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Here is the reply from tech support on the PREV function.
Hope it helps.
Lionel Issen
lissen@xxxxxxxxx
----- Original Message -----
From: <formulas@xxxxxxxxx>
To: <lissen@xxxxxxxxx>
Sent: Tuesday, March 27, 2001 4:30 PM
Subject: RE: More PREV Stuff [FOR2001032600000111]
> To enable us to provide more efficient support, use the "Reply" feature in
your email program when responding to this email. Please send unrelated
issues in a new email.
>
> Your email was responded to by:
> William
> Equis Customer Support
> http://www.equis.com
>
>
> William's response is below the following line.
> -----------------------------------------------------------------------
>
>
> The +PREV-PREV logic is a way of forcing MetaStock to dynamically
> calculate some value that is normally static. The formula is suppose to
> sum the differences between the current value of pr and a series of
> previous values. I could have manually written each days value, but it
> would have been ugly to type and would have locked in a set time frame.
>
> The alterative was to nest the part of the equation that needed to
> change into the logic of:
>
> LastValue( data array +PREV-PREV )
>
> The LastValue() function forces MetaStock to see it as a constant value
> equal to its last calculated value in the chart. This effect also
> allows the second part to work.
>
> The PREV function forces MetaStock to recalculate the formula to find
> the formula's value for the previous bar. To illustrate, consider the
> following formula:
>
> ( H-L ) - PREV
>
> Because of the PREV function refers to the previous bar of data, this
> formula has no value for the first record loaded ( there is no previous
> bar to refer to ).
>
> On the second bar, since the formula could not be calculated on the
> first bar, PREV has the value of 0 so the formula is calculated as the
> High minus the Low. For this example, assume the result is 5.
>
> On the third bar of data, the High minus the Low is calculated again.
> This time the result is 4. Now the value of the formula for the
> previous bar is recalculated to get the result 5 and that number is
> subtracted from 4. The result of the formula for this bar is -1.
>
> On the fourth bar, the High minus the Low yields a result of 3. Now we
> subtract the previous value of the formula, -1, to get the final result
> of 4; ( 3 - (-1) ).
>
> The fact that PREV forces the recalculation negates the LastValue()
> function's only looking at the last bar. LastValue() returns the last
> calculated value, but PREV keeps changing which value is the last one
> calculated. Since the formula does not include the previous value in
> its calculation, we can negate the PREV function by subtracting it and
> adding it at the same time.
>
> Looking back at the formula you asked about:
>
> coef:=Sum(Power(Ref(LastValue(pr+PREV-PREV)-pr,-1),2),ti);
>
> The formula is suppose to take a certain time frame, say 5 days. Over
> those 5 days, it totals the square of the value of pr from 5 days ago
> minus it value today; plus the square of the value of pr from 4 days ago
> minus it value today; plus the square of the value of pr from 3 days ago
> minus it value today; plus the square of the value of pr from 2 days ago
> minus it value today; plus the square of the value of pr from 1 day ago
> minus it value today. By using the LastValue() +PREV-PREV trick, each
> of the previous values of pr is dynamically calculated for however many
> periods are being used.
>
> I know this concept can be confusing and I hope I explained it well. If
> any part is confusing, please respond and I will try and answer any
> questions you may still have
>
>
>
> -----Original Message-----
>
> This was posted to the listserver.
>
> Can you clarify the problem and give the correct usage of the PREV
> function?
>
> Lionel Issen
> lissen@xxxxxxxxx
> ----- Original Message -----
> From: "C.S." <csaxe@xxxxxxxxxxx>
> To: <metastock@xxxxxxxxxxxxx>
> Sent: Friday, March 23, 2001 4:49 PM
> Subject: Re: More PREV Stuff
>
>
> > Yes, I realize that, but addition and subtraction is on the same
> precedence
> > level. However I assume that (pr+PREV-PREV) should be the same as
> (pr),
> > because
> > Ref(LastValue(pr+PREV-PREV)-pr,-1) =
> Ref(LastValue(pr-PREV+PREV)-pr,-1).
