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The following code is supposed to generate a parse tree of the input expression, but the problem is that the output E,T,F,S (functions used in the code). I want it to be something like:

a+b*c => E*c => E+b*c => a+b*c

 #include <stdio.h>
 #include <stdlib.h>
 #include <ctype.h>
 char next;
 void E(void);void T(void);
 void S(void);void F(void);
 void error(int);void scan(void);
 void enter(char);
 void leave(char);
 void spaces(int);
 int level = 0;



 //The main should always be very simple
 //First scan the string
 //second check for end of string reached , if yes success and if not error.

 //P ---> E '#'
 int main(void)
 printf("Input:");
 scan(); E();
 if (next != '#') error(1);
 else printf("***** Successful parse *****n");
 

 //E ---> T '-') T
 void E(void) next == '-') 
 scan();
 T();
 
 leave('E');
 


 //T ---> S '/') S
 void T(void)
 

 //S ---> F '^' S | F
 void S(void)
 
 enter('S'); F();
 if (next == '^') 
 scan(); S();
 
 leave('S');
 

 //F ---> char | '(' E ')'
 void F(void)
 
 enter('F');
 if (isalpha(next)) 
 
 scan();
 
 else if (next == '(') 
 scan(); E();
 if (next == ')')
 scan();
 else
 error(2);
 
 else 
 error(3);
 
 leave('F');
 
 //Scan the entire input
 void scan(void)
 while (isspace(next = getchar()));
 

 void error(int n)
 
 printf("n*** ERROR: %in", n);
 exit(1);
 

 void enter(char name)
 
 spaces(level++);
 printf("+-%cn", name);
 

 void leave(char name)
 
 spaces(--level);
 printf("+-%cn", name);

 
 //TO display the parse tree
 void spaces(int local_level)
 ");
 

You don't get to choose the parse tree. I guess you don't understand the output, but (again) I guess you'd like

a+b*c => F+b*c => S+b*c => T+b*c => T+F*c => T+S*c => T+S*F => T+S*S => T+T => E

So here are a few questions to help.

1. what kind of parsing is going on there? bottom up or top down?


2. what state the parser is in when it prints enter/leave E, or T?

If you answered bottom up to the first question, what does E, T, S, F, F denotes (enter E, enter T, enter S, enter F, leave F)? When you have leave F that means the parser successfully recognised a non-terminal.
Try the input string 1+b*c. What do you get? Why do you get an error after E, T, S, F?

The output you seem to require can be easily produced if you understand what is produced at the moment. Hope this helps.

Looks like a recursive descent parser. First, work out your grammar by hand. What you are expecting is not what your grammar says. You've got, from your comments,

E ---> T '-') T expression
T ---> S '/') S term
S ---> F '^' S | F subexpression?
F ---> char | '(' E ')' factor

The definitions of E and T put * at a higher precedence than + so there is no way that you will get E*c. If you want that, you'll have to switch the grammar to

E ---> T '/') T expression
T ---> S ('+' term

If you just want the output to include the rest of the expression,

1. Get the whole line in


2. Change your scanner or lexer to get the next character from that scanned line. Mark this as the scanned point.


3. Change your Enter routine to print the the mnemonic as well as the line from the scanned point.