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<H3>Nested control structures and the special variables start and end</H3>
The way in which compound events are describedin a score is by nesting
control structures representing low-level events within higher level ones.
The following (trivial) script illustrates this technique, at the same time as
introducing two special variables called <CODE>start</CODE> and <CODE>end</CODE>, which
play a central role in describing compound events:
<P><PRE>
Audio rate: 44100;
Init:
...
Score 2 secs:
From 0 secs to 1 secs:
At start:
Print "For interval 0-1 seconds start=", Time, newline;
...
At end:
Print "For interval 0-1 seconds end=", Time, newline;
...
...
From 1 secs to 2 secs:
At start:
Print "For interval 1-2 seconds start=", Time, newline;
...
At end:
Print "For interval 1-2 seconds end=", Time, newline;
...
...
...
</PRE>
<P>When invoked this script produces the following output:
<P><PRE>
Sample rate=44100 Hz
Score duration=2 seconds
For interval 0-1 seconds start=0
For interval 0-1 seconds end=1
For interval 1-2 seconds start=1
For interval 1-2 seconds end=2
</PRE>
<P>The four <CODE>Print</CODE> statements in this example print out the values of
the <CODE>start</CODE><A NAME="1"> and</A> <CODE>end</CODE><A NAME="2"> variables</A> at various points during the
performance. Note that the values change depending on where the variables
are actually accessed. This is due to the concept of <EM>scope</EM><A NAME="3">.</A> Each
control structure which defines a time interval during the performance
- i.e. each instance of <CODE>At..for</CODE>, <CODE>From..to</CODE>, <CODE>Before</CODE>
or <CODE>After</CODE> - has its own scope. Within that scope the values of
<CODE>start</CODE> and <CODE>end</CODE> are set to refer to the start and end times
of that particular time interval. This is useful for defining sub-events
in terms of the higher-level event in which they are enclosed.
<P>Another example is given below to clarify this point:
<P><PRE>
Audio rate: 44100;
Init:
...
Score 5 secs:
At start:
Print "For score, start=", Time, newline;
...
At end:
Print "For score, end=", Time, newline;
...
From 1 secs to 4 secs:
At start:
Print "For interval 1-4 seconds, start=", Time, newline;
...
At end:
Print "For interval 1-4 seconds, end=", Time, newline;
...
...
...
</PRE>
<P>In this example the first pair of <CODE>At</CODE> structures are enclosed within the scope
of the top-level <CODE>Score</CODE> structure, whilst the second pair of <CODE>At</CODE>
structures are enclosed or nested within the <CODE>From..to</CODE> structure. As you can
see from the output from this script, the values of <CODE>start</CODE> and <CODE>end</CODE>
are altered accordingly depending on their scope:
<P><PRE>
Sample rate=44100 KHz
Score duration=5 seconds
For score, start=0
For interval 1-4 seconds, start=1
For interval 1-4 seconds, end=4
For score, end=5
</PRE>
<P>Note also that it doesn't matter in which textual order events are given in
a score, the only thing which matters is the instant in time, or time interval
defined by the values in the head of the control structure.
<P>The ability to nest events and define the start and end times of a
sub-event in relative rather than absolute terms provides a rudimentary
mechanism for describing compound events <sup><A HREF="UserManual_156.html#footnote-1">1</A></sup>.
<P>
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<tr><td colspan="3" class="addr"><!-- bottom panel --><ADDRESS><FONT SIZE=-1>©1999,2000 Mark Pearson
<A HREF="mailto:m.pearson@ukonline.co.uk">m.pearson@ukonline.co.uk</A> April 30, 2000</ADDRESS><BR></td></tr><!-- end bottom panel --></table></BODY></HTML>
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