/* The program from Aho, Sethi, Ullman section 2.5 */ /* Without the optimizations on page 52-53 */ #include <stdio.h> #include <ctype.h> #include <stdlib.h> int lookahead; void error() { printf("syntax error\n"); exit(EXIT_FAILURE); } void match(int t) { if (lookahead == t) lookahead = getchar(); else error(); } void term () { if (isdigit(lookahead)) { putchar(lookahead); match(lookahead); } else error(); } void rest() { if (lookahead == '+') { match('+'); term(); putchar('+'); rest(); } else if (lookahead == '-') { match('-'); term(); putchar('-'); rest(); } else ; } void expr() { term(); rest(); } int main(void) { lookahead = getchar(); expr(); putchar('\n'); return EXIT_SUCCESS; }