STL sorting of list

[atticusr5]

Hello all,

I know there is a "sort" method in the STL for list. However I am not sure how to use it, and my research was unable to help me out. What I am trying to do is sort my master list by the averages, lowest to highest.

Can anyone give me clues as how to use the STL sort to do this?

Thanks

/*******************************************************************************************
Devang N. Joshi										   *
CSCI 271 										   *
Assignment Three									   *
February 19th, 2011									   *
STL Double Linked List									   *
											   *
The purpose of this assignment is to gain experiance using C++ built in List STL	   *
/******************************************************************************************/
#include <iostream>									// |
#include <iomanip>									// |
#include <fstream>									// |
#include <cstdlib>									// |
#include <cstring>									// |
#include <cmath>									// |
#include <list>										// |
using namespace std;									// |
//-----------------------------------------------------------------------------------------|
struct cProg3										// |
{											// |
	char lname[21];									// |
	float avg;									// |
	int gSize;									// |
	cProg3();									// |
	void Read_m(fstream &Infile);							// |
	void Read_g(fstream &Infile);							// |	
											// |
											// |
};											// |
					/***Functions**/				// |
//-----------------------------------------------------------------------------------------|
cProg3::cProg3()									// |
{											// |
	strcpy(lname,"");								// |
	avg = 0;									// |
	gSize = 0;									// |
}											// |
//-----------------------------------------------------------------------------------------|
void cProg3::Read_m(fstream &Infile)							// |
{ Infile>>lname>>avg; }									// |
//-----------------------------------------------------------------------------------------|
void cProg3::Read_g(fstream &Infile)							// |
{ Infile>>lname>>gSize; }								// |
//-----------------------------------------------------------------------------------------|					
					/***MAIN***/					// |
//-----------------------------------------------------------------------------------------|
int main()										// |
{											// |
//-----------------------------------------------------------------------------------------|
	/***File Handlers***/								// |
	fstream Infile;									// |	
	fstream Outfile;								// |
											// |
	/***Class Decleration***/							// |
	cProg3 Prog3;									// |
											// |
	/***List Decleration***/							// |
	list<cProg3>theList;		     //the list for master.dat			// |
	list<cProg3>groupList;		     //the list for groups.dat			// |
	list<cProg3>::iterator it_m;	     //master list iterator			// |
	list<cProg3>::iterator it_g;         //groups list iterator			// |
	list<cProg3>::iterator loopCount_m;  //for read & write loops in master list	// |
	list<cProg3>::iterator loopCount_g;  //for read & write loops in groups list    // |
//-----------------------------------------------------------------------------------------|
	/***Open Master Data and error check***/					// |				
	Infile.open("master.dat",ios::in);						// |
	if(!Infile)									// |
	{										// |
		cerr<<"FILE NOT FOUND!!!!!!!!!!!!!"<<endl;				// |
		exit(1);								// |
											// |
	}										// |
	else										// |
	{										// |
											// |
		cout<<"Master Data File Found & Opened"<<endl;				// |
											// |
		/***Load Data into List***/						// |
		while(!Infile.eof())							// |
		{									// |	
			Prog3.Read_m(Infile);						// |
			theList.push_back(Prog3);					// |
											// |
		};//while								// |
											// |
		cout<<"Master Data File Loaded into List"<<endl<<endl;			// |
		Infile.close();								// |
											// |
	}										// |
//-----------------------------------------------------------------------------------------|
	/***Open Groups Data and error check***/					// |					
	Infile.open("groups.dat",ios::in);						// |
	if(!Infile)									// |
	{										// |
		cerr<<"FILE NOT FOUND!!!!!!!!!!!!!"<<endl;				// |
		exit(1);								// |
											// |
	}										// |
	else										// |
	{										// |
											// |
		cout<<"Groups Data File Found & Opened"<<endl;				// |
											// |
		/***Load Data into Array***/						// |
		while(!Infile.eof())							// |
		{									// |
			Prog3.Read_g(Infile);						// |
			groupList.push_back(Prog3);					// |
											// |
		};//while								// |
											// |
		cout<<"Groups Data File Loaded into List"<<endl<<endl;			// |
		Infile.close();								// |
											// |
	}										// |
//-----------------------------------------------------------------------------------------|
	/***Sort List***/
	it_m = theList.begin();
	loopCount_m = theList.end();
	theList.sort();
		
