gistfile1.c

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

#define MAX_INPUT_SIZE 1024

#define TK_NOTHING      0
#define TK_NUM          1
#define TK_OP_PLUS     10
#define TK_OP_MINUS    11
#define TK_OP_DIV      12
#define TK_OP_MULTI    13


int POS = 0;
char *CONTENT = NULL;
char TOKEN[MAX_INPUT_SIZE];

void init(char *buf) {
    CONTENT = buf;
    POS = 0;
}

int next_token() {
    char c;
    int sub_pos;
    if (CONTENT == NULL)      return TK_NOTHING;

    while (CONTENT[POS] != '¥0' && POS < MAX_INPUT_SIZE) {
        c = CONTENT[POS];
        if ( isdigit(c) ) {
            sub_pos = 0;
            while (isdigit(c) && CONTENT[POS] != '¥0' && POS < MAX_INPUT_SIZE) {
                TOKEN[sub_pos++] = c;
                c = CONTENT[++POS];
            }
            TOKEN[sub_pos] = '¥0';

            return TK_NUM;
        }
        else if (c == '+') {
            POS++;
            return TK_OP_PLUS;
        }
        else if (c == '-') {
            POS++;
            return TK_OP_MINUS;
        }
        else if (c == '/') {
            POS++;
            return TK_OP_DIV;
        }
        else if (c == '*') {
            POS++;
            return TK_OP_MULTI;
        }
        else {
            POS++;
            // do nothing
        }
    }
    return TK_NOTHING;
}

char *get_token() {
    return TOKEN;
}

int main() {
    int x, y, result;
    int op;
    char buf[MAX_INPUT_SIZE];

    fgets(buf, MAX_INPUT_SIZE, stdin);
    if (buf == NULL) {
        fprintf(stderr, "input error¥n");
        exit(EXIT_FAILURE);
    }

    init(buf);

    if (next_token() != TK_NUM) {
        fprintf(stderr, "parse error¥n");
        exit(EXIT_FAILURE);
    }
    x = atoi(get_token());


    op = next_token();
    if ( !(
            op == TK_OP_PLUS    ||
            op == TK_OP_MINUS   ||
            op == TK_OP_DIV     ||
            op == TK_OP_MULTI   
          )
    ) {
        fprintf(stderr, "parse error¥n");
        exit(EXIT_FAILURE);
    }

    if (next_token() != TK_NUM) {
        fprintf(stderr, "parse error¥n");
        exit(EXIT_FAILURE);
    }
    y = atoi(get_token());

    switch (op) {
        case TK_OP_PLUS:
            result = x + y;
            break;
        case TK_OP_MINUS:
            result = x - y;
            break;
        case TK_OP_DIV:
            if (y == 0) {
                fprintf(stderr, "0 division error¥n");
                exit(EXIT_FAILURE);
            }
            result = x / y;
            break;
        case TK_OP_MULTI:
            result = x * y;
            break;
    }

    printf("result: %d¥n", result);

    return EXIT_SUCCESS;
}