Arrays

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Revision as of 07:21, 3 August 2013 by Jaberwocky6669 (talk | contribs) (fixed syntaxhighlighting, oops and reworded intro.)
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An array is an container data structure which stores elements (variables) aligned in the computer's memory which are referenced by the array's identifier and an index specifying a desired element. The array concept seems complex but the concept can be easily grapsed. Why are arrays so important? The array is a fundamental data structure which is often found in a vast amount of programs.

This tutorial targets people who are beginners. To understand how arrays work, it is imperative to try out and modify the provided samples. Make sure that each concept is understood before the next concept is attempted. This tutorial assumes the use of the SciTE editor. There is a minimal version of SciTE included in the latest stable release of AutoIt (version 3.2.0.1 and above).

Declaring Arrays in AutoIt

You declare an array in the same manner as you would declare any variable in AutoIt. But when you know it is an array, you add information on how many elements you want to have in the array. This information is provided by adding brackets after the identifier and a number indicating how many elements you want the array to hold. 

 Global $arr[4] ; Will make space for 4 elements.
 Local  $arr[1] ; Will make space for 1 element.
 ;NOTE! Avoid using Dim.  Use Global or Local instead.
 Dim    $arr[3] ; Will make space for 3 elements.

Making space for one element may seem ridiculous, but further down the road we shall see how we can have arrays grow and shrink. So when we declare an array with one element, it is usually just a place holder.

In AutoIt, you can also declare an array as a normal variable and later get an array assigned to it from a function. Note about the "from a function" part: this can refer to a user-defined function or an AutoIt internal built-in function. The point is, a variable does not need to hold an array before the array is assigned to it. 

 Local $arr = StringSplit("This is my string. I want to split it in sentences.", ".")

Now to make really certain we have an array from StringSplit, we should check it with the IsArray built-in function. 

 If IsArray($arr) Then 
     ; Do work on the array
 EndIf

Assigning Data to Array Elements

When we declare the array we make some room in memory for future data. We want to assign some data to the items in the array. Now here is the catch. The array always starts at index zero. So, the first element in the array will be accessed by zero, the second element in the array is accessed at by one and so on. 

 Local $arr[3] ; Make room for three elements
 ;Assign some data
 $arr[0]="Element 1"
 $arr[1]="Element 2"
 $arr[2]="Element 3"

You can also assign all the data in one smack like this: 

 Local $arr[3] = ["element 1", "element 2", "element 3"]

This zero-based indexing is quite common in most computer languages, but it can be a source of headaches to beginners until it becomes second nature to them. For example, every time you want to loop through a range of elements and the range includes the last element, you have to subtract one from the number of items your array is holding, to get the index of the last item. I.E., An array with three elements has a last index of two.

So if you don't take zero-based indexing into consideration in your code, you may ask for something outside the memory area set aside for the array. When you do, you get an error message (Array variable has incorrect number of subscripts or subscript dimension range exceeded) and your script will cease execution.

Accessing Data in Arrays

Let's walk all elements in the previous sample: 

 Local $arr[3] = ["element 1", "element 2", "element 3"] 

 For $i = 0 to 3 - 1 ; We have an array with three elements but the last index is two.
     ConsoleWrite($arr[$i] & @LF)
 Next

Determine array size with UBound

The "3 - 1" construct used in the last sample looked strange. It is not a good idea to hard-code size like that. So lets improve our sample a little. 

 Local $iMax = 3
 Local $arr[$iMax] = ["Element 1", "Element 2", "Element 3"]
 For $i = 0 to $iMax - 1
     ConsoleWrite($arr[$i] & @LF)
 Next

Now that's a bit cleaner. It's also a lot easier to increase or decrease the size of the array.
But say you don't know the size of the array upfront because it may come in a variable size when created dynamically. 

Local $iMax 

Local $data="Element 1|Element 2|Element 3" 

; The string in data will be split into an array everywhere | is encountered 
Local $arr = StringSplit($data, "|") 

If IsArray($arr) Then
     $iMax = UBound($arr); get array size

     ConsoleWrite("Items in the array: " & $iMax & @LF)

     For $i = 0 to $iMax - 1; subtract 1 from size to prevent an out of bounds error
         ConsoleWrite($arr[$i] & @LF)
     Next
EndIf

When you run the above code you will see that $iMax is four and not three as you might have expected. The reason for this is that the developer of the StringSplit() function thought it was a good idea to use the first item (item zero) to keep a count of valid items in the array. This makes sense in many situations as you now have an array containing data with an index starting at one. So our sample code can now be rewritten like this. 

