\chapter Control Statements

The flow--of--control in \QS programs is controlled by control
statements, for example, \l if and \l switch, \l for and \l while. \QS
also supports exceptions with \l try and \l catch.

\section1 break

\code
label:
for ( var i = 0; i < limit; i++ ) {
    if ( condition ) {
	break label;
    }
}

switch ( expression ) {
    case 1:
	Statements1;
	break;
    default:
	DefaultStatements;
	break;
}
\endcode

This keyword is used in \l for loops, \l do loops, \l while loops and
\l{switch} statements. When a \c break statement is encountered in a
loop, control is passed to the statement following the end of the
inner-most loop that contains the \c break statement; unless the \c
break statement is followed by the name of a \e label, in which case
control passes to the statement governed by the label.

A \c break statement is usually placed at the end of each \l{case} in
a switch statement to prevent the interpreter "falling through" to the
next case. When the interpreter encounters a \c break statement, it
passes control to the statement that follows the inner-most enclosing
\c switch statement. If every \c case has a corresponding \c break,
then at most one \c case's statements will be executed. If the \c
break statement is followed by a label name (\e label) then when the
\c break is encountered, control will pass to the statement marked
with that label; this is useful, for example, for breaking out of
deeply nested loops.

Example:
\code
    red:
    for ( x = 0; x < object.width; x++ ) {
	for ( y = 0; y < object.height; y++ ) {
	    if ( color[x][y] == 0xFF0000 ) {
		break red;
	    }
	}
    }
\endcode

See \l{switch} for another example.
See also \l do, \l while, \l for and \l break.

\section1 case

\code
switch ( expression ) {
    case Value:
	Statements;
	break;
    default:
	DefaultStatements;
	break;
}
\endcode

This keyword is used in \l{switch} statements. For each possible value
that a \c switch statement's expression may evaluate to, one \c case
may be written, (but see \l{default}.) If a case's literal value (\e
Value) matches the value of the \c switch statement's \e expression,
then that \c case's statements (\e Statements) are executed.

Normally a \c case's statements are terminated by a \l{break}
statement which causes execution to pass to the end of the \c switch
statement.

See \l{switch} for an example.

\section1 catch

\code
try {
    Statements;
}
catch( exception ) {
    ExceptionStatements;
}
\endcode

This keyword is used in \l{try} statements. If an \e exception occurs,
then the \e ExceptionStatements in a matching \c catch block are
executed.

Catch blocks come in two varieties, unqualified and qualified. An
unqualified \c catch block has the form:
\code
    catch ( e ) {
	/* statements */
    }
\endcode
and a qualified \c catch block has the form:
\code
    catch ( e if e instanceOf RangeError ) {
	/* statements */
    }
\endcode

The possible exception types are:
\list
\i EvalError -- the result of a failed \l{eval()} call.
\i RangeError -- a result that is out of range, e.g. an array index to
an element that isn't in the array.
\i TypeError -- an attempt to perform an operation on an object of an
inappropriate type.
\i User defined exceptions -- exceptions that are objects of a
user-defined type.
\endlist

See \l{try} for an example.

\omit
(See also \l{Built-in Exceptions}.)
\endomit

\section1 continue

\code
for ( var i = 0; i < limit; i++ ) {
    if ( condition ) {
	continue;
    }
    Statements;
}
\endcode

This keyword is used within the context of a \l for, \l while or \l do
loop.

If a \c continue statement is encountered in a \l for loop, execution
immediately passes to the third part of the \l for loop (normally
where a counter is incremented or decremented), and then execution
continues normally, i.e. the middle part of the \l for loop (the
conditional) is tested, and if true, the body of the loop is executed.

If a \c continue statement is encountered in a \l while or \l do loop,
execution immediately passes to the conditional, which is retested;
the body of the loop will be executed if the conditional is still
true.

See \l do for an example.
See also \l do, \l while, \l for and \l break.

\section1 default

\code
switch ( expression ) {
    case 1 :
	Statements1;
	break;
    default :
	DefaultStatements;
	break;
}
\endcode

This keyword is used in \l{switch} statements. It is used instead of
\l{case} to match \e anything that the \c switch statement's
expression has evaluated to. If no \c default is used, and none of the
cases match, then the \c switch statement will not execute anything
and control will pass on to the following statement. If \c default is
used, it \e must be the last case in the \c switch statement. This is
because each \c case is evaluated in order, and since \c default
matches any value, it will always be executed if the interpreter
reaches it, and any following cases would always be ignored. When the
\c default case is encountered its \e DefaultStatements are executed.
It is customary to end a \c default statement with a \l break.

See \l{switch} for an example.

\section1 do

\code
do {
    Statements;
} while ( condition );
\endcode

This keyword is used in conjunction with \l while to form a loop which
is guaranteed to execute at least once.

The \e Statements in the braces following the \c do are executed once.
If the \l while \e condition evaluates to \c true, execution passes
back to the \c do, and the whole process repeats. If the \l while
loop's conditional ever becomes \c false, execution continues from the
statement following the \l while statement.

