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Hello,
Please note however that IIF is a *function* and it
does not work like
if-else statement.
In other words IIF function evaluates ALL
parameters and does not change
the program flow.
IIF evaluates all arguments and RETURNS the value
that needs to be assigned
to some variable.
Quite opposite if-else statement provides control
flow and skips
entirely parts that do not match
condition.
See "COMMON misunderstandings" section of
<FONT face=Arial
size=2>http://www.amibroker.com/guide/a_language.html
Conditional function IIF()
The iif() function is used to create conditional
assignments. It contains three parameters as shown in the following
example. dynamicrsi = IIf( Close > MA(C,10), RSI(9), RSI(14) );
The above "iif" statement reads (in English) as follows: If today's close is
greater than today's 10-day simple moving average of the close, then assign a
9-day RSI to the dynamicrsi variable, otherwise, assign a 14-day RSI.
The next formula assigns “positive volume” to volresult variable if the
close is greater than the median price. Otherwise, "negative volume" is
assigned. volresult = IIf( Close > (High+Low)/2, Volume, -Volume );
If you simply want an expression to be evaluated as either true or false, it
can be done without the use of the iif() function. The following formula will
result in either a 1 (true) or a 0 (false):result = RSI(14) > 70;
The same done with iif() gives the same results, but the formula is longer.
result = IIf(RSI(14) > 70, 1, 0 );
Please note that IIF is a function - so the result of evaluation is returned
by that function and should be assigned to some variable.
IIf always evaluates both TRUE_PART and FALSE_PART, even though it returns
only one of them. Because of this, you should watch for undesirable side
effects. IIF function is NOT a flow-control statement. If you
need flow control (conditional execution of some code parts) you should look for
if-else conditional statement described later in this
document.
The following example shows one common error made with IIF
function:
IIf<FONT
color=#000000>( condition, result = 7<FONT
color=#000000>, result = 9<FONT
color=#000000> ); // THIS IS
WRONG
Correct usage is:
result = <FONT
color=#0000ff>IIf( condition, <FONT
color=#ff00ff>7, <FONT
color=#ff00ff>9 );
/* 7 or 9 is *returned* and assigned to result variable depending on
condition */
if, else Statements
if( expression )statement1 [else
statement2]
The if keyword executes statement1 if expression is true
(nonzero); if else is present and expression is false (zero), it
executes statement2. After executing statement1 or
statement2, control passes to the next statement.
Example 1if ( i > 0 )
y = x / i;
else
{
x = i;
y = abs( x );
}
In this example, the statement y = x/i; is executed if
i is greater than 0. If i is less than or equal to 0,
i is assigned to x and <FONT
face="Courier New">abs( x ) is assigned to y.
Note that the statement forming the if clause ends with a semicolon.
When nesting if statements and else clauses, use braces to
group the statements and clauses into compound statements that clarify your
intent. If no braces are present, the compiler resolves ambiguities by
associating each else with the closest if that lacks an
else.
Example 2if ( i > 0 ) /* Without braces */
if ( j > i )
x = j;
else
x = i;
The else clause is associated with the inner if statement in
this example. If i is less than or equal to 0, no value is assigned
to x.
Example 3if ( i > 0 )
{ /* With braces */
if ( j > i )
x = j;
}
else
x = i;
The braces surrounding the inner if statement in this example make the
else clause part of the outer if statement. If i is
less than or equal to 0, i is assigned to x.
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
write<FONT
color=#800000>if ( <FONT
color=#000000>H ><FONT
color=#0000ff>Ref(<FONT
color=#000000>H,-<FONT
color=#ff00ff>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:<FONT
color=#800000>for( i = <FONT
color=#ff00ff>1; i < <FONT
color=#000000>BarCount; i++ ) {
if<FONT
color=#000000> ( High<FONT
color=#000000>[ i ] > High<FONT
color=#000000>[ i - 1 ] )
{ x[ i ] =
High[ i ];
} <FONT
color=#800000>else {
x[ i ] = <FONT
color=#000000>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<FONT
color=#000000>( High<FONT
color=#000000> > Ref(
High, -<FONT
color=#ff00ff>1 ), <FONT
color=#000000>High, <FONT
color=#000000>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.Best regards,Tomasz
Janeczkoamibroker.com
----- Original Message -----
From: "Anthony Faragasso" <<A
href=""><FONT face=Arial
size=2>ajf1111@xxxxxxxx>
To: <<A
href=""><FONT face=Arial
size=2>amibroker@xxxxxxxxxxxxxxx<FONT face=Arial
size=2>>
Sent: Wednesday, August 04, 2004 2:51
PM
Subject: Re: [amibroker] IIF
statement
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