using set of class obj to sort by obj member

lawrenced · 07-22-2009, 10:15 PM

hi,

I have a Class which one of its attributes is a date -

Class reservations
members - name
- date

lets say I have multiple restaurants and want to insert class reservations into the set - but have it automatically sort by date.

can I insert pointers to the class object into the set and have it automatically sort by the date attribute?

many thanks

tabstop · 07-22-2009, 10:41 PM

You are using this magic word "set". Do you have a set, somewhere? If so, then yes it is sorted, although you will have to define the ordering yourself. (If you really mean that you have a set of pointers, then you will have to define a function object; if you are inserting the class it would be possible for you to define operator < for the class.)

lawrenced · 07-22-2009, 10:50 PM

I am new to c++ so I don't fully understand -

I will be creating set<Reservations*> restaurant1;
I want to randomly allocate

vector<Reservations*> reservationsmade;

to set<Reservations*> restaurant1; or
set<Reservations*> restaurant2;

but want it to automatically sort by

reservationsmade[rsvpnumber]->Getdate();

when i insert it into the set.

so that when I later get

for(int table = 0; table < totalrsvpsatrestaurant; ++table)
{
restaurant1[table]->Getdate()
}

it will display dates in sequential order...

tabstop · 07-22-2009, 11:01 PM

You'll have to decide whether you want a set or a vector. Sets automatically sort, but you don't get to use [brackets]. Vectors allow you to use [brackets], but you will have to keep them sorted yourself.

Either way, you will need to write a function that takes two pointers-to-Reservations and "compares" them -- decides which one comes first.

(I am assuming that you have made a conscious design decision to use pointers-to-Reservations; at the moment it seems supremely silly but I'm trusting you that there's a reason.)

lawrenced · 07-22-2009, 11:14 PM

Quote:

Originally Posted by tabstop

You'll have to decide whether you want a set or a vector. Sets automatically sort, but you don't get to use [brackets]. Vectors allow you to use [brackets], but you will have to keep them sorted yourself.

Either way, you will need to write a function that takes two pointers-to-Reservations and "compares" them -- decides which one comes first.

(I am assuming that you have made a conscious design decision to use pointers-to-Reservations; at the moment it seems supremely silly but I'm trusting you that there's a reason.)

c++ does not have an STL that will find the best place to insert the object in a list based on some sortable criteria? i was originally going to use list, and run through the list comparing dates, but speed is very critical. set will not automatically sort once i insert?

tabstop · 07-22-2009, 11:20 PM

Quote:

Originally Posted by lawrenced

c++ does not have an STL that will find the best place to insert the object in a list based on some sortable criteria? i was originally going to use list, and run through the list comparing dates, but speed is very critical. set will not automatically sort once i insert?

When I said "sets automatically sort" I meant exactly what I said. If you were storing, say, ints or floats it would be completely automatic. But since you just made up a Reservation object out of your own head, you also have to specify how they get sorted. (After all, presumably there's more than one member of this Reservation object. How is the machine to know it isn't supposed to sort by number in party? Or last name? Or the order they were created?) That's why you are allowed to define "<" for new objects like that.

(However, if you are storing pointers, pointers already come with a "<" defined -- whichever one comes first in memory. You will have to specify a function object if you wish to store pointers in your set.)

lawrenced · 07-22-2009, 11:27 PM

i am confused on syntax. may you offer some example code as help please?

tabstop · 07-22-2009, 11:32 PM

Well. The most obvious thing would be this:

Code:

class Reservation {
    //whatever
};

bool operator< (Reservation left, Reservation right) {
    /* I'm assuming that getDate gets some sort of timestamp */
    /* If not, you can put your date logic here */
    return left.getDate() < right.getDate();
}

std::set<Reservation> reservationsmade;

Sebastiani · 07-22-2009, 11:33 PM

Example.

tabstop · 07-22-2009, 11:37 PM

Quote:

Originally Posted by Sebastiani

Example.

That's the most ... instructive ... example I've ever seen.

