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Sorting STL lists

Hi,

I need some help with STL lists.

I've a list with pointers to some objects,

let's say std::list< ObjectA* myList.

The objects of the class ObjectA all have a
function "int getIntValue()". What I need
now is to sort all elements of "myList" by the
return value of "getIntValue()", i.e. after sorting
the first element in the list will be the one with
the smallest return value of "getIntValue()" ... and
the last obviously with the largest.

Could you give me some hints how to do that?

Regards,
Tim
Jul 14 '07 #1
6 2165
On 2007-07-14 16:29, Tim Frink wrote:
Hi,

I need some help with STL lists.

I've a list with pointers to some objects,

let's say std::list< ObjectA* myList.

The objects of the class ObjectA all have a
function "int getIntValue()". What I need
now is to sort all elements of "myList" by the
return value of "getIntValue()", i.e. after sorting
the first element in the list will be the one with
the smallest return value of "getIntValue()" ... and
the last obviously with the largest.

Could you give me some hints how to do that?
You have to create a comparator function, something like

bool cmp(const ObjectA* a, const ObjectA* b)
{
return a->getIntValue() < b->getIntValeu();
}
and then use

std::sort(list.begin(), list.end(), cmp);

--
Erik Wikström
Jul 14 '07 #2
Tim Frink wrote:
I've a list with pointers to some objects,

let's say std::list< ObjectA* myList.

The objects of the class ObjectA all have a
function "int getIntValue()". What I need
now is to sort all elements of "myList" by the
return value of "getIntValue()", i.e. after sorting
the first element in the list will be the one with
the smallest return value of "getIntValue()" ... and
the last obviously with the largest.

#include <list>
#include <cassert>
#include <iostream>
#include <ostream>

struct Object {

int i;

Object ( int k )
: i ( k )
{}

int getIntValue ( void ) {
return ( i );
}

};

bool ptr_compare( Object * lhs, Object * rhs ) {
assert( lhs != 0 );
assert( rhs != 0 );
return ( lhs->getIntValue() < rhs->getIntValue() );
}

typedef std::list< Object* PtrList;

int main ( void ) {
PtrList ptr_list;
ptr_list.push_back( new Object ( 4 ) );
ptr_list.push_back( new Object ( 3 ) );
ptr_list.push_back( new Object ( 1 ) );
ptr_list.push_back( new Object ( 7 ) );
ptr_list.push_back( new Object ( 4 ) );
ptr_list.push_back( new Object ( 5 ) );
for ( PtrList::const_iterator iter = ptr_list.begin();
iter != ptr_list.end(); ++ iter ) {
std::cout << (*iter)->getIntValue() << '\n';
}
std::cout << '\n';

ptr_list.sort( &ptr_compare ); // !!!!!!!!!!!!!!!!!!!!

for ( PtrList::const_iterator iter = ptr_list.begin();
iter != ptr_list.end(); ++ iter ) {
std::cout << (*iter)->getIntValue() << '\n';
}
}

Best

Kai-Uwe Bux
Jul 14 '07 #3
Erik Wikström wrote:
On 2007-07-14 16:29, Tim Frink wrote:
>Hi,

I need some help with STL lists.

I've a list with pointers to some objects,

let's say std::list< ObjectA* myList.

The objects of the class ObjectA all have a
function "int getIntValue()". What I need
now is to sort all elements of "myList" by the
return value of "getIntValue()", i.e. after sorting
the first element in the list will be the one with
the smallest return value of "getIntValue()" ... and
the last obviously with the largest.

Could you give me some hints how to do that?

You have to create a comparator function, something like

bool cmp(const ObjectA* a, const ObjectA* b)
{
return a->getIntValue() < b->getIntValeu();
}
and then use

std::sort(list.begin(), list.end(), cmp);
I think, std::sort() needs random access iterators. You want:

list.sort( cmp );
Best

Kai-Uwe Bux

Jul 14 '07 #4
Tim Frink wrote:
Hi,

I need some help with STL lists.

I've a list with pointers to some objects,

let's say std::list< ObjectA* myList.

The objects of the class ObjectA all have a
function "int getIntValue()". What I need
now is to sort all elements of "myList" by the
return value of "getIntValue()", i.e. after sorting
the first element in the list will be the one with
the smallest return value of "getIntValue()" ... and
the last obviously with the largest.

Could you give me some hints how to do that?

Regards,
Tim
As stated earlier you need a comparator function to use with
std::sort(). Myself I created a template to handle iterator type
comparisons like that below

template <typename T>
inline bool DRefCompare(T i1,T i2)
{
return *i1 < *i2;
}

class Compartment
{
public:
typedef std::vector<MyObjectvector;
typedef std::vector<vector::iteratorivector;
typedef std::vector<vector::const_iteratorcivector;

bool operator < (const MyObject &inp) const;
/*
Rest of the definition here
*/
};

MyObject::ivector viCmp;
/*
Other code here
Create sorted list of MyObject iterators
*/
MyObject::vector::iterator iC = vCmp.begin();
do
{
viCmp.push_back(iC);
}
while (++iC != vCmp.end());

std::sort(viCmp.begin(),viCmp.end(),&DRefCompare<M yObject::vector::const_iterator>);

Naturally this relies on the operator < () exists for type T, so changes
to make it more general might be more suitable

JB
Jul 14 '07 #5
bnonaj wrote:
Tim Frink wrote:
>Hi,

I need some help with STL lists.

I've a list with pointers to some objects,

let's say std::list< ObjectA* myList.

The objects of the class ObjectA all have a
function "int getIntValue()". What I need
now is to sort all elements of "myList" by the
return value of "getIntValue()", i.e. after sorting
the first element in the list will be the one with
the smallest return value of "getIntValue()" ... and
the last obviously with the largest.

Could you give me some hints how to do that?

Regards,
Tim

As stated earlier you need a comparator function to use with
std::sort(). Myself I created a template to handle iterator type
comparisons like that below

template <typename T>
inline bool DRefCompare(T i1,T i2)
{
return *i1 < *i2;
}

class Compartment
{
public:
typedef std::vector<MyObjectvector;
typedef std::vector<vector::iteratorivector;
typedef std::vector<vector::const_iteratorcivector;

bool operator < (const MyObject &inp) const;
/*
Rest of the definition here
*/
};

MyObject::ivector viCmp;
/*
Other code here
Create sorted list of MyObject iterators
*/
MyObject::vector::iterator iC = vCmp.begin();
do
{
viCmp.push_back(iC);
}
while (++iC != vCmp.end());

std::sort(viCmp.begin(),viCmp.end(),&DRefCompare<M yObject::vector::const_iterator>);
Naturally this relies on the operator < () exists for type T, so changes
to make it more general might be more suitable

JB
That would be using std::list::sort for std::list types as pointed out
earlier

JB
Jul 14 '07 #6
On Sat, 14 Jul 2007 16:53:50 +0200, Kai-Uwe Bux wrote:
>
ptr_list.sort( &ptr_compare ); // !!!!!!!!!!!!!!!!!!!!

for ( PtrList::const_iterator iter = ptr_list.begin();
iter != ptr_list.end(); ++ iter ) {
std::cout << (*iter)->getIntValue() << '\n';
}
}
Many thanks.

Regards,
Tim

Jul 16 '07 #7

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