DaniWeb Forum Index > Software Development > C++

working with array of structs

Please support our C++ advertiser: Intel Parallel Studio Home

•

Join Date: Dec 2006

Posts: 14

Reputation: DynamitMsk is an unknown quantity at this point

Solved Threads: 0

DynamitMsk DynamitMsk is offline

Offline

Newbie Poster

working with array of structs

Dec 22nd, 2006

For my project I'm using an array of structs that I declared in the header file

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
    struct City{
        string name;
        int visits;
    };
 
    City* map[200];
    struct City{
        string name;
        int visits;
    };
    
    City* map[200];

now when I'm trying to initialize it in a constructor, like

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
for (int ii=0; ii<200; ii++)
     map[ii].visits = 0;
for (int ii=0; ii<200; ii++)
     map[ii].visits = 0;

I get an error:

left of '.visits' must have class/struct/union

Is that due to the map being a pointer? Anyone can point me in a right direction for solving this problem?

•

Join Date: Aug 2005

Posts: 5,273

Reputation: iamthwee is a splendid one to behold

Solved Threads: 378

iamthwee

Offline

Posting Expert

Re: working with array of structs

Dec 22nd, 2006

Are we using c or c++?

*Voted best profile in the world*

•

Join Date: Jun 2006

Posts: 7,649

Reputation: ~s.o.s~ has much to be proud of

Solved Threads: 474

~s.o.s~

Offline

Failure as a human

Re: working with array of structs

Dec 22nd, 2006

When you are doing City* map[200] , map is an array of 200 pointers to the type City. So in order to access the members of the City struct using its pointers you need to use the -> operator.

Here are the two ways you can do it:

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
*map[i].visits = 100 ;
// OR
map[i]->visits = 100 ;
*map[i].visits = 100 ;
// OR
map[i]->visits = 100 ;

But, even if you do so your program won't work. Why ? Because you are not allocating memory to the pointers so acutally what they are poniting to is junk i.e. memory which doesn't belong to you. Consider allocating memory to the pointer before using it in your program.

I don't accept change; I don't deserve to live.

Jo Tujhe Jagaaye, Nindein Teri Udaaye Khwaab Hai Sachcha Wahi.
Nindon Mein Jo Aaye Jise To Bhul Jaaye Khawab Woh Sachcha Nahi.
Khwaab Ko Raag De, Nind Ko Aag De

•

Join Date: May 2006

Posts: 3,124

Reputation: WaltP has much to be proud of

Solved Threads: 283

WaltP

Offline

Posting Sensei

Re: working with array of structs

Dec 22nd, 2006

Do, to piggy back on the answer by Mr. ~s.o.s~ :mrgreen: you should probably just declare the map as City map[200];. That would fix your problem.

I also hope this line is not in your header file. It should be in your code file only.

The 3 Laws of the Procrastination Society:
1) Never do today that which can be put off until tomorrow
2) Tomorrow never comes

•

Join Date: Jun 2006

Posts: 7,649

Reputation: ~s.o.s~ has much to be proud of

Solved Threads: 474

~s.o.s~

Offline

Failure as a human

Re: working with array of structs

Dec 22nd, 2006

•

Originally Posted by WaltP

Do, to piggy back on the answer by Mr. ~s.o.s~ :mrgreen: you should probably just declare the map as City map[200];. That would fix your problem.

I also hope this line is not in your header file. It should be in your code file only.

Bleh..I wrongly assumed that the project required him to dynamically allocate memory, otherwise would have told him the simple fix

•

Join Date: Dec 2006

Posts: 14

Reputation: DynamitMsk is an unknown quantity at this point

Solved Threads: 0

DynamitMsk DynamitMsk is offline

Offline

Newbie Poster

Re: working with array of structs

Dec 22nd, 2006

~s.o.s~

The project will require me to dynamically allocate memory, not for this array, but for a link list I'll also need to have.

Here's what I've done for this array:

in the header file I created a pointer to the struct

Help with Code Tags

C++ Syntax (Toggle Plain Text)

City* map;

and in the constructor I did the following:

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
    map = new City[200];
    for (int ii=0; ii<200; ii++)
     map[ii].visits = 0;
    map = new City[200];
    for (int ii=0; ii<200; ii++)
     map[ii].visits = 0;

How is this solution? I believe this is how I'd be alocating memory dynamically, if I needed to. Just have City[array_size_variable]. Is this correct? Thank you for your help.

•

Join Date: Jun 2006

Posts: 7,649

Reputation: ~s.o.s~ has much to be proud of

Solved Threads: 474

~s.o.s~

Offline

Failure as a human

Re: working with array of structs

Dec 22nd, 2006

One important thing in programming is to learn to evaluate your own code by writing it down and testing it with the help of the compiler.

Does the code which you pasted give you any errors ? No, then you are right atleast for the time being. Keep experimenting and you would learn two fold than what you are learning now.

And btw, yes your code is fine for the time being.