Example:
\code
    var x = 0;
    do {
	x += 5;
	if ( x == 50 )
	    continue;
	System.println( x );
    } while ( x < 100 );
\endcode
The example prints 5, 10, 15, ..., 45, 55, 60, 65, ..., 95 on the
console.

See also \l while, \l for, \l continue and \l break.

\section1 else

\code
if ( condition ) {
    Statements;
}
else {
    ElseStatements;
}
\endcode

The \c else keyword is used in conjunction with \l if. See \l if for
details.

\section1 for

\code
for ( i = 0; i < limit; i++ ) {
    Statements;
}
\endcode

This keyword is used to create a loop that executes a fixed number of
times.

The \c for statement is broken into parts as follows: the keyword \c
for, an opening parentheses, zero or more statements (the \e{first
part}), a semi-colon, a conditional expression (the \e{second part}),
a semi-colon, zero or more statements (the \e{third part}), a closing
parentheses, and finally a statement or block that is governed by the
\c for loop.

The first part of the \c for statement is typically used to initialize
(and possibly declare) the variable used in the conditional in the
second part of the \c for loop statement. This part is executed once
before the loop begins. This part may be empty.

The second part contains a conditional expression. The expression is
evaluated before each iteration of the loop (including before the
first iteration). If this expression evaluates to \c false, the
statement or block governed by the \c for loop is not executed and
control passes to the statement that follows. If the condition is \e
never \c true, the statement or block governed by the \c for loop will
never be executed. If the condition expression is \c true, the
statement or block governed by the \c for loop is executed, and then
the third part of the \c for statement is executed, before control is
passed back to the conditional expression with the whole process being
repeated. This part should not be empty.

The third part contains statements which must be executed at the end
of every iteration of the loop. This part is typically used to
increment a variable that was initialized in the first part, and whose
value is tested in the second part. This part may be empty.

Example:
\code
    var a = [ "a", "b", "c", "d", "e" ];
    for( var i = 0; i < a.length; i++ ) {
	System.println( a[ i ] );
    }
\endcode
See also \l do, \l while, \l continue and \l break.

\section1 if

\code
if ( expression1 ) {
    // statements1
else {
    // elsestatements
}

if ( expression1 ) {
    // statements1
else if ( expression2 ) {
    // statements2
}
// else if ...
else {
    // elsestatementsN
}
\endcode

An \c if statement provides a two-way branch. A multi-way branch is
achieved using \c{else if}s. (See also \l switch.)

\list 1
\i If the first expression, \e expression1, is \c true, then the
statements governed by that expression (\e statements1) will be
executed, after which control will pass to the statement following the
\c if block.

\i If \e expression1 is \c false, control passes to the \c else
statement. If the \c else has no following \c if, the \c else
statements (\e elsestatements) are executed, after which control will
pass to the statement following the \c if block. If the \c else has a
following \c if, then step 1 or step 2 (this step) is repeated for
that \c if statement depending on whether its expression is \c true or
\c false.
\endlist

\section1 finally

\code
try {
    Statements;
}
finally {
    finalStatements;
}
\endcode

A \c finally block is a block of code that is executed after the
governing \l try block. If no exceptions occur within the \l try
block, control will pass to the \c finally block after the last
statement in the \l try block has been executed. If an exception
occurs within the \l try block, control is passed immediately to the
\c finally block.

See also \l try.

\omit
    \section1 import

    \PRELIMINARY

    \code
    import Module;
    import Relative.Path.To.Module;
    \endcode

    This keyword is used to import a module (a \QS \File .qs file).
    Modules may class definitions.

    If the module is not in one of the \c $QTSCRIPT_PACKAGES
    directories, then it \e must be in a \c $QTSCRIPT_PACKAGES
    subdirectory (at any depth), in which case the full (case-sensitive)
    path must be given, using '.' as the path separator.

    See \l Importing for full details.
\endomit

\section1 label

\c{labelname: Statement;}

Statements may be labelled; the syntax is \e labelname followed by a
colon, followed by the \e Statement. The \e labelname is any valid
(unique) identifier.

See \l break for an example.

\section1 return

\c{return optExpression;}

A \c return statement may occur anywhere within a function, including
member functions, but not within constructors. When a \c return
statement is encountered, control leaves the function immediately, and
execution resumes at the statement following the call that invoked the
function. If the \c return statement is followed by an expression (\e
optExpression) the value of the expression is returned to the caller.

Example:
\code
    function inRange( v, min, max )
    {
	if ( v >= min && v <= max ) {
	    return true;
	}
	else {
	    return false;
	}
    }
\endcode

\section1 switch

\code
switch( expression ) {
    case Value :
	Statements;
	break;
    default :
	DefaultStatements;
	break;
}
\endcode

A \c switch statement provides a multi-way branch. The \e expression
is evaluated once, then each \l case is checked in order to find one
with a \e Value that matches the value of the \e expression. If a
match is made, the \e Statements of the matching case are executed,
after which control passes to the statement following the \c switch
block. If no matching \l case is found, and there is a \l default
case, the \e DefaultStatements are executed, after which control
passes to the statement following the \c switch block. If there is no
matching case and no \l default, no statements within the \c switch
block are executed, and control passes to the statement following the
\c switch block.

