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<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
~/tcltk$ <b>cat input01.txt</b>
1 + 2
4 + 5
7 - 9

~/tcltk$ <b>tclsh</b>
% <b>set InFile [open input01.txt r]</b>
file3
% <b>while {[gets $InFile Op] &gt;= 0} {
    set Operation "expr $Op"
    set Result [eval $Operation]
    puts stdout "$Op = $Result\n"
}</b>
1 + 2 = 3
 
4 + 5 = 9
 
7 - 9 = -2
 
%
			</code>
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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>Using <i>eval</i> for dynamic scripting</b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr>   第 15 页（共15 页）</nobr></font></td>
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<p>In this example, you can sense the power of the <i>eval</i> command. Under normal circumstances, the Tcl interpreter operates in a one-pass mode: It first parses the input command line (possibly stretched over several physical lines), performing any substitutions. Then execution takes place, unless a bad or malformed command is found. <i>eval</i> permits a second pass (or perhaps more correctly, a pre-pass). Thus a Tcl command can be first dynamically constructed, then parsed and executed.</p>
<p>In the listing at the left, the input file consists of three lines, each with one arithmetic operation shown per line. After invoking <b>tclsh</b>, the file is opened read only, and associated with the <code>$InFile</code> variable. The <code>while</code> loop reads in one line at a time, to <code>$Op</code>. Then a complete Tcl command is constructed by pre-pending <code>expr</code> to the $Op variable. This is then expanded, evaluated, and the result assigned accordingly. Finally, each operation and result is displayed on <code>stdout</code>.</p>
<p>While this sample demonstrates a relatively trivial application of <i>eval</i>, conceptually it can be easily extended to dynamic file and/or directory processing based on the input of an input file of known syntax or to base operations on file type, permissions, access time or any variety of testable elements.</p>
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