What's wrong with Multidimensional Array?

[Dominik]

--- Example 1
var myarray=[[],[]];
myarray[0][1]="NA"
print(myarray[0][1]); // output "NA"

--- Example 2
var myarray=[[],[]];
for (var x = 0; x < 5; x++) {
for (var y = 0; y < 5; y++) {
myarray[x][y]=0;
}
}

for (var x = 0; x < 5; x++)
print(myarray[x]); // output error myarray[x] is undefined !?!

What's wrong with example 2 ????

 

[Dominik]

var myarray=[[],[]];

myarray now has (empty) arrays at index 0 and 1.

--- Example 1.1
var myarray=[[],[]];
myarray[10][10]="NA"
print(myarray[10][10]); // output error myarray[10] is undefined

OMG What a mistake-a to make-a.
Thank you!

[snip]

At the third iteration, you're trying to set myarray[2][0] = 0.
myarray[2] is undefined, so you get an error.

You could create the child arrays when you need them, instead of doing
it in advance:

OK, I see it now.
But I'm curius, is it possible to create empty multidimensional array in
advance?

 

[Dominik]

Well, strictly speaking there's no such thing as a multidimensional
array in JS. There are only arrays of arrays. The two are often used in
a similar way, but there are differences. One of them is that there's no
way to initialize the child arrays with a simple statement*.

Thank you very much for your help.

 

[Thomas 'PointedEars' Lahn]

In JS 1.7, you might use array comprehension to make the initialization
more concise:

var myarray = [[] for each (i in range(0, 4))];
// assuming a suitable range() generator function

Not very portable, though. If you need such arrays a lot, you may want
to define a function to handle the initializations.

Given that, according to my current research, Array comprehension is *only*
supported in (Mozilla) JavaScript 1.7 and later, I do not think it deserves
the attribute "portable" at all.


PointedEars

 

[Evertjan.]

Thomas 'PointedEars' Lahn wrote on 20 feb 2012 in comp.lang.javascript:

Given that, according to my current research, Array comprehension is
*only* supported in (Mozilla) JavaScript 1.7 and later, I do not think
it deserves the attribute "portable" at all.

I fail to comprehend the meaning of such atributable comprehension, Thomas.

Could you divulge?

 

[Dr J R Stockton]

In comp.lang.javascript message <[email protected]

OK, I see it now.
But I'm curius, is it possible to create empty multidimensional array in
advance?

One can assign a value to a variable which does not already exist.

For counting purposes, it would be useful to be able to increment a
variable which currently is value=undefined or is non-existent, with the
pre-increment value taken as either 0 or "", depending on the nature of
the increment. Possibly with X+++ and X ++= 7.

 

[Scott Sauyet]

Thomas 'PointedEars' Lahn wrote on 20 feb 2012 in comp.lang.javascript:


I fail to comprehend the meaning of such atributable comprehension, Thomas.

<https://developer.mozilla.org/en/JavaScript/Guide/
Predefined_Core_Objects#Array_comprehensions>

-- Scott