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  1. PO
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    <p>Your lex rule <code>("[0-9A-Za-z_]+")</code> will match (only) the literal string <code>[0-9A-Za-z_]+</code> -- get rid of the <code>"</code> characters to have it be a pattern to match any identifier or number.</p> <p>Your yacc code does not match your lex code for punctuation -- the lex code returns <code>AND</code> for <code>&amp;</code> while the yacc code is expecting an <code>&amp;</code> -- so either change the lex code to return <code>'&amp;'</code> or change the yacc code to use the token <code>AND</code>, and similarly for <code>|</code>, <code>(</code>, and <code>)</code>. You might also want to ignore spaces in the lex code (rather than treating them as errors). You also have no lex rule to match and return <code>'\n'</code>, even though you use that in your yacc grammar.</p> <p>Your yacc code is otherwise correct, but is ambiguous, thus giving you shift/reduce conflicts. That's because your grammar is ambiguous -- an input like <code>a&amp;b|c</code> can be parsed as either <code>(a&amp;b)|c</code> or <code>a&amp;(b|c)</code>. You need to decide how that ambiguity should be resolved and reflect that in your grammar -- either by using more non-terminals, or by using yacc's built-in precedence support for resolving this kind of ambiguity. If you stick the declarations:</p> <pre><code>%left '|' %left '&amp;' </code></pre> <p>in the top of your yacc file, that will resolve the ambiguity by making both <code>&amp;</code> and <code>|</code> left associative, and <code>&amp;</code> higher precedence than <code>|</code>, which would be the normal interpretation.</p> <p><strong>Edit</strong></p> <p>The problem you have now is that you never define YYSTYPE (either directly or with %union) in your .y file and you never set yylval in your .l file. The first problem means that <code>$1</code> etc are just <code>int</code>s, not pointers (so it makes no sense to try to print them with <code>%s</code> -- you should be getting a warning from your C compiler over that). The second problem means that they never have a value anyways, so its just always the default 0 value of an uninitialized global variable</p> <p>The easiest fix would be to add</p> <pre><code>%union { const char *name; } %token &lt;name&gt; VAR LB RB LN %left &lt;name&gt; AND OR %left &lt;name&gt; EQ %type &lt;name&gt; expr </code></pre> <p>to the top of the yacc file. Then change the all the lex rules to be something like</p> <pre><code>([A-Za-z0-9_]+) { yylval.name = strdup(yytext); return VAR;} </code></pre> <p>Finally, you also need to change the bison actions for expr to set <code>$$</code>, eg:</p> <pre><code>| LB exp RB { asprintf(&amp;$$, "%s %s %s",$1,$2,$3); printf("abstract: %s\n", $$); } </code></pre> <p>This will at least work, though it will leak lots of memory for the allocated strings.</p> <p>The last problem you have is that your <code>line</code> rule only matches a single line, so a second line of input causes an error. You need a recursive rule like:</p> <pre><code>line: /* empty */ | line exp LN { printf.... </code></pre>
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    1. POHow to write flex and bison file parsing this language?
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    1. USChris Dodd
    1. USChris Dodd
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      1. VTUpMod
    1. COThank you,I edited .l and .y file as you said,But the new problem comes,see the description above
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      1. USAlex Chan
 

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