1. Recursive decent parsing

2. Recursive Decent parsing
Recursive descent parsing associates a procedure with each nonterminal in the grammar, it may require backtracking of the input string.

3. Example S [ U$ U] W| [U]W W[U]|ε Program involves function s , u, v
Calls on these three functions corresponds to substitution for respective variable during a derivation
There is a variable curr_ch whose value is assigned before any of the three functions is called.

4. Example Char curr_ch; void s(),u(),w() void match(char);
void get_ch();
void error(char);
void main()
{ get_ch();s();
Cout<<endl<<”parsing complete” }

5. Example Void s() { Match(‘[‘);u(); match(‘$’);} Void u()
{ switch(curr_ch)
Case’]’: match(‘]’); w(); break;
Case ‘[‘: match(‘[‘); u(); match(‘]’); w(); break
Default error }

6. void w()
{if(curr_ch==’[‘{) match(‘[‘); u();}}
void get_ch()
{ if(cin>>curr_ch)cout<<curr_ch;
If(cin.eof() && curr_ch!=$)
{ cout<<”(end of input)}

7. void match(char this_ch)
{if(curr_ch==this_ch) getch(); else error(this_ch);else error(this_ch);}
void error(char * some_char)
{ cout<< error}

8. Example 2
Example: <type> -> <simple> | ^ id | array [<sample>] of <type>
<simple> -> integer | char | num dotdot num
void type() {
if (lookahead == INTEGER || lookahead == CHAR || lookahead==NUM)
simple();
else if (lookahead == ‘^’) {
match (‘^’);
match(ID);
} else if (lookahead == ARRAY) {
match (ARRAY);
match(‘[‘);
match (‘]’);
match (OF);
type();
} else error(); }

9. void simple() {
if (lookahead == INTEGER) match (INTEGER);
else if (lookahead == CHAR) match (CHAR);
else if (lookahead == NUM) {
match(NUM);
match(DOTDOT);
} else error(); }
void match(token t) {
if (lookahead == t) {lookahead = nexttoken();}
else error();