											// |
//-----------------------------------------------------------------------------------------|
	/***Print Lists Back Out**/							// |
	Outfile.open("output.dat",ios::out);						// |
//-----------------------------------------------------------------------------------------|
	/***Print Master List***/							// |
											// |
	loopCount_m = theList.end();         //sent loop counter to end of list		// |
	loopCount_m--;		             //move one position back for proper output	// |
	it_m = theList.begin();	             //set iterator to begining of list		// |
											// |
	while(it_m != loopCount_m)							// |
	{										// |
		cout<<it_m->lname<<" "<<it_m->avg<<endl;				// |
		Outfile<<it_m->lname<<" "<<it_m->avg<<endl;				// |
		it_m++;									// |
											// |
	};//end loop									// |
	cout<<endl<<endl;								// |
	Outfile<<endl<<endl;								// |
//-----------------------------------------------------------------------------------------|
	/***Print Group List***/							// |					
											// |
	loopCount_g = groupList.end();       //sent loop counter to end of list		// |
	loopCount_g--;		             //move one position back for proper output // |
	it_g = groupList.begin();	     //set iterator to begining of list		// |
											// |
	while(it_g != loopCount_g)							// |
	{										// |
		cout<<it_g->lname<<" "<<it_g->gSize<<endl;				// |
		Outfile<<it_g->lname<<" "<<it_g->gSize<<endl;				// |
		it_g++;									// |
											// |
	};//end loop									// |
	cout<<endl<<endl;								// |
	Outfile<<endl<<endl;								// |
//-----------------------------------------------------------------------------------------|
	Outfile.close();								// |
											// |
	return 0;									// |
											// |
}//end main										// |
/******************************************************************************************/

[NathanOliver]

to sort the list using sort you would have to do

sort(theList.first(), theList.end(), compareFunction);

the compareFunction is one that you would have to write to compare the avg of one object to the avg of another object. this should help you out.

[mike_2000_17]

@NathanOliver: The <algorithm> version of sort cannot be used for the list container because it requires a random-access iterator (and list is a linked-list so it only provides a Forward and Backward Iterator concept). The way the OP did is correct.

@atticusr:
The sorting algorithm relies on the comparison (by default) of two elements using the < (less-than) comparison. So, the underlying type has to have that operator defined. In other words, if you have "list<MyType> l; MyType a, b;", you have to be able to write "a < b". To be able to do that for a custom type, you need to overload the operator <. As so:

struct cProg3
{
  char lname[21];
  float avg;
  int gSize;
  cProg3();
  void Read_m(fstream &Infile);
  void Read_g(fstream &Infile);
  //Overload operator < here:
  bool operator < (const cProg3& other) const {
    return (avg < other.avg); //compare by average.
  };
};

That should work.

[atticusr5]

@mike_2000_17, just to make sure I understand what you did with the overload,

bool operator overload < as a bool variable, that works on the struct cProg3 (and is a const). This will allow it to access the struct member 'avg' and make the comparison and return either a true or false, which the .sort() function can then use?

Does that sound about right? I just want to make sure I have the understanding locked down.

@nathanOliver, thanks to you too for the link and the example!

atticusr5

[Fbody]

>>Does that sound about right? I just want to make sure I have the understanding locked down.
It looks like you have the right idea.

If you make an operator function a member of a class/struct (or a friend of the class/struct) it will have access to the data members just like any other member function and the compiler will automatically call it under the correct circumstances.

[atticusr5]

Great good trick to know. Thanks again everyone!