MultiMap不打印列表

Question

我的代码有两个问题。

第一个问题是我的multimap不能打印，我不知道为什么。

第二个问题是，我不知道如何创建一个函数来搜索和查找我的multimap中的特定项目。

您可以随意更改代码，但我需要保留类或将其更改为结构。

#include<iostream>
#include<string> 
#include<iterator>
#include<map>
#include<algorithm>
#include<iomanip>
using namespace std;

class SongNode {
public:
    string title;
    string artist;
    float rating;
};

//Constants
#define INFINITE_LOOP 1
#define LEFT_JUSTIFIED 30

//Prototypes

void sortByArtist(multimap<string, SongNode>& newSongNode); 
void sortByRating(multimap<string, SongNode>& newSongNode); 


//FUNCTION      : void sortByArtist
//PARAMETER     : multimap<string, SongNode>&newSongNode
//RETURN        : void
//DESCRIPTION   : This function will sort the multimap by the artist of the song
void sortByArtist(multimap<string, SongNode>&newSongNode) {
    multimap<string, SongNode>temp; 

    for (auto i = newSongNode.begin(); i != newSongNode.end(); i++) 
        temp.insert(pair<string, SongNode>(i->second.artist, i->second)); 
    newSongNode.clear(); 

    for (auto i = temp.begin(); i != temp.end(); i++)
        newSongNode.insert(pair<string, SongNode>(i->second.artist, i->second)); 
    temp.clear(); 
}

//FUNCTION      : void sortByRating
//PARAMETER     : (multimap<string, SongNode>& newSongNode
//RETURN        : void
//DESCRIPTION   : This function will sort the multimap by the rating. 
void sortByRating(multimap<string, SongNode>& newSongNode) {

    multimap<float, SongNode, greater<int>> temp; 

    for (auto i = newSongNode.begin(); i != newSongNode.end(); i++)
        temp.insert(pair<int, SongNode>(i->second.rating, i->second)); 
    newSongNode.clear(); 

    for (auto i = temp.begin(); i != temp.end(); i++)
        newSongNode.insert(pair<string, SongNode>(" ", i->second)); 
    temp.clear(); 
}
    
int main() {

    multimap<string, SongNode> songList; 
    multimap<string, SongNode>::iterator itr; 
    string title; 
    string artist; 
    int rating; 
    SongNode* newSongNode = new SongNode; 

    cout << "Please enter a title artist and rating\n"; 

    while (INFINITE_LOOP) {
        cout << "Title: "; 
        cin >> title;

        if (title == ".") {
            break; 
        }

        cout << "Artist: "; 
        cin >> artist; 

        cout << "Rating: ";
        cin >> rating; 

        //Error check  for rating 
        while (INFINITE_LOOP) {     

            if (rating < 1) {
                cout << "The rating you have entered is less then 1\n";
                cout << "Rating: ";
                cin >> rating;
                break;
            }

            else if (rating > 5) {
                cout << "The rating you have to enterd is greater then 5\n";
                cout << "Rating: ";
                cin >> rating;
                break; 
            }

            else
                break;
        }

        SongNode* newNode = new SongNode; 

        newNode->title = title;
        newNode->artist = artist; 
        newNode->rating = rating; 

        songList.insert(pair<string, SongNode>(title, *newSongNode));
    }

    //display the two lists here. 
    cout << "Artist Sorted List\n"; 
    sortByArtist(songList); 
    for (itr = songList.begin(); itr != songList.end(); ++itr) {
        cout << "Title" << left << setw(LEFT_JUSTIFIED) << itr->second.title; 
        cout << "Artist" << left << setw(LEFT_JUSTIFIED) << itr->second.artist; 
        cout << "Rating" << left << setw(LEFT_JUSTIFIED) << itr->second.rating<<endl; 
    }

    cout << "\n"; 
    cout << "Rating Sorted List\n"; 
    sortByRating(songList); 

    delete newSongNode; 
    return 0;
}

Answer 1

首先，您有两个名称相似的变量，并且您肯定会滥用它们：

    SongNode* newSongNode = new SongNode; 
    // ...
    SongNode* newNode = new SongNode; 
   
    newNode->title = title;
    newNode->artist = artist; 
    newNode->rating = rating; 

    songList.insert(pair<string, SongNode>(title, *newSongNode));