Note that if a \l default is used it \Bold must be come after all the
\l{case}s; this is because once \l default is encountered it is
treated as a matching \l case regardless of what follows.

Every \l case, and the \l default (if used) should have a \l break as
their last statement. If \l break is not present, control will "drop
through" to the following statements, which is not usually the desired
behavior.

The \e expression may be any arbitrary \Q expression that evaluates to
an object that can be strictly compared. For example, an expression
that evaluates to a \l{Boolean}, \l{Date}, \l{Number} or \l{String}
value.

Example:
\quotefile langtest.qs
\skipto QUOTE SWITCH
\skipline
\printto QUOTE SWITCH
In the example, if \c expr has the value 1, the \e doActionOne()
function will be called. But if \c expr has the value 3, both \e
doActionThree() and \e doActionFour() will be called, because case 3
doesn't have a \c break statement, and execution "falls through" to
case 4. If \c expr is not 1, "two", 3 or 4 then the \l{default} case
will be matched and \e doDefaultAction() will be executed.

See also \l break.

\section1 throw

\code
try {
    Statements;
    throw "an exception";
}
catch ( e ) {
    if ( e == "an exception" ) {
	ExceptionStatements;
    }
    else {
	OtherExceptionStatements
    }
}
\endcode

The \c throw keyword is used to raise user-defined exceptions.

Example:
\code
    function monthToName( i )
    {
	var IndexToMonth = [ "Jan", "Feb", "Mar", "Apr", "May", "Jun",
                             "Jul", "Aug", "Sep", "Oct", "Nov", "Dec" ];
	
	if ( i < 0 || i > 11 ) {
	    throw "month number out of range";
	}
	else {
	    return IndexToMonth[ i ];
	}
    }
\endcode

It is also possible to define a user-defined exception class and throw
an object of that type, e.g.
\code
    throw new AUserDefinedException( "month number out of range" );
\endcode

See also \l try.

\section1 try

\code
try {
    Statements;
}
catch ( e ) {
}

try {
    Statements;
}
finally {
}
\endcode

The \c try keyword is used to identify a statement block for which
exceptions will be caught. There are two kinds of \c try block,
\c{try...catch} and \c{try...finally}.

\Bold{try...catch}

If an exception occurs within a \c{try...catch} block, control is
passed to the first \l{catch} block. If that \c catch block does not
accept the exception, control is passed on to the next \c catch block
(if any), and so on, until there are no more \c catch blocks, in which
case the exception is passed up the call chain until an enclosing \c
catch block is found to accept it, or if none accept it, the program
will terminate.

Catch blocks come in two varieties, unqualified and qualified. An
unqualified \c catch block has the form:
\code
    catch ( e ) { /* statements */ }
\endcode
and a qualified \c catch block has the form:
\code
    catch ( e if e instanceOf RangeError ) { /* statements */ }
\endcode
See \l catch for details of the qualifiers.

\Bold{try...finally}

If an exception occurs within a \c{try...catch} block, control is
passed to the \l finally block. This is useful if you want to ensure
that something happens at the end of the block, no matter what.

Examples:

\code
    try {
	file = new File;
	file.open( filename );
	process( file );
    }
    finally {
	file.close();	
    }
\endcode
In this example, the file is always closed, even if an exception
occurred.

\code
    try {
	var a = monthToName( 11 );
	var b = monthToName( 2 );
    }
    catch ( e ) {
	if ( e == "month number out of range" ) {
	    debug( "Code error: " + e );
	}
	else {
	    throw e;
	}
    }
\endcode
In this example, the \Func monthToName() function is called to set two
variables. If the function fails, it throws an exception rather than
returns an error value, so no explicit check for an error value is
necessary. If one of the function calls failed \l debug() is called;
otherwise the exception is re-thrown so that it can be handled at a
higher level. (See \l throw for the definition of \Func
monthToName().)

\section1 while

\code
while ( condition ) {
    Statements;
}
\endcode

This keyword is used to repeat a block of code zero or more times.
When the \c while statement is encountered the \e condition is
evaluated. If the \e condition is \c true, the \e Statements in the
\c while block are executed; otherwise control passes to the statement
following the \c while block. If the \e condition is \c true, after
the \e Statements have been executed, the \e condition is again
evaluated, and the whole process repeats.

Example:
\code
    var i = 10;
    while ( i > 0 ) {
	debug( i );
	i--;
    }
\endcode

See also \l do, \l for, \l continue and \l break.

\section1 with

\code
with ( QualifyingName ) {
    Statements;
}
\endcode

This keyword is used to indicate that any unqualified names in the \e
Statements should be qualified by \e QualifyingName. 

Example:
\code
// Without with
    System.print "one ";
    System.print "two ";
    System.println "three";
// With with
    with ( System ) {
	print "one ";
	print "two ";
	println "three";
    }
\endcode

If multiple qualifying names are required, with statements can be
nested, e.g.
\code
    with ( System ) {
	with ( Math ) {
	    print abs( -4 );
	    print pow( 2, 3 );
	    print random();
	}
    }
\endcode

Forcing the interpreter to do the lookup may be slower than using the
fully qualified names.

