parser_by_c

source.c

#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
#include<ctype.h>
#include<stdlib.h>
#include<string.h>
char str[512];
int digit(int*pos) {
	int ret = 0;
	while (isdigit(str[*pos])) {
		ret *= 10;
		ret += (str[*pos] - '0');
		(*pos)++;
	}
	return ret;
}
void skip(int*pos) {
	while (str[*pos] == ' ' || str[*pos] == '\t') {
		(*pos)++;
	}
}
void next(int* pos) {
	(*pos)++;
	skip(pos);
}

int expr(int*pos,int val);

int func(int* pos,int val) {
	int locpos = *pos;
	int ret = val;
	skip(&locpos);
	
	if (str[locpos] == '(') {
		next(&locpos);
		ret = expr(&locpos,0);
		skip(&locpos);
		if (str[locpos] != ')') {
			goto err;
		}
		next(&locpos);
	}
	else {
		int digitposend = locpos;
		ret = digit(&digitposend);
		if (digitposend == locpos) {
			goto err;
		}
		locpos = digitposend;
	}

	*pos = locpos;
	return ret;

err:
	*pos = -1;
	return 0;
}

int muldiv(int*pos, int val) {
	int locpos = *pos;
	int ret = val;
	int flag = str[locpos] == '*' ? 1 : (str[locpos] == '/' ? -1 : 0);
	if (!flag) {
		goto end;
	}
	next(&locpos);
	int tmpret = func(&locpos, ret);
	flag > 0 ? (ret *= tmpret) : (ret /= tmpret);
	if (locpos == -1) {
		goto err;
	}
	skip(&locpos);
	ret = muldiv(&locpos, ret);
	goto end;

end:
	*pos = locpos;
	return ret;

err:
	*pos = -1;
	return 0;
}

int term(int* pos,int val) {
	int locpos = *pos;
	skip(&locpos);
	int ret = func(&locpos,val);
	if (locpos == -1) {
		goto err;
	}
	skip(&locpos);
	ret = muldiv(&locpos, ret);
	goto end;
	
end:
	*pos = locpos;
	return ret;

err:
	*pos = -1;
	return 0;
}

int addsub(int*pos, int val) {
	int locpos = *pos;
	int ret = val;
	skip(&locpos);
	int flag = str[locpos] == '+' ? 1 : (str[locpos] == '-' ? -1 : 0);
	if (!flag) {
		goto end;
	}
	next(&locpos);
	ret += flag*term(&locpos, val);
	if (locpos == -1) {
		goto err;
	}
	skip(&locpos);
	ret = addsub(&locpos, ret);
	goto end;

end:
	*pos = locpos;
	return ret;

err:
	*pos = -1;
	return 0;
}

int expr(int* pos,int val) {
	int locpos = *pos;
	skip(&locpos);
	int ret = term(&locpos,val);
	if (locpos == -1) {
		goto err;
	}

	skip(&locpos);
	ret = addsub(&locpos, ret);
	goto end;

end:
	*pos = locpos;
	return ret;

err:
	*pos = -1;
	return 0;
}

int main() {
	scanf("%511s[^\n] ", str);
	int len = strlen(str);
	int pos = 0;
	int ret = expr(&pos,0);
	if (pos >= 0){
		printf("%d\n", ret); 
	}
	else {
		printf("err\n");
	}
	return 0;
}