 Local $iMax

 Local $data = "Element 1|Element 2|Element 3"

 ; The string in data will be split into an array everywhere | is encountered
 Local $arr = StringSplit($data, "|")  

 If IsArray($arr) Then 
     For $i = 1 to $arr[0]
         ConsoleWrite($arr[$i] & @LF)
     Next
 EndIf

There is another good reason for keeping the count in $arr[0]. When you start to use arrays extensively, you will encounter situations where you have to create an array without knowing how many of the elements you will use. Resizing the array is a relatively expensive operation (in CPU cycles) in most languages.

Now consider our example if our initial array has reserved space for ten items but we end up only using three. In this case iterating the array using UBound will force us to check for empty elements. While iterating with $arr[0] needs no other change than maintaining the correct count in $arr[0]. 

 Local $iMax=10

 ;NOTE: We have added the count in the first element

 Local $arr[$iMax] = [3, "Element 1", "Element 2", "Element 3"]

 ;NOTE: We use the count in $arr[0] to indicate the last item and we start from index=1
 For $i = 1 to $arr[0]
     ConsoleWrite($arr[$i] & @LF)
 Next

Changing array sizes with ReDim

As arrays are critical to algorithm implementations, and in AutoIt even more so as there is no other means of grouping data, we have to understand how to let it grow and shrink. This is where the keyword ReDim comes into the picture.

Let's make an example. We want our array to hold data lines but we don't know how many items we need. We make a guess, in this case five. Now, we use that array to hold data we get from a loop with a random number of iterations. If the array is too small it should automatically be increased. Before we dump the array to output, we should adjust it to the exact size it is supposed to be. 

Local Const $iMax = 5

; NOTE: We have added the count in the first element
Local $arr[$iMax] = [0] ; Initiate the array and place a counter in the first element.

; Generate a random number between 0 and 20
Local Const $iRandom = Random(0, 20, 1)

For $i = 1 to $iRandom
	; Check that the array is big enough
	If UBound($arr) = $i Then
		; Resize the array when $i is equal to the element count in the array to prevent subscript error
		ReDim $arr[$arr[0] + $iMax]
	EndIf

	$arr[$i] = "Item " & $i ; safely add data to new index element

	$arr[0] = $i ; update the index count for future reference
Next

; Adjust the array size. This time it is probably downward to the size of
; $arr[0] + 1 (remember the first item is $arr[0])
ReDim $arr[$arr[0] + 1] 

; Now dump the results
For $i = 1 to $arr[0]
	ConsoleWrite("$arr[" & $i & "]:=" & $arr[$i] &  @LF)
Next

; Visually check that the values are sound
ConsoleWrite("Ubound($arr):=" & UBound($arr) & ", $arr[0]:=" & $arr[0] & ", $iRandom:=" & $iRandom & @LF)

Note how the array now has first been adjusted to a multiple of $iMax and in the last part adjusted down to a size matching the data items.

Multi dimensional arrays

Now what is a good explanation of an multi-dimensional array? It could be a table where you access one item in the table at a time. 

 Local  $arr[3][3] = [[1, 2, 3], [2, 3, 4], [3, 4, 5]]

 For $i = 0 to UBound( $arr, 1) - 1
   For $j = 0 to UBound($arr, 2) - 1
	  ConsoleWrite("$arr[" & $i & "][" & $j & "]:=" & $arr[$i][$j] & @LF)
   Next 

   ConsoleWrite(@LF)
 Next

You can add a number of dimensions not to exceed 64 as stated in the help file section "AutoIt>Appendix>AutoIt3 limits/Defaults". Drop me a note if you encounter any circumstances where it might be simpler to add dimensions rather than using other techniques.

Here is a four dimensional sample. You tell me how that initializer is for readability. 

 
; NOTE: The following is supposed to be all on one line
; but we use the "_" character to split it into multiple lines for readability

Local $arr[3][3][3][3] = [_
		[[[1, 2, 3], [2, 3, 4], [3, 4, 5]], [[1, 2, 3], [2, 3, 4], [3, 4, 5]], [[1, 2, 3], [2, 3, 4], [3, 4, 5]]], _
		[[[1, 2, 3], [2, 3, 4], [3, 4, 5]], [[1, 2, 3], [2, 3, 4], [3, 4, 5]], [[1, 2, 3], [2, 3, 4], [3, 4, 5]]], _
		[[[1, 2, 3], [2, 3, 4], [3, 4, 5]], [[1, 2, 3], [2, 3, 4], [3, 4, 5]], [[1, 2, 3], [2, 3, 4], [3, 4, 5]]]]

For $i = 0 To UBound($arr, 1) - 1
	For $j = 0 To UBound($arr, 2) - 1
		For $k = 0 To UBound($arr, 3) - 1
			For $l = 0 To UBound($arr, 4) - 1
				ConsoleWrite("$arr[" & $i & "][" & $j & "][" & $k & "][" & $l & "]:=" & $arr[$i][$j][$k][$l] & @LF)
			Next
		Next
	Next
Next

ConsoleWrite(@LF)

Arrays in Arrays

You may save an array in an array element (item). The thing is, there is no way to directly access that array stored in the element. You have to go through a variable to get access to the embedded array which may make your code overly complicated and difficult to debug.