•

Join Date: May 2006

Posts: 3,124

Reputation: WaltP has much to be proud of

Solved Threads: 283

WaltP

Offline

Posting Sensei

Re: working with array of structs

Dec 22nd, 2006

•

Originally Posted by DynamitMsk

~s.o.s~

The project will require me to dynamically allocate memory, not for this array, but for a link list I'll also need to have.

Here's what I've done for this array:

in the header file I created a pointer to the struct
Help with Code Tags

C++ Syntax (Toggle Plain Text)
City* map;
City* map;
and in the constructor I did the following:
Help with Code Tags

C++ Syntax (Toggle Plain Text)
    map = new City[200];
    for (int ii=0; ii<200; ii++)
     map[ii].visits = 0;
map = new City[200]; for (int ii=0; ii<200; ii++) map[ii].visits = 0;
How is this solution? I believe this is how I'd be alocating memory dynamically, if I needed to. Just have City[array_size_variable]. Is this correct? Thank you for your help.

No. You probably want:

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
City* map[200];    // declare an array of pointers
 
...
 
    for (int ii=0; ii<200; ii++)
    {
        map[ii] = new City;    // load each pointer with a structure
    }
City* map[200];    // declare an array of pointers

...

    for (int ii=0; ii<200; ii++)
    {
        map[ii] = new City;    // load each pointer with a structure
    }

•

Originally Posted by s.o.s.

And btw, yes your code is fine for the time being.

It is? If so, I stand corrected. I've never seen it done that way.

Last edited by WaltP; Dec 22nd, 2006 at 4:53 pm.

The 3 Laws of the Procrastination Society:
1) Never do today that which can be put off until tomorrow
2) Tomorrow never comes

•

Join Date: Jun 2006

Posts: 7,649

Reputation: ~s.o.s~ has much to be proud of

Solved Threads: 474

~s.o.s~

Offline

Failure as a human

Re: working with array of structs

Dec 22nd, 2006

•

Originally Posted by WaltP

No. You probably want:
Help with Code Tags

C++ Syntax (Toggle Plain Text)
City* map[200];    // declare an array of pointers
 
...
 
    for (int ii=0; ii<200; ii++)
    {
        map[ii] = new City;    // load each pointer with a structure
    }
City* map[200]; // declare an array of pointers ... for (int ii=0; ii<200; ii++) { map[ii] = new City; // load each pointer with a structure }
It is? If so, I stand corrected. I've never seen it done that way.

If it is about the semantics, then yes I agree, what I presented is not array of pointers, but if going implementation wise, both the solutions are the same. But mine with less overhead since the constructor is not getting called in the loop.

Just an example of pointer being used to emulate an array...

An eg.

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#include <iostream>
#include <string>
using namespace std ;
 
struct City
{
    string name ;
    int visit ;
} ;
 
int main( )
{
    cout << endl << "~s.o.s~'s method" << endl ;
    City* map = new City[3] ;
    for( int i = 0; i < 3; ++i )
    {
        cout << "Enter name: " ;
        cin >> map[i].name ;
        getchar( ) ;
    }
 
    for( int i = 0; i < 3; ++i )
    {
        cout << map[i].name << endl ;
    }
 
    cout << endl << "Mr. WaltP's method" << endl ;
 
    City* mapWalt[3] ;
 
    for( int i = 0; i < 3; ++i )
    {
        mapWalt[i] = new City ;
        cout << "Enter name: " ;
        cin >> mapWalt[i]->name ;
        getchar( ) ;
    }
 
    for( int i = 0; i < 3; ++i )
    {
        cout << mapWalt[i]->name << endl ;
    }
 
}
#include <iostream>
#include <string>
using namespace std ;

struct City
{
    string name ;
    int visit ;
} ;

int main( )
{
    cout << endl << "~s.o.s~'s method" << endl ;
    City* map = new City[3] ;
    for( int i = 0; i < 3; ++i )
    {
        cout << "Enter name: " ;
        cin >> map[i].name ;
        getchar( ) ;
    }

    for( int i = 0; i < 3; ++i )
    {
        cout << map[i].name << endl ;
    }

    cout << endl << "Mr. WaltP's method" << endl ;

    City* mapWalt[3] ;

    for( int i = 0; i < 3; ++i )
    {
        mapWalt[i] = new City ;
        cout << "Enter name: " ;
        cin >> mapWalt[i]->name ;
        getchar( ) ;
    }

    for( int i = 0; i < 3; ++i )
    {
        cout << mapWalt[i]->name << endl ;
    }

}

My output:

•

~s.o.s~'s method
Enter name: abc
Enter name: efg
Enter name: hij

abc
efg
hij

Mr. WaltP's method
Enter name: abc
Enter name: efg
Enter name: hij
abc
efg
hij

In the case of array of pointers, to delete the enteries, you will have to loop through the array once while in my case it would be a simple matter of using the delete[] syntax.

Last edited by ~s.o.s~; Dec 22nd, 2006 at 5:17 pm.