if List contains is null

diamond diamond

13y2.1kVisual Basic .NET

Hi all,

If I want to say history list was null matrix [][] be 0, How could be written?

the code blew gives an error.

List > history = new List >() ;

int [][] matrix= new int[5][];

if (history[y].Contains("192.168.2.160", StringComparer.OrdinalIgnoreCase))

matrix[x+4][y] = 1;

else if (history[y].Contains==null)

matrix[x][y] = 0;

thanks

9 Replies

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Vulpes13y ago
Accepted answer
Can I just check that you know how to fully initialize a two dimensional 'jagged' array.

This line on its own is not sufficient - it just tells the compiler than there are 5 single dimensional arrays within the jagged array.

int [][] matrix= new int[5][];

You have to initialize each of the 5 single dimensional arrays individually as they can be of different lengths.

So, if you want the array at index x of the jagged array to be of length 5, you'd do:

matrix[x] = new int[5];
diamond diamond13y ago
Thanks Vulpes
I correct it with this style
int [,] matrix=new int [5,1000]
diamond diamond13y ago
Vulpes,

I want to create matrix which has 5 rows and history.count columns.
for that as far as I know, it is sufficient to write, is it wrong?
Can you let me know how do I do for that?
diamond diamond13y ago
Vulpes,

int [][] matrix= new int[5][];

The exception message is as same as already message

check to determined if the object is null before calling the method
{"Object reference not set to an instance of an object."}
Vulpes13y ago
What exception are you getting?

Have you initialized matrix[x] somewhere?

matrix[x] = new int[5]; // or whatever
diamond diamond13y ago
Many thanks,

But this time for matrix[x][y] = 0; gives an exception ??!!!
Vulpes13y ago
If you're wanting to check whether the element of history at index y is either an empty List or null, you can use:

else if (history[y] == null || history[y].Count == 0)

Note that it's not possible for a List's Count property to be less than 0.
diamond diamond13y ago
Thanks, an error is solved, but code is not suitable for this work
I used history[].count<=0

this time is run but the command of if gives an exception:

else if (history[y].Count<=0)
matrix[x][y] = 0;
check to determined if the object is null before calling the method
{"Object reference not set to an instance of an object."}
Vulpes13y ago
If you want to test that the element of history at index y is null, then it's:

else if (history[y] ==null)
matrix[x][y] = 0;

However, if you want to test that the List object at index y of history contains a null value, then it's:

else if (history[y].Contains(null))
matrix[x][y] = 0;