Remember that there may be issues if you pass an array containing arrays into a function and the embedded array is changed inside that function. So, to conclude this, try not to embed arrays within arrays unless you absolutely need to and are prepared to do rigorous testing to make sure your code will always work as expected.

That said, here is an example. 

Local $arr[3]
Local $a1[3] = [2, "a1-1","a1-2"]
Local $a2[3] = [2, "a2-1","a2-2"]
$arr[1] = $a1
$arr[2] = $a2
$arr[0] = 2
Local $dumy

For $i = 1 to $arr[0]
    $dumy = $arr[$i]

    If IsArray($dumy) Then
        For $j = 1 to $dumy[0]
            ConsoleWrite("$i:=" & $i & ", $dumy[" & $j & "]:=" & $dumy[$j] & @LF)
        Next
    Else
        ConsoleWrite("!>Oops!, What happened? Expected an array!" & @LF)
    EndIf
Next

Passing arrays to a function

There is no hocus pocus about this. Only a few rules.

  • You can not declare the variable to hold the array in the function declaration as an array. So, users could pass on a variable. So you have to check that the variable holds an array before you do array specific operations on it.
  • You don't have to, but you should, specify the variable to hold the array as ByRef.


During the tutorial you have probably noticed that there is a lot of code that is equal in each sample. I'm especially thinking about the code we have used to output the array content. Let's make life easier and create a debug function. 

 Func dbgArray(ByRef $arr, $msg="")
     If $msg <> "" Then 
         ConsoleWrite("*** " & $msg & " ***" & @LF)
     EndIf

     For $i = 0 to UBound($arr) - 1
        ConsoleWrite("$arr[" & $i & "]:=" & $arr[$i] &  @LF)
     Next

     ConsoleWrite( "Ubound($arr)=:=" & UBound($arr) & ", $arr[0]:=" & $arr[0] & @LF)   
 EndFunc

And let's make a little function to fill our arrays with something. Note how the ArrayFiller makes sure it works on an array. 

 Func ArrayFiller(ByRef $arr)
     If IsArray($arr) Then 
         ReDim $arr[3] ;Notice we might discard content in this operation
     Else
         Local $foo[3]
         $arr = $foo
     EndIf 
     ;Fill the array
     $arr[0] = 2
     $arr[1] = "Fill 1"
     $arr[2] = "Fill 2"
 EndFunc

And finally some code using the new functions 

   Local $arr1[1]
   ArrayFiller($arr1)
   dbgArray($arr1)

This code is a test on what happens when we pass a regular variable. 

   Local $arr2
   ArrayFiller($arr2)
   dbgArray($arr2)

Passing arrays from a function

As you could observe, in the previous samples, an array will be passed back and forth with the ByRef keyword infront of the variable holding the array in the function declaration.

We could also have used the Return keyword in a function. Lets re-work the ArrayFiller function to do this rather than using a variable ByRef. 

 Func ArrayFiller2()
   Local $arr = [3, "Fill 1", "Fill 2"]
   Return $arr
 EndFunc

And now we can use the function like this 

 Local $foo = ArrayFiller2()
 dbgArray($foo)

Forum FAQ about arrays

Feel free to add questions and answers you have encountered in the forum.

Accessing Arrays

Obviously one can assign arrays to variables: 

$NewArray = $OldArray

One can also assign single array elements: 

$Array[1] = "Element 1"

Or 

$Content = $Array[1]

Comparing Arrays

You can not, however, compare complete arrays:

Local $Array1[3] = [1, 2, 3]
Local $Array2[3] = [1, 2, 3]
Local $Array3[4] = [1, 2, 3, 4]

If $Array1 == $Array2 Then ConsoleWrite("1.) $Array1 is equal to $Array2! which might be correct in some sense." & @LF); while they contain the same data, the comparison does not work.

If $Array1 == $Array3 Then ConsoleWrite("2.) $Array1 is equal to $Array3! which is incorrect." & @LF); even though they're different in size, it's incorrectly determined that they're equal.

I have the impression that such comparisons compare the memory address of the arrays instead of the array elements values. And the addresses are always different for different arrays. You have to instead, compare all elements one after the other. It might be a good idea to first compare array sizes if that can vary for both the compared arrays!

Array UDF: array.au3

AutoIt features a large list of User-Defined Functions (UDF), among which is a module supplying extra array functions. You can find a reference on those functions in AutoIt's Help file as the last main chapter named 'User Defined Functions Reference'.

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