For years I have emptied arrays by just setting each element to 0 with
For loops. That's time consuming for big arrays and today when I was
looking at some old code I wrote a couple of years ago I ran into
those lines, and I thought that there must be a much better way to do
this, so I started thinking… And yes, of course there is!
How many times didn't I scratch my head when my array is suddenly
empty when I forgot ”Preserve” in ReDim? So why not just forget it by
purpose…?
Here's my example code:
REM ***** BASIC *****
Option Explicit
Sub Main
Const MaxElements=1000000
' Create an array.
Dim A(1 To MaxElements) As Integer
Dim i As Long
' Fill the array with some data.
Randomize
For i=1 To MaxElements
A(i)=Rnd*1000
Next i
' Empty the array.
Dim Time As Long
Time=GetSystemTicks()
For i=1 To MaxElements
A(i)=0
Next i
Print GetSystemTicks()-Time
' Fill it again.
For i=1 To MaxElements
A(i)=Rnd*1000
Next i
' Empty it again.
Time=GetSystemTicks()
ReDim A(1 To MaxElements) As Integer
Print GetSystemTicks()-Time
End Sub
Running this, two message boxes (from the Print command) pops upp.
Typically the first one displays around 14000 (14213 when I tested a
few seconds ago) and the second one around 600 – 800 (797 in this
case, 638 is the lowest value so far). After trying a couple of times
it seems like the ReDim statement is about 20 times faster than the
For loop.
It doesn't really matter for small arrays, but the ReDim is a
one-liner, For requires three lines.
So now I feel really stupid that I didn't think of this before…
By the way, the getSystemTicks() function seems to return the time in
ms, at least on my system (did some experiments with it a while ago).
Try this one, for example:
REM ***** BASIC *****
Sub Main
Dim A As Long, B As Long, C As Long
A=getSystemTicks()
Wait 100
B=getSystemTicks()
Wait 100
C=getSystemTicks()
Print A, B, C
End Sub
Kind regards
Johnny Rosenberg
ジョニー・ローゼンバーグ
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