recur down expr

#define _CRT_SECURE_NO_WARNINGS
#include <ctype.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
char *input = NULL, operator= 0;
int pos = 0;
int oprands[80], top_opr = 0;
void dp() {
    printf("[");
    for (int i = 0; i < top_opr; ++i) {
        printf(i == top_opr - 1 ? "%d" : "%d, ", oprands[i]);
    }
    printf("] [%s]\n", input + pos);
}
bool ws() {
    if (!input || !input[pos]) return false;
    while (input[pos] && isspace(input[pos])) pos++;
    if (!input[pos]) return false;
    return true;
}
bool factor() {
    if (!ws()) return false;
    char *end = NULL;
    int i = strtol(input + pos, &end, 10);
    if (errno == ERANGE) {
        printf("error: expected factor, got %c\n", input[pos]);
        return false;
    }
    oprands[top_opr++] = i;
    pos = end - input;
    return true;
}
bool match(char *s) {
    if (!ws()) return false;
    char *p;
    if ((p = strchr(s, input[pos]))) {
        operator= *p;
        pos++;
        return true;
    }
    return false;
}
bool mul() {
    if (!factor()) return false;
    while (match("*/")) {
        char op = operator;
        if (!factor()) {
            printf("error: expected factor, got eof\n");
            return false;
        }
        int b = oprands[--top_opr];
        int a = oprands[--top_opr];
        if (op == '*') oprands[top_opr++] = a * b;
        if (op == '/') oprands[top_opr++] = a / b;
        printf(". %d %c %d = %d\n", a, op, b, oprands[top_opr - 1]);
    }
    return true;
}
bool add() {
    if (!mul()) return false;
    while (match("+-")) {
        char op = operator;
        if (!mul()) {
            printf("error: expected factor, got eof\n");
            return false;
        }
        int b = oprands[--top_opr];
        int a = oprands[--top_opr];
        if (op == '+') oprands[top_opr++] = a + b;
        if (op == '-') oprands[top_opr++] = a - b;
        printf(". %d %c %d = %d\n", a, op, b, oprands[top_opr - 1]);
    }
    return true;
}
bool expr() { return add(); }
int main() {
    input = "3 + 4 * 5 / 7 - 1";
    pos = 0;
    printf("? %s\n", input);
    expr();
    printf("= %d\n", oprands[--top_opr]);
}