RECURSIVE DESCENT PARSER.txt

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

#define MAX_INPUT_SIZE 100

typedef enum {
    LPAREN,
    RPAREN,
    PLUS,
    MULT,
    ID,
    END,
    ERROR
} Token;

Token lookahead;
char input[MAX_INPUT_SIZE];
int index = 0;
int success = 1;

Token get_token() {
    char c = input[index++];
    while (isspace(c)) {
        c = input[index++];
    }
    switch (c) {
        case '(':
            return LPAREN;
        case ')':
            return RPAREN;
        case '+':
            return PLUS;
        case '*':
            return MULT;
        case 'i':
            if (input[index] == 'd') {
                index++;
                return ID;
            } else {
                break;
            }
        case '\0':
            return END;
    }
    success = 0;
    return ERROR;
}

void E();
void Eprime();
void T();
void Tprime();
void F();

void match(Token expected) {
    if (lookahead == expected) {
        lookahead = get_token();
    } else {
        printf("Error: expected %d, got %d\n", expected, lookahead);
        success = 0;
    }
}

void E() {
    T();
    Eprime();
}

void Eprime() {
    if (lookahead == PLUS) {
        match(PLUS);
        T();
        Eprime();
    }
}

void T() {
    F();
    Tprime();
}

void Tprime() {
    if (lookahead == MULT) {
        match(MULT);
        F();
        Tprime();
    }
}

void F() {
    if (lookahead == LPAREN) {
        match(LPAREN);
        E();
        match(RPAREN);
    } else if (lookahead == ID) {
        match(ID);
    } else {
        printf("Error: expected LPAREN or ID, got %d\n", lookahead);
        success = 0;
    }
}

int main() {
    printf("Enter an expression: ");
    fgets(input, MAX_INPUT_SIZE, stdin);
    lookahead = get_token();
    E();
    if (success && lookahead == END) {
        printf("Parsing successful\n");
    } else {
        printf("Parsing unsuccessful\n");
    }
    return 0;
}