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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>Variable scoping rules</b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr>   第 11 页（共15 页）</nobr></font></td>
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<p>Rules of scope describe the visibility of procedure and variable 
				names and values at different levels of the program. For 
				instance, variables defined at the outermost level of a script 
				are global variables. By default, these are not visible, nor 
				are their values available from within procedures. This 
				permits procedure writers to freely define variable names and 
				assign values without fear of overwriting important variables 
				that are unknown at the local scope. To make a global variable 
				visible inside a procedure, it must be declared as such 
				<i>within the procedure</i>, as I did for PI (in the example 
				on the previous panel) using the <i>global</i> command.</p>
<p>The <i>upvar</i> command provides a facility for associating 
				a local-level variable with the value of a variable from 
				another scope. This permits calling by name into procedures, 
				which is handy when the procedure needs to be able to modify 
				the value at another scope, rather than just using it. The 
				command syntax is <b>upvar <i>level $VarName LocalVar</i></b>, 
				where level is the number of steps up out of the current scope. 
				"#0" is the representation of the global scope level.</p>
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