Declare an array of queues

Question

What is the language grammatical problem in my code? I want to declare an array of queues. Is this the right way to declare and use them?

   public static void Main(string[] args)
    {
        Queue<int>[] downBoolArray = new Queue<int>[8]();
        downBoolArray[0].Enqueue(1);
    }

As the error tells you, you cannot put the parentheses after the closing `]` - though removing the parentheses (`new Queue[8]`) will give you an array of `null` references — UnholySheep, Feb 09 '18 at 15:16
Your array consist of null object. First instantiate the queue's in the array. — Ravenix, Feb 09 '18 at 15:16
`Queue[] downBoolArray = new Queue[8]; downBoolArray[0] = new Queue(); downBoolArray[0].Enqueue(1);` — Leo, Feb 09 '18 at 15:21

Stephen Kennedy · Accepted Answer · 2018-02-09T17:54:52.040

Your first problem is a syntax error: new Queue<int>[8]() should be new Queue<int>[8].

Once declared with the correct syntax, when you attempt to use an element of the array (downBoolArray[0].Enqueue(1)) you will encounter a NullReferenceException because array elements initialise to their default values which in the case of a reference type is null.

You could instead initialise your array with non-null seed values using a single line of LINQ:

Queue<int>[] downBoolArray = Enumerable.Range(1,8).Select(i => new Queue<int>()).ToArray();

The arguments to Range specify that we need 8 'entries' in our sequence; the Select statement creates a new Queue<int> for each item; and the ToArray call outputs our sequence as an array.

santosh singh · Answer 2 · 2018-02-09T15:24:02.467

0

You need to initialize each element in your array

void Main()
{
    Queue<int>[] downBoolArray =new Queue<int>[10];

    for (int i = 0; i < downBoolArray.Length; i++)
        downBoolArray[i] = new Queue<int>();
    downBoolArray[0].Enqueue(1);
}

edited Feb 09 '18 at 15:24

answered Feb 09 '18 at 15:17

santosh singh

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please add what your code is doing and why, otherwise it wouldn't add much. – Ravenix Feb 09 '18 at 15:18
Thanks A lot!!!!!!!!!!!!!!!!!!!!!!!! – deepsigner Feb 09 '18 at 15:18
You mean `downBoolArray.Length;` instead of `downBoolArray.Count();` – Dennis_E Feb 09 '18 at 15:19
1

@Ravenix : Added the explanation – santosh singh Feb 09 '18 at 15:19
@Dennis_E : Length will also work – santosh singh Feb 09 '18 at 15:21
1

@Dennis_E An array is an IEnumerable, so both Length and Count() work in this case. – SimonC Feb 09 '18 at 15:21
`Count()` also works, but that is not the point. `Count()` does unnecessary work. We always use `.Length` when accessing the number of items in an array. – Dennis_E Feb 09 '18 at 15:23
@Dennis_E:Updated the answer – santosh singh Feb 09 '18 at 15:24
1

@Dennis_E In case the the underlying collection implements `ICollection` which array does, it is not more costly to call `Count()` than `Length` – Magnus Feb 09 '18 at 15:34
@Magnus Still doesn't matter. `Count()` still does an unnecessary cast. We use `.Length` to access the number of items in an array. There's no reason to use `.Count()`. It's weird. – Dennis_E Feb 09 '18 at 15:39

SimonC · Answer 3 · 2018-10-12T08:57:50.273

-2

You've created an array of null values.

What you want is something like this:

public static void Main(string[] args) {

    var queues = new Queue<int>[8];

    // Possibly some other stuff

    // Initialise all values
    for (var i = 0; i < queues.Length; i++) {
        // Accounting for maybe already sporadically initialising values
        queues[i] = (queues[i]) ?? new Queue<int>();

    }
    // Do whatever

}

edited Oct 12 '18 at 08:57

answered Feb 09 '18 at 15:19

SimonC

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2

That `foreach` loop is not going to work. – Dennis_E Feb 09 '18 at 15:27

Declare an array of queues

3 Answers3