new Student[5];` assigns a 5-element array of `Student`s to `students` but does not actually create any `Student` instances. Instead, every element of `students` is `null` until you make it otherwise. You need to do the same thing you did with `Grades`: assign some instances to its elements. — Lance U. Matthews, Jul 14 '20 at 15:56
One way I learned to fix NullReferenceEx is by stepping through the code with a breakpoint and seeing what is null. then once you find what is null set a condition to prevent it from being null or skipping it if is null depending on what you want your code to do. — MrLu, Jul 14 '20 at 15:58

score 0 · Answer 1 · answered Jul 14 '20 at 15:56

0

You're using the same index (i) for the student and the student's grade. What you really need is a nested loop, one to iterate through the students, and an inner one to loop through that student's grades:

for (int i = 0; i < students.Length; ++i)
{
    for (int j = 0; j < students[i].Grades.Length; ++j)
    {
        Console.Write(" {0}: ", students[i].Grades[j].Subject);
    }
}

Or you can use foreach loops which are arguably cleaner:

foreach (var student in students)
{
    foreach (var grade in student.Grades)
    {
        Console.Write(" {0}: ", grade.Subject);
    }
}

You also need to actually create Student objects in your student array. You can't just instantiate the array itself or all of the elements will be null.

public Student[] students = new Student[5];  // Array of 5 null elements
for (int i = 0; i < students.Length; ++i)
    students[i] = new Student();

answered Jul 14 '20 at 15:56

itsme86

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Thanks! This make it iterate through grades five times though. I only want Art, Geography and Math to be printed one time. https://i.ibb.co/ZMLjtnX/fdf.png – Sandro Jul 14 '20 at 16:14
@Sandro You have 5 students. If each student has a grade for each of those subjects, how can you only show each subject once? What is the actual end result you're looking for? If you just want 1 student, then change the `new Student[5]` to `new Student[1]`. – itsme86 Jul 14 '20 at 16:17
1

Yes you're right about that. I might need to rethink this. Cause for every student I create the grades will be displayed just as many times which is wrong... Hmm. – Sandro Jul 14 '20 at 16:26

Iterate through an array in an object

1 Answers1