Charlotte Foust
cfoust at infostatsystems.com
Mon Feb 23 18:14:52 CST 2009
I didn't say functions HAD to return a value, just that you had to have a function in order to return the value in this case. Actually, using ByRef for arguments passed into a sub allows you to get values back too, but it isn't as tidy. Charlotte Foust -----Original Message----- From: accessd-bounces at databaseadvisors.com [mailto:accessd-bounces at databaseadvisors.com] On Behalf Of Stuart McLachlan Sent: Monday, February 23, 2009 3:43 PM To: Access Developers discussion and problem solving Subject: Re: [AccessD] Classes and Events - EVENTS NOT REQUIRED That's the theory. In practice Functions do not *need* to return a value, you can replace "Sub" with "Function" anywhere you like, but you can only replace "Function" with "Sub" if there is no return value. I tend to use "Function" all the time, the only "Sub"s you'll see in my apps are the built in events in Forms etc. It's also good practice to "type" Functions which do return a value, otherwise you have the overhead of converting from/to variants - it also makes it clear whether a Function does in fact return a value. Here it would be preferable to use: "Function EndTimer() As Long" -- Stuart On 23 Feb 2009 at 15:16, Charlotte Foust wrote: > EndTimer has to be a function to return the elapsed time. StartTimer > doesn't need to return a value. > > Charlotte Foust > > -----Original Message----- > From: accessd-bounces at databaseadvisors.com > [mailto:accessd-bounces at databaseadvisors.com] On Behalf Of Tina Norris > Fields > Sent: Monday, February 23, 2009 2:52 PM > To: Access Developers discussion and problem solving > Subject: Re: [AccessD] Classes and Events - EVENTS NOT REQUIRED > > John, > > My question is why is StartTimer() a Sub and EndTimer() a Function? > It is not clear to me what the distinction is between a Sub and a > Function > - they both look like "methods" or "functions" as found in Java. So, > please explain that difference to me. Thanks. > > Tina > > jwcolby wrote: > > To this point the classes we have looked at were specifically > > designed > > > to allow you to "wrap" an object that generates events and add code > > and variables to process those events. This lecture will > > demonstrate > that classes have other uses and do not have to wrap other objects. > > > > The class introduced today will be clsTimer, a means of timing > > events (things happening) in your code. The class is perhaps the > > simplest class I have ever written, and perhaps the simplest class > > you will > ever see. > > > > * Click Insert / Class > > * Save immediately as clsTimer > > * Insert the following code into the class: > > > > Private Declare Function apiGetTime Lib "winmm.dll" _ > > Alias "timeGetTime" () As Long > > > > Private lngStartTime As Long > > > > Private Sub Class_Initialize() > > StartTimer > > End Sub > > > > Function EndTimer() > > EndTimer = apiGetTime() - lngStartTime End Function > > > > Sub StartTimer() > > lngStartTime = apiGetTime() > > End Sub > > > > * Compile and save the class. > > > > Notice that in the header of the class we have a function definition > > apiGetTime that calls out to Windows. This function gets the > > windows tick timer and has a resolution of 1 millisecond, or one > > thousandth of > > > a second. This simply means that we can't time anything that takes > > less than one thousandth of a second without resorting to timing it > > several times. It returns a long integer that is simply an absolute > number of "ticks". Since when? It doesn't matter, it is just "this > is the tick count RIGHT NOW". > > > > To compute a "time" (and we aren't really doing that, we are > > calculating a time since the first time), you get the tick count and > > store it, then later you get another tick and compare it to the > > first > tick. The difference is the number of 1000ths of a second since the > first tick count. > > > > Notice that we have no mInit() method in this class. Notice also > > that > > > the Class_Initialize calls the StartTimer() function. As you know > > now, the Class_Initialize is a class event that fires as the class > loads, so this tells the class to load the first tick time as soon as > the class instance loads. > > > > In the header of the class we dimensioned a long variable > > lngStartTime. This will be used to store the starting tick count. > > StartTimer() simply calls out to Windows, gets the current tick > > count > from Windows, and stores that count to lngStartTime. > > > > EndTimer() simply calls out to Windows again to get the current tick > > count, subtracts the current count to the previous count stored in > lngStartTime and returns that count to you the programmer. > > > > That's it folks! This class has in the header a function definition > > to call Windows and a place to store the count. In the body of the > > class it then has two methods to start the "timer" and to return the > ticks since the timer started. You are not going to see many classes > simpler than that. > > > > So let's discuss why we need to encapsulate this in a class. You > > might be saying that you can do the same thing without the class but > > a > > > class allows you to create as many of these timers as you want. > > Let's > build some test code to see how this thing works and why we might need > several. > > > > * In the tools menu click Insert / MODULE. We are building a > normal module this time, NOT a class > > module. > > * Immediately save the module as basTimerTest > > * Into this new module insert the following code: > > > > Function TmrTest() > > Dim lngCtr1 As Long > > Dim lngCtr2 As Long > > Dim clsTmr1 As clsTimer > > Dim clsTmr2 As clsTimer > > > > Set clsTmr1 = New clsTimer > > For lngCtr1 = 1 To 5 > > Set clsTmr2 = New clsTimer > > For lngCtr2 = 1 To 100000 > > Pi > > Next lngCtr2 > > Debug.Print clsTmr2.EndTimer > > Next lngCtr1 > > Debug.Print clsTmr1.EndTimer > > End Function > > > > Function Pi() As Double > > Dim dblPi As Double > > dblPi = 4 * Atn(1) > > Pi = dblPi > > End Function > > > > Notice that we dim two timers, then we SET the timers on the outside > > of their respective loops. As you know, the SET statement loads the > > class, at which point the Class_Initialize fires which grabs the > > first > timer tick from windows. > > > > The Debug.Print statement simply calls the .EndTimer method of the > > class and prints it to the debug window. > > > > Voila, a timer, with a resolution of one thousandth of a second. > > > > TmrTest simulates a real world code where you have two loops, and > inner loop and an outer loop. > > > > The inner loop times how long it takes to calculate Pi. Notice that > > modern computers are so fast that I have to do it a hundred thousand > > times in order to get enough "tick counts" (thousandths of a > > second) to even get a number to use. The outer loop simply times > > how long it takes to run the inner loop 5 times. > > > > I have intentionally kept this thing simple, but your outer loop > > might > > > time how long it takes to read a thousand records and the inner loop > > might be replaced with timing how long it takes to ... transform a > string from comma delimited to pipe delimited or something like that. > > > > In this lecture we have demonstrated that a class encapsulates all > > of the code required to perform its function, plus the variables > > required > > > to store its data. It also demonstrates that you can use as many > > instances of the class as you need. If you need one or a hundred > timers, you just dim and SET the variables and you are off to the > races so to speak. > > > > Classes are used to encapsulate code and data required to implement > > a system. You're your imagination is the only limit to what that > > system > can be. > > > > > -- > AccessD mailing list > AccessD at databaseadvisors.com > http://databaseadvisors.com/mailman/listinfo/accessd > Website: http://www.databaseadvisors.com > > -- > AccessD mailing list > AccessD at databaseadvisors.com > http://databaseadvisors.com/mailman/listinfo/accessd > Website: http://www.databaseadvisors.com -- AccessD mailing list AccessD at databaseadvisors.com http://databaseadvisors.com/mailman/listinfo/accessd Website: http://www.databaseadvisors.com