LN : JSON (利用C++实现JSON)

• Appreciation to our TA, 王毅峰, who designed this task.

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问题描述

JSON, JavaScript Object Notation,is an flexible format that uses human-readable text to transmit data objects consisting of key-value pairs(键值对)
To construct a json object, we need to parse a raw string

For example

// {"name":"lilei","country":"china","age":"20"}
// in constructor, we parse the string to map
// that is, we find the first key "name", and correspoding value "lilei"
// then we modify our private data member map<string, string> _data
// _data["name"] = "lilei"
// don't stop until all the key-value pairs are stored in _data
json test("{\"name\":\"lilei\",\"country\":\"china\",\"age\":\"20\"}");

NOTE:

To simplify the problem

1. You just need to finish the constructor,which find out the key/value pairs and store in _data
2. all the string doesn't consist of space(空格), and it is strictly formed like
3. all the key and value have double quotation marks(双引号)
4. in front of them and after them(所有键的前后和值的前后都有双引号)
5. read json.h and main.cpp for more details

问题解析

问题的关键是如何从一个长字符串中获取对应的键值对，并且运用make_pair组成一组map。

json.h

#ifndef JSON_H
#define JSON_H
#include <iostream>
#include <string>
#include <map>

using std::ostream;
using std::string;
using std::map;

class json {
private:
    // store the relationship between key and value
    map<string, string> _data;
public:
    // parse the raw string to map<string, string>
    explicit json(string);

    // return mutable value according to key
    string& operator[](string key) {
        return _data[key];
    }

    // return the number of key/value
    int count() const {
        return _data.size();
    }

    // output
    friend ostream& operator<<(ostream& os, const json& obj) {
        map<string, string>::iterator it;
        map<string, string> data = obj._data;
        int num = 0;
        os << "{\n";
        for (it = data.begin(); it != data.end(); it++) {
            num++;
            os << "    \"" << it->first << "\": \"" << it->second << "\"";
            if (num != obj.count()) {
                os << ",";
            }
            os << "\n";
        }
        os << "}";
        return os;
    }
};
#endif  // JSON_H

json.cpp

#include "json.h"
using namespace std;

json::json(string a) {
    int len = a.length();
    string m, n;
    int famen = 0;
    for (int i = 0; i < len; i++) {
        if (a[i] == '"') {
            famen++;
            continue;
        }
        if (famen%4 == 1) {
            m.push_back(a[i]);
        } else if (famen%4 == 3) {
            n.push_back(a[i]);
        } else if (famen%4 == 0 && famen != 0) {
            _data.insert(make_pair(m, n));
            m.clear();
            n.clear();
        }
    }
}

main.cpp

#include <iostream>
#include <string>
#include "json.h"

using std::cin;
using std::string;
using std::cout;
using std::endl;

int main(void) {
    {
        // {"name":"lilei","country":"china","age":"20"}
        json test("{\"name\":\"lilei\",\"country\":\"china\",\"age\":\"20\"}");
        cout << test << endl;
        test["name"] = "mike";
        test["country"] = "USA";
        cout << test << endl;
    }
    {
        // {"book_name":"c++ primer 5th","price":"$19.99"}
        json test("{\"book_name\":\"c++ primer 5th\",\"price\":\"$19.99\"}");
        cout << test << endl;
        test["page"] = "345";
        test["ISBN"] = "978-962";
        cout << test << endl;
    }
    {
        int AvoidRepeatedData;
        cin >> AvoidRepeatedData;
        string rawString;
        cin >> rawString;
        json test(rawString);
        cout << test << endl;
    }
    return 0;
}