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Arold,
1. Please read:
Tutorial: Understanding how AFL works.
It is fundamental reading. It explains that AFL is array based
language.
2. To get close 15 days ago use Ref( C, -15 );
Buy = Ref( C, -15 ) < MA( C, 50 );
3. Your problem is explained
in the User's Guide: AFL language reference :
More information
Please read also <A
href="">Understanding
how AFL works article to learn more.
Common misunderstandings
"New if-else problem"
Question:
Why I get the syntax error when I write: if( H > Ref(H,-1) )
Answer:
if-else statement changes flow of execution (opposite to IIF function that
evaluates all arguments and works on arrays) and you can not really
writeif ( H >Ref(H,-1) )because it has no meaning. It would
translate to " If high array is higher than high array shifted one bar" (see
tutorial below). Flow control statement (such as if-else) has to get SINGLE
boolean value to make decision which execution path should be taken. If you
write H (or High) it means ARRAY (entire array).if you write H[ i ] - it
means i-th element of the array. The subscript operator [ ] allows you to access
individual array elements.Instead you should write:for( i = 1; i
< BarCount; i++ ){ if ( High[ i ] > High[ i - 1 ]
) { x[ i ] = High[ i
]; } else {
x[ i ] = Low[ i
]; } }this will translate to correct one
"for EVERY BAR 'i' assign i-th element of high array to the i-th element of x
array if i-th element of high array is higher than the previous element,
otherwise assign i-th of low array to the i-th element of x array". The rule is:
new if-else and while statements need single boolean value
(not array) to decide which execution path should be taken. If you want to use
them with arrays you have to iterate through bars using for loop (as
shown above).
On the other hand this can be implemented in single line using old-style
array operations and IIF function:
x = IIF( High > Ref( High, -1 ), High, Low );
This works because IIF operates on ARRAYS as described in the <A
href="">tutorial.As
you can see in many cases old-style AFL provides much more compact form. I
always tried to explain this advantage of AFL but only a few realised that. New
control statements should be used where it is better to use them. As I tried to
explain during last years in 80% of cases 'old-style' AFL provides the shortest
formula. Only remaining 20% of cases needed <A
href="">script.
Those 'script-only' cases now can be coded in native AFL thanks to new
for/while/if-else statements. And this is correct usage of them - to replace
script parts.
Hope this helps.
Best regards,Tomasz Janeczkoamibroker.com
----- Original Message -----
From: "Arold" <<A
href=""><FONT
size=2>arold_ite@xxxxxxxxx>
To: <<A
href=""><FONT
size=2>amibroker@xxxxxxxxxxxxxxx>
Sent: Wednesday, November 12, 2003 11:59 AM
Subject: [amibroker] Newbie, basic AFL
questions
First of all, this discussion
group (and the software) is GREAT. It's so encouraging that such a
group of like-minded peopleexist.As soon as I'm up to speed with
AFL, I will make sure tocontribute in any way that I can.Anyway, I'm
just starting out with AFL and I'm having troublegetting my head around a
few basic concepts. I should say thatI've been programming for 15
years, but that obviously hasn'thelped me here!I've trawled through
the historical discussions and also read the help files, etc, but I still
can't seem to grasp the language properly.Here are my
questions…1. Boolean ExpressionsI'm trying to write an
Automatic Analysis Script (sorry, Idon't know the correct technical term for
this) for issuing buyand sell signals.I understand that I need to
initiate a `Buy' signal byassigning a `True' Boolean expression to the
implicit`Buy'variable such as this…Buy = Close[15] >
MA(Close,50);I think I have understood this concept, but please correct
me if not.However, what I can't grasp is why AFL considers the
aboveBoolean expression to be valid in the given scenario, but it does NOT
consider it to be a valid Boolean expression in the following
scenario…if ( Close[15] > MA(Close,50) )Buy =
1;For this `If' statement, I am given the following
error…Error 3.Condition in IF, WHILE, FOR statementshas to be
Numeric or Boolean type.You can not use array here,please use [] (array
subscript operator)to access array elementsCan somebody explain to
me why AFL considers the same expression to be a valid Boolean expression in
the first example but not the second?2. Previous Closing
PriceHow do I retrieve the closing price for 15 bars ago? I want
to say… "Is this closing price of 15 bars ago greater than the50 day
moving average of today's bar?"I appreciate that the code I've given so
far is clearly wrongbecause the it's using the index 15 (i.e.
Close[15]). What Iwant to do is something like this…If (
Close[CurrentBar – 15] > MA(Close,50) )But I can't figure out how to
get a handle to the index of the current bar so that I can subtract 15 from
it?3. Boolean FunctionsHow do I write a Boolean
function?I want to write something like this…function
myBooleanFunction(){myReturnValue = 0;if ( expression )if (
expression )if ( expression )myReturnValue = 1;return
myReturnValue;}if ( myBooleanFunction() == 1 )Buy =
1;elseSell = 1;Is this the correct way to write and use a
boolean function?Thanks for your
helpArold------------------------ Yahoo! Groups
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