编译器实践三 之 针对算术表达式的语法分析器

该算术表达式的上下文无关文法是：

E -> E + T

   | E – T

   | T

T -> T * F

   | T / F

   | F

F -> num

   | (E)

部分代码来自MOOC

#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>

void parse_F();
void parse_T();
void parse_E();
void error (char *want, char got);

int i;
char *str = 0;

void error (char *want, char got)
{
  fprintf (stderr, "Compling this expression:\n%s\n", str);
  int j = i;
  while (j--)
    fprintf (stderr, " ");
  fprintf (stderr, "^\n");
  fprintf (stderr, "Syntax error at position: %d\n"
           "\texpecting: %s\n"
           "\tbut got  : %c\n",
           i, want, got);
  exit (0);
  return;
}

void parse_F()
{
  char c = str[i];
  if (isdigit(c)){
    i++;
    return;
  }
  if (c=='('){
    i++;
    parse_E();
    c = str[i];
    if (c==')'){
      i++;
      return;
    }
    error ("\')\'", c);
    return;
  }
  error ("\'0-9\' or \'(\'", c);
  return;
}


void parse_T()
{
  parse_F();
  char c = str[i];
  while (c=='*' || c =='/'){
    i++;
    parse_F();
    c = str[i];
  }
  return;
}

void parse_E()
{
  parse_T();
  char c = str[i];
  while (c=='+' || c == '-'){
    i++;
    parse_T();
    c = str[i];
  }
  return;
}

void parse (char *e)
{
  str = e;
  i = 0;
  parse_E();
  if (str[i]=='
#include <ctype.h> #include <stdio.h> #include <stdlib.h> void parse_F(); void parse_T(); void parse_E(); void error (char *want, char got); int i; char *str = 0; void error (char *want, char got) { fprintf (stderr, "Compling this expression:\n%s\n", str); int j = i; while (j--) fprintf (stderr, " "); fprintf (stderr, "^\n"); fprintf (stderr, "Syntax error at position: %d\n" "\texpecting: %s\n" "\tbut got : %c\n", i, want, got); exit (0); return; } void parse_F() { char c = str[i]; if (isdigit(c)){ i++; return; } if (c=='('){ i++; parse_E(); c = str[i]; if (c==')'){ i++; return; } error ("\')\'", c); return; } error ("\'0-9\' or \'(\'", c); return; } void parse_T() { parse_F(); char c = str[i]; while (c=='*' || c =='/'){ i++; parse_F(); c = str[i]; } return; } void parse_E() { parse_T(); char c = str[i]; while (c=='+' || c == '-'){ i++; parse_T(); c = str[i]; } return; } void parse (char *e) { str = e; i = 0; parse_E(); if (str[i]=='\0') return; error ("\'+\' or '\\0\'", str[i]); return; } /////////////////////////////////////////////// // Your job: // Add some code into the function parse_E() and // parse_T to parse "-" and "/" correctly. // When you finish your task, NO error message // should be generated. // Enjoy! :-P int main (char argc, char **argv) { // There are the following rules on an expression: // 1. Every expression is represented as a string; // 2. integers are non-negative; // 3. integers are between 0-9. char *e; e = "(2)"; parse(e); e = "(3+4*5))"; parse(e); e = "(8-2)*3"; parse(e); e = "(8-2)/3"; parse(e); return 0; }
')
    return;
  error ("\'+\' or '\
#include <ctype.h> #include <stdio.h> #include <stdlib.h> void parse_F(); void parse_T(); void parse_E(); void error (char *want, char got); int i; char *str = 0; void error (char *want, char got) { fprintf (stderr, "Compling this expression:\n%s\n", str); int j = i; while (j--) fprintf (stderr, " "); fprintf (stderr, "^\n"); fprintf (stderr, "Syntax error at position: %d\n" "\texpecting: %s\n" "\tbut got : %c\n", i, want, got); exit (0); return; } void parse_F() { char c = str[i]; if (isdigit(c)){ i++; return; } if (c=='('){ i++; parse_E(); c = str[i]; if (c==')'){ i++; return; } error ("\')\'", c); return; } error ("\'0-9\' or \'(\'", c); return; } void parse_T() { parse_F(); char c = str[i]; while (c=='*' || c =='/'){ i++; parse_F(); c = str[i]; } return; } void parse_E() { parse_T(); char c = str[i]; while (c=='+' || c == '-'){ i++; parse_T(); c = str[i]; } return; } void parse (char *e) { str = e; i = 0; parse_E(); if (str[i]=='\0') return; error ("\'+\' or '\\0\'", str[i]); return; } /////////////////////////////////////////////// // Your job: // Add some code into the function parse_E() and // parse_T to parse "-" and "/" correctly. // When you finish your task, NO error message // should be generated. // Enjoy! :-P int main (char argc, char **argv) { // There are the following rules on an expression: // 1. Every expression is represented as a string; // 2. integers are non-negative; // 3. integers are between 0-9. char *e; e = "(2)"; parse(e); e = "(3+4*5))"; parse(e); e = "(8-2)*3"; parse(e); e = "(8-2)/3"; parse(e); return 0; }
\'", str[i]);
  return;
}
///////////////////////////////////////////////
// Your job:
// Add some code into the function parse_E() and
// parse_T to parse "-" and "/" correctly.
// When you finish your task, NO error message
// should be generated.
// Enjoy! :-P
int main (char argc, char **argv)
{
  // There are the following rules on an expression:
  //   1. Every expression is represented as a string;
  //   2. integers are non-negative;
  //   3. integers are between 0-9.
  char *e;

  e = "(2)";
  parse(e);

  e = "(3+4*5))";
  parse(e);

  e = "(8-2)*3";
  parse(e);

  e = "(8-2)/3";
  parse(e);
  
  return 0;
}

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