LEX & YACC

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void graphFinish() {int i, j;for (i = 0; i < lmax; i++) {for (j = cmax-1; j > 0 && graph[i][j] == ' '; j--);graph[i][cmax-1] = 0;if (j < cmax-1) graph[i][j+1] = 0;if (graph[i][j] == ' ') graph[i][j] = 0;}for (i = lmax-1; i > 0 && graph[i][0] == 0; i--);printf ("\n\nGraph %d:\n", graphNumber++);for (j = 0; j
More LexStringsQuoted strings frequently appear in programming languages. Here is one way to match a string inlex:%{char *yylval;#include %}%%\"[^"\n]*["\n] {yylval = strdup(yytext+1);if (yylval[yyleng-2] != '"')warning("improperly terminated string");elseyylval[yyleng-2] = 0;printf("found '%s'\n", yylval);}The above example ensures that strings don’t cross line boundaries, and removes enclosingquotes. If we wish to add escape sequences, such as \n or \", start states simplify matters:%{char buf[100];char *s;%}%x STRING%%\" { BEGIN STRING; s = buf; }\\n { *s++ = '\n'; }\\t { *s++ = '\t'; }\\\" { *s++ = '\"'; }\" {*s = 0;BEGIN 0;printf("found '%s'\n", buf);}\n { printf("invalid string"); exit(1); }. { *s++ = *yytext; }Exclusive start state STRING is defined in the definition section. When the scanner detects aquote, the BEGIN macro shifts lex into the STRING state. Lex stays in the STRING state,recognizing only patterns that begin with , until another BEGIN is executed. Thus, wehave a mini-environment for scanning strings. When the trailing quote is recognized, we switchback to state 0, the initial state.33
Page 1 and 2: A COMPACT GUIDE TOLEX & YACCby Thom
Page 3 and 4: PrefaceThis document explains how t
Page 5 and 6: Lex generates C code for a lexical
Page 7 and 8: Now we can easily understand some o
Page 9 and 10: Nameint yylex(void)char *yytextyyle
Page 11 and 12: YaccTheoryGrammars for yacc are des
Page 13 and 14: Lex includes this file and utilizes
Page 15 and 16: Numeric values are initially entere
Page 17 and 18: %%void yyerror(char *s) {fprintf(st
Page 19 and 20: a version that generates a syntax t
Page 21 and 22: Include Filetypedef enum { typeCon,
Page 23 and 24: Yacc Input%{#include #include #incl
Page 25 and 26: if ((p = malloc(nodeSize)) == NULL)
Page 27 and 28: Compiler#include #include "calc3.h"
Page 29 and 30: Graph/* source code courtesy of Fra
Page 31: * node has children */cs = c;for (k
Page 35 and 36: int dbgToken(int tok, char *s) {pri
Page 37 and 38: | expr ';'| PRINT expr ';'| VARIABL
Page 39: BibliographyAho, Alfred V., Ravi Se

LEX & YACC

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