Sebastiani · 07-23-2009, 01:43 AM

>> That's the most ... instructive ... example I've ever seen.

What? It looks ok to me.

Elysia · 07-23-2009, 03:01 AM

Quote:

Originally Posted by tabstop

Well. The most obvious thing would be this:

Code:

class Reservation {
    //whatever
};

bool operator< (Reservation left, Reservation right) {
    /* I'm assuming that getDate gets some sort of timestamp */
    /* If not, you can put your date logic here */
    return left.getDate() < right.getDate();
}

std::set<Reservation> reservationsmade;

operator < should accept two const references for efficiency's sake.
And lawrenced: avoid pointers in the first place. Just create your Reservations and put them into the set. No new. No delete.

lawrenced · 07-23-2009, 08:47 PM

Elysia - thank you very much for suggestion - Reservation will grown to have many characteristics - very large class with many private data members - i think i will want to keep it as pointer for efficiency?

Elysia · 07-24-2009, 03:48 AM

Pointers are only necessary if you're going to move around data a lot, for example assigning a set to another.
However, as the rule goes, start without pointers first, and if you application is running slow (you can use a profiler), then optimize it.

07-22-2009, 10:15 PM	#1
lawrenced Registered User Join Date: Jul 2009 Posts: 24	using set of class obj to sort by obj member hi, I have a Class which one of its attributes is a date - Class reservations members - name - date lets say I have multiple restaurants and want to insert class reservations into the set - but have it automatically sort by date. can I insert pointers to the class object into the set and have it automatically sort by the date attribute? many thanks
lawrenced is offline

07-22-2009, 10:41 PM	#2
tabstop and the Hat of Guessing Join Date: Nov 2007 Posts: 8,740	You are using this magic word "set". Do you have a set, somewhere? If so, then yes it is sorted, although you will have to define the ordering yourself. (If you really mean that you have a set of pointers, then you will have to define a function object; if you are inserting the class it would be possible for you to define operator < for the class.)
tabstop is offline

07-22-2009, 10:50 PM	#3
lawrenced Registered User Join Date: Jul 2009 Posts: 24	I am new to c++ so I don't fully understand - I will be creating set<Reservations> restaurant1; I want to randomly allocate vector<Reservations> reservationsmade; to set<Reservations> restaurant1; or set<Reservations> restaurant2; but want it to automatically sort by reservationsmade[rsvpnumber]->Getdate(); when i insert it into the set. so that when I later get for(int table = 0; table < totalrsvpsatrestaurant; ++table) { restaurant1[table]->Getdate() } it will display dates in sequential order...
lawrenced is offline

07-22-2009, 11:01 PM	#4
tabstop and the Hat of Guessing Join Date: Nov 2007 Posts: 8,740	You'll have to decide whether you want a set or a vector. Sets automatically sort, but you don't get to use [brackets]. Vectors allow you to use [brackets], but you will have to keep them sorted yourself. Either way, you will need to write a function that takes two pointers-to-Reservations and "compares" them -- decides which one comes first. (I am assuming that you have made a conscious design decision to use pointers-to-Reservations; at the moment it seems supremely silly but I'm trusting you that there's a reason.)
tabstop is offline

07-22-2009, 11:27 PM	#7
lawrenced Registered User Join Date: Jul 2009 Posts: 24	i am confused on syntax. may you offer some example code as help please?
lawrenced is offline

07-22-2009, 11:33 PM	#9
Sebastiani Guest Join Date: Aug 2001 Posts: 4,923	Example.
Sebastiani is offline

07-23-2009, 01:43 AM	#11
Sebastiani Guest Join Date: Aug 2001 Posts: 4,923	>> That's the most ... instructive ... example I've ever seen. What? It looks ok to me.
Sebastiani is offline

07-23-2009, 08:47 PM	#13
lawrenced Registered User Join Date: Jul 2009 Posts: 24	Elysia - thank you very much for suggestion - Reservation will grown to have many characteristics - very large class with many private data members - i think i will want to keep it as pointer for efficiency?
lawrenced is offline

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