您正在初始化newNode，但插入了newSongNode。这些值是用空字符串初始化的，因此可以解释不打印的原因。

至于“在多地图中搜索”，要更具体地说明你的问题是什么。

Answer 2

您不会用用数据填充的SongNode对象填充songList。您有一个不包含任何数据的额外SongNode，它是您要放入songList的唯一对象。因此，没有什么可打印的。

试着这样做：

#include <iostream>
#include <string> 
#include <iterator>
#include <map>
#include <algorithm>
#include <iomanip>
#include <limits>
using namespace std;

struct SongNode {
    string title;
    string artist;
    float rating;
};

//Constants
#define INFINITE_LOOP 1
#define LEFT_JUSTIFIED 30

//Prototypes

void sortByArtist(multimap<string, SongNode>& newSongNode); 
void sortByRating(multimap<string, SongNode>& newSongNode); 


//FUNCTION      : void sortByArtist
//PARAMETER     : multimap<string, SongNode>&newSongNode
//RETURN        : void
//DESCRIPTION   : This function will sort the multimap by the artist of the song
void sortByArtist(multimap<string, SongNode> &newSongNode) {
    multimap<string, SongNode> temp; 

    for (auto &elem : newSongNode) 
        temp.insert(make_pair(elem.second.artist, elem.second)); 
    newSongNode.clear(); 

    for (auto &elem : temp)
        newSongNode.insert(make_pair(elem.second.artist, elem.second)); 
}

//FUNCTION      : void sortByRating
//PARAMETER     : (multimap<string, SongNode>& newSongNode
//RETURN        : void
//DESCRIPTION   : This function will sort the multimap by the rating. 
void sortByRating(multimap<string, SongNode>& newSongNode) {

    multimap<float, SongNode, greater<int>> temp; 

    for (auto &elem : newSongNode)
        temp.insert(make_pair(elem.second.rating, elem.second)); 
    newSongNode.clear(); 

    for (auto &elem : temp)
        newSongNode.insert(make_pair(" ", elem.second)); 
}
    
int main() {

    multimap<string, SongNode> songList; 
    string title, artist;
    float rating; 

    cout << "Please enter a title artist and rating\n"; 

    while (INFINITE_LOOP) {
        cout << "Title: "; 
        cin >> title;

        if (title == ".") {
            break; 
        }

        cout << "Artist: "; 
        cin >> artist; 

        cout << "Rating: ";
        cin >> rating; 

        //Error check for rating
        while (INFINITE_LOOP) {     
            if (!cin) {
                cin.clear();
                cin.ignore(numeric_limits<streamsize>::max(), '\n');
                cout << "The rating you have entered is invalid\n";
            }
            else if (rating < 1.0f) {
                cout << "The rating you have entered is less then 1\n";
            }
            else if (rating > 5.0f) {
                cout << "The rating you have to entered is greater then 5\n";
            }
            else
                break;

            cout << "Rating: ";
            cin >> rating;
        }

        SongNode newNode; 

        newNode.title = title;
        newNode.artist = artist; 
        newNode.rating = rating; 

        songList.insert(make_pair(title, newNode));
    }

    //display the two lists here. 

    cout << "Artist Sorted List\n"; 
    sortByArtist(songList); 
    for (auto &elem : songList) {
        cout << "Title" << left << setw(LEFT_JUSTIFIED) << elem.second.title; 
        cout << "Artist" << left << setw(LEFT_JUSTIFIED) << elem.second.artist; 
        cout << "Rating" << left << setw(LEFT_JUSTIFIED) << elem.second.rating << endl; 
    }
    cout << "\n";

    cout << "Rating Sorted List\n"; 
    sortByRating(songList); 
    for (auto &elem : songList) {
        cout << "Title" << left << setw(LEFT_JUSTIFIED) << elem.second.title; 
        cout << "Artist" << left << setw(LEFT_JUSTIFIED) << elem.second.artist; 
        cout << "Rating" << left << setw(LEFT_JUSTIFIED) << elem.second.rating << endl; 
    }
    cout << "\n";

    return 0;
}