Proper way of incrementing an array's size by 1

[TheDcoder]

Hello , I know about ReDim but I am confused

Local $aArray[4] = [0, 1, 2, 3]

Func ExpandArray(ByRef $aArray)
    ; How?
EndFunc

Thanks in Advance, TD

[Jos]

Confused about what?
Did you open the helpfile on Redim and try it?
What didn't work?

Jos
 

[TheDcoder]

@Jos I can't understand what is this:

ReDim $aArray[subscript 1]...[subscript n]

[TheDcoder]

Sorry for posting this in GUI help section

[Jos]

I'll move it.

So can in the mean time play a little with ReDim so you understand it.

Jos

[TheDcoder]

I found out my answer

Local $aArray[4] = [0, 1, 2, 3]

Func ExpandArray(ByRef $aArray, $iExpandCount)
    ReDim $aArray[UBound($aArray) + ($iExpandCount - 1)]
EndFunc

 

[Jos]

I knew you would, you just need a little push to sort it out by yourself in stead of always asking everything that looks difficult.

Jos

Edited May 30, 2015 by Jos

[TheDcoder]

Thanks!

[Melba23]

TheDcoder,

If you do not know the final size of an array, incrementing it by one each time you add an element is not the best way to go as ReDim is a very slow function. Look at the final example in the Recursion tutorial in the Wiki and the "A little added extra" section below it to see how best you can manage this situation.

M23

[TheDcoder]

@Melba23 Multiply it?

[Melba23]

TheDcoder,

Did you read it the explanation?

ReDim is among the slowest of the AutoIt functions, so you want to limit its use as much as possible. If we were to increase the array size by just the one element each time [...], we would slow down the function enormously [...]. Instead of adding a single additional element we double the array in size if it is already full to make sure we get plenty of extra space. You will need to have a count variable available to do this - so why not in the [0] element as is the case for many AutoIt arrays? Just remember that if you want to use the array subsequently [...] you will need one final ReDim to get rid of any unused elements left over after the last increase in size.

So yes, you set a reasonable initial size and then multiply the array size by 2 each time you fill it - getting rid of any unused empty elements once you have got the array to the final size.  It really does work - give it a try.

M23

[TheDcoder]

@Melba23 Will implement that Melba

[TheDcoder]

I found this awesome function called _ArrayAdd which does the same thing as ExpandArray... I don't know about internal workings of it but It does the job (internal workings should be fine b/c its coded by AutoIt pros)

 

TD

[Melba23]

TheDcoder,

If you look inside _ArrayAdd you will see that in essence it is almost the same as your function - and so it suffers the same problem (lots of slow ReDim calls) if you use it to add lots of single elements in a loop. As explained above, the best way to deal with an array of unknown final size increasing by one element at a time is a "large increase in size" algorithm such as the "doubling" one I used in the Recursion tutorial.

M23

[czardas]

I found this awesome function called _ArrayAdd which does the same thing as ExpandArray... I don't know about internal workings of it but It does the job (internal workings should be fine b/c its coded by AutoIt pros)

 

TD

​Coded by Melba and working quite nicely too. You should calculate the size of the array you need before creating it if that is possible. Otherwise there are various approaches you could adopt depending on the situation. I differ slightly with Melba on one thing though: doubling the size of an array is not always recommended, especially when dealing with large arrays. Sometimes it's better to make an educated guess at the maximum size when using ReDim. Doubling is a good idea in certain circumstances as is the case with the example Melba mentioned.

Edited May 31, 2015 by czardas

[TheDcoder]

@Melba23 I see... ReDim works fine for me in this case (ReDim take 0.01 ms to expand an array by 1 with 200 elements ). So I will stick to _ArrayAdd because I need to expand an array only ~10 times

 

TD

[jchd]

So it's OK in this case. Otherwise, I'd say an expansion factor of ~1.5 is mostly correct in a large number of cases. Doubling the size just seems a little bit too much in my limited experience.

[TheDcoder]

@jchfd Yeah, 2x seems too much

[jchd]

Here are the first 20 iterations for geometric progressions by 1.5 and 2, both starting at 1:
 

1, 2, 3, 5, 8, 12, 18, 27, 41, 62, 93, 140, 210, 315, 473, 710, 1065, 1598, 2397, 3596, ...
1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192, 16384, 32768, 65536, 131072, 262144, 524288, ...

As always the success of a strategy depends on your precise context.

[TheDcoder]

An array with 524288 elements (all empty) would take 3.4 MB of RAM