List sorting problem

[jjordan33]

Hi, I'm not sure how the sort() function for list objects works.
I'm trying to sort a list of album titles alphabetically, but I'm having very little success.

It's hw. Can anyone give me a few tips or help?

Thank you.

.h

#ifndef COLL_COLLECTION_H
#define COLL_COLLECTION_H

#include <iostream>
#include <iomanip>
#include <fstream>
#include <string>
#include <list>

struct CD {
    std::string title;
    float price;
    int year;
};

class Collection {
private:
    std::list<CD> cdList;
    std::list<CD>::iterator it;

public:
    Collection();

    void insert_sorted(std::string tempName, int tempYear, float tempPrice);

    void read_from_file(std::string fileName);

    void print();

    void find_cd(std::string fileName);

    void remove(std::string fileName);

    void write_to_file(std::string fileName);
};


#endif //COLL_COLLECTION_H 

#include <iostream>
#include <string>
#include <list>

#include "Collection.h"

using namespace std;

Collection::Collection() {

    list<CD> temp;
    cdList = temp;

}

void Collection::insert_sorted(string tempName, int tempYear, float tempPrice){

    CD x;

    x.title = tempName;
    x.price = tempPrice;
    x.year = tempYear;

    if(cdList.empty()){
        cdList.push_front(x);
    }

    else{

        it = cdList.begin();
        while(x.title.compare(it->title) <= 0 && it != cdList.end()){
            it++;

        }

        cdList.insert(it, x);

    }


}

void Collection::read_from_file(string fileName){

    ifstream fin;
    string alName;
    int alYear;
    double alPrice;

    fin.open(fileName);

    while(fin){

        getline(fin, alName);

        fin >> alPrice;
        fin >> alYear;
        fin.get();

        insert_sorted(alName, alYear, alPrice);

    }

    fin.close();
    cout << endl;

}

void Collection::print(){

    for(it = cdList.begin(); it != cdList.end(); it++) {
        cout << "Album Name: " << it -> title << endl;
        cout << "Album Year: " << it -> year << endl;
        cout << "Alum Price: $" << it -> price << endl;
    }

    cout << endl;
}

void Collection::find_cd(string fileName){

    for(it = cdList.begin(); it != cdList.end(); it++){

        if(it -> title == fileName){
            cout << "Album Name: " << it -> title << endl;
            cout << "Album Year: " << it -> year << endl;
            cout << "Alum Price: $" << it -> price << endl;
        }

    }

    cout << endl;

}

void Collection::remove(std::string fileName){

    for(it = cdList.begin(); it != cdList.end(); it++){

        if(it -> title == fileName){

            cout << "Now removing item!" << endl;

            cdList.erase(it++);

        }

    }

    cout << endl;

}

void Collection::write_to_file(std::string fileName){

    ofstream outputFile; 

    outputFile.open(fileName);

    if(outputFile.is_open()){

        for(it = cdList.begin(); it != cdList.end(); it++){
            outputFile << it -> title << endl;
            outputFile << it -> price << " " << it -> year << endl;
        }

    }

    else{
        cout << "ERROR! Opening write file!" << endl;
    }

    cout << "The file has now been created..." << endl << endl;
    outputFile.close();

}

Side note. I've also tried doing a string compare to dynamically insert them in the right location, but it doesn't even add anything at all.

void Collection::insert_sorted(string tempName, int tempYear, float tempPrice){

    CD x;

    x.title = tempName;
    x.price = tempPrice;
    x.year = tempYear;

    if(cdList.empty()){
        cdList.push_front(x);
    }

    else{

        it = cdList.begin();
        while(x.title.compare(it->title) <= 0 && it != cdList.end()){
            it++;

        }

        cdList.insert(it, x);

    }


}

[lastchance]

jjordan33 wrote:

Can anyone give me a few tips?

Supply enough code to give us a chance of answering your question.

