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POWhy am I getting conflicts: 1 shift/reduce
 

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  1. POWhy am I getting conflicts: 1 shift/reduce
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    <p>I'm new to bison and I'm getting a "conflicts: 1 shift/reduce" error. Can anyone shed some light on this?</p> <p>Here's the y file.</p> <p>test.y:</p> <pre><code>%{ #include &lt;stdio.h&gt; #include &lt;string.h&gt; #define YYERROR_VERBOSE #define YYDEBUG 1 void yyerror(const char *str); int yywrap(); %} %union { int integer; char *string; } %token &lt;string&gt; VAR_LOCAL %token &lt;integer&gt; LIT_NUMBER %token &lt;string&gt; LIT_STRING %token WS_LINEBRK //%token SYMB_EQL %token SYMB_PLUS %token SYMB_MINUS %token SYMB_MUL %token SYMB_DIV %% /* // Sample input num = 10 str = "this is a string" */ inputs: /* empty token */ | literal | variable | inputs stmt WS_LINEBRK ; stmt: variable "=" exps ; exps: variable op literal | variable op variable | literal op literal | literal op variable ; op: SYMB_PLUS | SYMB_MINUS | SYMB_MUL | SYMB_DIV ; variable: VAR_LOCAL { printf("variable: %s\n", $1); } ; literal: number | string ; string: LIT_STRING { printf("word: %s\n", $1); } ; number: LIT_NUMBER { printf("number: %d\n", $1); } ; %% void yyerror(const char *str) { fprintf(stderr,"error: %s\n",str); } int yywrap() { return 1; } main() { yyparse(); } </code></pre> <p>Here's the lex file test.l:</p> <pre><code>%{ #include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; #include "y.tab.h" int line_no = 0; %} %% [a-z][a-zA-Z0-9]* { // local variable yylval.string=strdup(yytext); return VAR_LOCAL; } [0-9]+ { //number literal yylval.integer=atoi(yytext); return LIT_NUMBER; } = return SYMB_EQL; \+ return SYMB_PLUS; \- return SYMB_MINUS; \* return SYMB_MUL; \/ return SYMB_DIV; \"[-+\!\.a-zA-Z0-9' ]+\" { // word literal yylval.string=strdup(yytext); return LIT_STRING; } \n { // line break printf("\n"); return WS_LINEBRK; } [ \t]+ /* ignore whitespace */; %% </code></pre>
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    1. PO
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      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USJonathan Leffler
    1. USharoldcampbell
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    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
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      1. PTAnswer
    2. PO
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      1. PTAnswer
    1. This table or related slice is empty.
    1. COI've noticed that moving the "| literal" and "| variable" rules from the inputs production to stmt production makes the error go away...but I don't understand why...
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      1. POWhy am I getting conflicts: 1 shift/reduce
      1. USharoldcampbell
    2. COSee my reply. Moving the `variable` rule to `stmt` allows bison to resolve all situations with just one lookahead token: Now it just matches `variable`. If it's followed by `"="` (lookahead token used here), it matches the assignment rule. If not, it matches the `variable` rule.
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      1. POWhy am I getting conflicts: 1 shift/reduce
      1. USFabian Giesen
 

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