Multidimensional array javascript with index starting not from 0

Question

I need to fill multidimensional array but not from zeroth element but from i.e. 3rd, but got errors when trying to do that:

code is:

var matrix = [ [] ];
matrix[3][0] = 10;
print(matrix[3][0]);

then got error: "Unable to set value of the property '0': object is null or undefined"

but when do same from zeroth element then it works:

var matrix = [ [] ];
matrix[0][0] = 10;
print(matrix[0][0]);

No errors here - why?

you have to initialize array from zeroth index. you can not access directly third index before initialize. — Yagnesh Agola
– Yagnesh Agola, Commented Dec 3, 2013 at 14:01
var matrix = []; matrix[3] = [10]; print(matrix[3][0]); JS doesn't have multidimensional arrays in the sense that, say, Pascal does - JS has arrays of arrays. — nnnnnn
– nnnnnn, Commented Dec 3, 2013 at 14:12

Pointy · Accepted Answer · 2013-12-03 14:00:52Z

3

When you create your initial array:

var matrix = [ [] ];

you've got an array with a single zero-element array in it. At that point, matrix[3] is undefined.

You can initialize your matrix in several ways, depending on the nature of your problem. Here's one:

var matrix = [];
for (var i = 0; i < 10; ++i)
  matrix[i] = [];

Now you've got 10 rows, each one empty.

answered Dec 3, 2013 at 14:00

Pointy

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2 Comments

user1337 Over a year ago

Well I have one more question. In case when I need to use more than one matrix array element then I have to initialize all of them in the loop or somwhere in the code initialize each matrix array element as array?

Pointy Over a year ago

@user2968398 I'm not sure I understand you completely, but yes - before you use an element of any array as an array, it must be initialized. You don't have to do it ahead of time - you can check before trying to use it, and initialize it at that time.

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