[dutch]

You need to use the sort algorithm (include <algorithm>).

    sort(cdList.begin(), cdList.end());

Assuming cdList is a vector of CD, you will need to supply a comparison function to indicate how to sort CD's. Maybe something like this inside the CD struct/class:

    bool operator<(const CD& cd) {
        return title < cd.title;
    }

[jjordan33]

>> lastchance

I updated the code. My mistake!

>> dutch

Hmm. Okay.
I was afraid I'd need another library. That will resort in a loss of a couple points, but it's a great last resort. Thank you for the advice!

[jjordan33]

I got it to work with this code, so I'll mark this as completed, but if anyone has a better solution, or how to fit it inside one function, please feel free to help me improve.

Thanks y'all.

bool comp (const CD& a,const CD& b) // helper function
{
    return a.title < b.title; // returns the object valued lowest in ASCII
}


void Collection::insert_sorted(string tempName, int tempYear, float tempPrice){

    CD x;

    x.title = tempName;
    x.price = tempPrice;
    x.year = tempYear;

    cdList.push_front(x);

    cdList.sort(comp);
}

[JLBorges]

Resorting the list on each insert is expensive.
Simpler and faster to locate the right position in the list and directly insert it there.

#include <iostream>
#include <string>
#include <list>
#include <algorithm>
#include <fstream>

struct cd {

    std::string title ;
    double price = 0 ;
    int year = 0 ;

    // compare two cds with the < operator; compare (case-sensitive) titles
    friend bool operator< ( const cd& a, const cd& b ) {

            return a.title < b.title ;
    }

    friend std::ostream& operator<< ( std::ostream& stm, const cd& c ) {

            return stm << "cd{ " << c.title << ", "
                       << c.price << ", " << c.year << " }" ;
    }
};

struct cd_collection {

    std::list<cd> cd_list ;

    std::ostream& print( std::ostream& stm = std::cout ) const {

        for( const cd& c : cd_list ) stm << c << '\n' ;
        return stm ;
    }

    void insert_sorted( const cd& this_cd ) {

        // get an iterator to the first element in cd_list that is greater than this_cd
        // https://en.cppreference.com/w/cpp/algorithm/upper_bound
        const auto where = std::upper_bound( cd_list.begin(), cd_list.end(), this_cd ) ;

        // insert this_cd at that position
        cd_list.insert( where, this_cd ) ;
    }
};

int main() {

    cd_collection coll ;

    coll.insert_sorted( { "ijkl", 2.4, 2007 } ) ;
    coll.insert_sorted( { "abcd", 5.6, 2117 } ) ;
    coll.insert_sorted( { "mnop", 8.2, 2110 } ) ;
    coll.insert_sorted( { "efgh", 1.5, 2019 } ) ;
    coll.insert_sorted( { "qrst", 3.8, 2001 } ) ;

    coll.print() ; // print to stdout

    // print to file
    std::ofstream file { "my_cds.txt" } ;
    coll.print(file) ;
}

http://coliru.stacked-crooked.com/a/89ac6195b6660387

[jjordan33]

I didn't know you could declare a friend inside a struct like that at all.

But this brings up something that is a little confusing....

How did my code know how to act on the < operator with objects without it being overloaded?

I got pretty lucky there, seeing as how, if I had this thought before I tried it, I would've discarded it thinking it wouldn't work.

[JLBorges]

> How did my code know how to act on the < operator with objects without it being overloaded?

It is overloaded; line 14: friend bool operator< ( const cd& a, const cd& b )

[jjordan33]

It's not overloaded in mine though right? I'm confused about how my code worked at all now that I am looking at yours.

[JLBorges]

> I'm confused about how my code worked at all

You are passing a comparison function to sort: cdList.sort(comp);

" to compare the elements, the second version uses the given comparison function"
https://en.cppreference.com/w/cpp/container/list/sort

[jjordan33]

Thanks for the documentation. I learned several things.