I'm trying something like that:
class point
{
public int x;
public int y;
}
point[] array = new point[100];
array[0].x = 5;
and here's the error: Object reference not set to an instance of an object. (@ the last line)
whats wrong? :P
6 Answers
It only creates the array, but all elements are initialized with null.
You need a loop or something similar to create instances of your class.
(foreach loops dont work in this case)
Example:
point[] array = new point[100];
for(int i = 0; i < 100; ++i)
{
array[i] = new point();
}
array[0].x = 5;
3 Comments
Chris Shain
To add to @Skalli's answer here, if your
Point were a struct instead of a class (as the Point type is in the BCL), then you wouldn't need to worry about this. Class's need to be "new'd" when used in arrays, structs don't.
Skalli
Whoever downvoted: Care to explain what you don't like about the question and how I can improve it?
ggurov
That was not me, but your example is incorrect. One cannot assign foreach iteration variable. There have to be another loop type: for (int i = 0; i < array.Length; i++) array[i] = new point();
When you do
point[] array = new point[100];
you create an array, not 100 objects. Elements of the array are null. At that point you have to create each element:
array[0] = new point();
array[0].x = 5;
Comments
You can change class point to struct point in that case new point[500] will create an array of points initialized to 0,0 (rather than array of null's).
Comments
As the other answers explain, you need to initialize the objects at each array location. You can use a method such as the following to create pre-initialized arrays
T[] CreateInitializedArray<T>(int size) where T : new()
{
var arr = new T[size];
for (int i = 0; i < size; i++)
arr[i] = new T();
return arr;
}
If your class doesn't have a parameterless constructor you could use something like:
T[] CreateInitializedArray<T>(int size, Func<T> factory)
{
var arr = new T[size];
for (int i = 0; i < size; i++)
arr[i] = factory();
return arr;
}
LINQ versions for both methods are trivial, but slightly less efficient I believe
Comments
int[] asd = new int[99];
for (int i = 0; i < 100; i++)
asd[i] = i;
Something like that?
1 Comment
Skalli
If you are going to use an index, you better use
for (int i=0; i < asd.Length; ++i)We don't want to risk an IndexOutOfRangeException. :)Sometimes LINQ comes in handy. It may provide some extra readability and reduce boilerplate and repetition. The downside is that extra allocations are required: enumerators are created and ToArray does not know the array size beforehand, so it might need to reallocate the internal buffer several times. Use only in code whose maintainability is much more critical than its performance.
using System.Linq;
const int pointsCount = 100;
point[] array = Enumerable.Range(0, pointsCount)
.Select(_ => new point())
.ToArray()
array[0], so how can you set its value to5?