How can I initialize an array of arrays (multidimensional) in PowerShell 5.0

Question

I'm trying to partition out chunks of work using PowerShell (5.0) and am having a hard time instantiating a multidimensional array.

$n = 456;
$MaxChunks = 6;

$Chunks = @();

for($x = 0; $x -lt $MaxChunks; $x++)
{    
    Write-Host "Creating chunk $x"    
    $Chunks += @();
}

$Chunks.Count always returns 0 and I cannot access anything in $Chunks by index (i.e. $Chunks[0] is null).

Ultimately, my goal is to access the array located at $Chunks[$i] and add multiple System.Data.DataRow objects to it. However, as I've said, I'm not able to access the array at that index because that array is never instantiated.

I've read through this and this but am not quite able to translate the hashtable scenario to my situation.

Did you see this answer from your second link? That's the probably the information you need. — briantist
– briantist, Commented Jan 31, 2017 at 18:36
@PetSerAl, it'll reuse the same object instance. Although it won't be a problem if the OP uses += to populate those sub-arrays since += recreates the array. — woxxom
– woxxom, Commented Jan 31, 2017 at 19:39
@wOxxOm Zero length arrays are effectively read-only, so (apart from reference equality) it does not matter if it reused or not. — user4003407
– user4003407, Commented Jan 31, 2017 at 19:41

Simon Catlin · Accepted Answer · 2017-01-31 18:45:45Z

5

Alternatively:

[System.Array] $chunks = [System.Array]::CreateInstance( [Int32], 3, 3 );

$chunks[0,0];
0

Not native PS, but works.

answered Jan 31, 2017 at 18:45

Simon Catlin

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Comments

woxxom · Accepted Answer · 2017-02-01 18:40:00Z

5

Replicate an empty array.
The same object is referenced in each element but in this particular case it's not a problem because standard arrays are read-only and recreated when elements are added via +=.
```
$Chunks = ,@() * $MaxChunks
```
Collect foreach output:
```
$Chunks = @(foreach ($x in 1..$MaxChunks) { ,@() })
```
The outer @() handles a theoretically possible case when $MaxChunks = 1.
You can use a more verbose (arguably slower) Write-Output -NoEnumerate:
```
$Chunks = @(foreach ($x in 1..$MaxChunks) { echo -NoEnumerate @() })
```

And in case the sub-arrays will be modified a lot, use ArrayList instead of +=:

$Chunks = @(foreach ($x in 1..$MaxChunks) { ,[Collections.ArrayList]@() })

and later:

$Chunks[0].Add($something) >$null

P.S. Don't use += to generate entire arrays from scratch regardless of its size as it's a terrible method that recreates and copies the entire array each time; there's a much faster and simpler output collection via loop statements such as foreach, for, while; there's ArrayList object for the case of frequent non-sequential modifications of an array.

edited Feb 1, 2017 at 18:40

answered Jan 31, 2017 at 18:57

woxxom

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4 Comments

user4003407 Over a year ago

BTW, why do you use echo -NoEnumerate instead of ,?

woxxom Over a year ago

Mainly because it's more verbose so less chance for a typo/overlook. Also, internally ,@() creates two arrays, the outer one is unrolled (the overhead is negligible though so I guess I'll add it in the answer).

user4003407 Over a year ago

Also, internally ,@() creates two arrays The same applies for echo -NoEnumerate @() too. And it also have overhead of instantiating Write-Output cmdlet, parameter binding, etc.

woxxom Over a year ago

Sounds convincing.

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