clr(1).txt

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

#define MAX_INPUT_SIZE 100

typedef enum {
    ID,
    PLUS,
    MINUS,
    MULT,
    DIV,
    LPAREN,
    RPAREN,
    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 PLUS;
        case '-':
            return MINUS;
        case '*':
            return MULT;
        case '/':
            return DIV;
        case '(':
            return LPAREN;
        case ')':
            return RPAREN;
        case '\0':
            return END;
        default:
            if (isalpha(c)) {
                return ID;
            }
            printf("Error: unexpected character %c\n", c);
            success = 0;
            return ERROR;
    }
}

void expr();
void term();
void factor();
void id();

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

void id() {
    match(ID);
}

void factor() {
    switch (lookahead) {
        case ID:
            id();
            break;
        case LPAREN:
            match(LPAREN);
            expr();
            match(RPAREN);
            break;
        default:
            printf("Error: expected ID or LPAREN, got %d\n", lookahead);
            success = 0;
            break;
    }
}

void term() {
    factor();
    while (lookahead == MULT || lookahead == DIV) {
        switch (lookahead) {
            case MULT:
                match(MULT);
                factor();
                break;
            case DIV:
                match(DIV);
                factor();
                break;
        }
    }
}

void expr() {
    term();
    while (lookahead == PLUS || lookahead == MINUS) {
        switch (lookahead) {
            case PLUS:
                match(PLUS);
                term();
                break;
            case MINUS:
                match(MINUS);
                term();
                break;
        }
    }
}

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