> >
> >
> > In examining the equation, I found another interesting phenomena,
> >
> > pr:=CLOSE;
> > Ref(LastValue(pr+PREV-PREV)-pr,-1)
> >
> > Is not the same as:
> >
> > Ref(LastValue(CLOSE+PREV-PREV)-CLOSE,-1)
> >
> > Even though
> >
> > Ref(LastValue(C)-C,-1) = Ref(LastValue(pr)-pr,-1)
> > and
> > LastValue(pr+PREV-PREV)-pr = LastValue(C+PREV-PREV)-C
> >
> > .....the "plot" thickens....
> > -Corey
> >
> >
> >
> > ----- Original Message -----
> > From: <wooglin.org@xxxxxxxxxxx>
> > To: <metastock@xxxxxxxxxxxxx>
> > Sent: Friday, March 23, 2001 10:49 AM
> > Subject: Re: More PREV Stuff
> >
> >
> > > I suspect the answer lies in the "Operator Precedence" in
> conjunction
> with
> > > the PREV function. If Operator Precedence is not involved, you must
> > > recognize that the PREV constant cannot be treated in a conventional
> > > algebraic context like any other constant or variable. That is, you
> cannot
> > > multiply or divide through an expression to eliminate a PREV. For
> instance
> > > (PREV(C+PREV) is not the same as C*PREV + PREV^2 or (Pr+PREV-PREV)
> is
> not
> > > the same as Pr.
> > >
> > > Jim Barone
> > >
> > > ----- Original Message -----
> > > From: C.S. <csaxe@xxxxxxxxxxx>
> > > To: MetaStock List <metastock@xxxxxxxxxxxxx>
> > > Sent: Friday, March 23, 2001 1:11 PM
> > > Subject: More PREV Stuff
> > >
> > >
> > > > Just when I thought that I was beginning to figure out Metastock
> formula
> > > > language, the following was listed in Apr 01 TASC Traders' Tips as
> the
> > > > solution to Ehler's nonlinear filter article. The first one is
> easy to
> > > > figure out from the article.
> > > >
> > > > {Ehlers Filters}
> > > > ti:=15;
> > > > pr:=MP();
> > > > coef:=Abs(pr-Ref(pr,-5));
> > > > Sum(coef*pr,ti)/Sum(coef,ti)
> > > >
> > > >
> > > > This one works, but has me stumped as to how it works according to
> the
> > > > article. You really have to read the article to understand what is
> being
> > > > calculated here.
> > > >
> > > > {Distant Coefficient Ehlers Filter}
> > > > ti:=15;
> > > > pr:=MP();
> > > > coef:=Sum(Power(Ref(LastValue(pr+PREV-PREV)-pr,-1),2),ti);
> > > > Sum(coef*pr,ti)/Sum(coef,ti)
> > > >
> > > > The two PREV would appear to be just adding a number and then
> > subtracting
> > > a
> > > > number, leaving "pr" but that isn't the case. The formulas were
> written
> > by
> > > > Cheryl Abram of Equis. It would be nice if someone at Equis would
> > consider
> > > > having her give a us a quick class on how this works and other
> formula
> > > > construction tips.
> > > >
> > > > I came up with:
> > > >
> > > > ti:=15;
> > > > pr:=MP();
> > > > coef:=Sum(Power(LastValue(pr)-Ref(pr,-1),2),ti);
> > > > Sum(coef*pr,ti)/Sum(coef,ti)
> > > >
> > > > This works too, but doesn't follow rapid price changes as quickly
> as
> > > > Cheryl's formula.
> > > >
> > > > Any ideas on how the two PREV act together?
> > > >
> > > > -Corey
> > > >
> > > >
> > > >
> > > >
> > >
> > >
> >
> >
> "MetaStock, Equis, OptionScope, and QuoteCenter are registered trademarks
of Equis International. Achelis Binary Wave, The DownLoader, Expert
Advisor, The Explorer, Visual Control, and Smart Charts are trademarks of
Equis International."
>
>
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