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C++ STL list.sort Function problem

Posted on 2013-11-11
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Last Modified: 2013-11-11
I am trying to sort an STL list using the list.sort function with a compare function argument.
The list is a list of pointers to a structure type and I wish to sort the list on the basis of the value of one of the structure members (called order).
The code compiles and runs but the sort function just reverses the existing list order regardless of the values of the structure member.
I am using borland C++ Builder 5.

The code is attached.

The displayed results are as follows:
10
11
40
30
2
15
sorted
15
2
30
40
11
10
re-sorted
10
11
40
30
2
15
Unit1.h
Unit1.cpp
0
Comment
Question by:alcindor
  • 2
3 Comments
 
LVL 86

Accepted Solution

by:
jkr earned 500 total points
ID: 39639688
Your comparator function is

int __fastcall Compare(void *item1,void *item2)
{   // used for ordering the list according to the order member of the structure
    Mystructure *ms1 = (Mystructure *)item1;
    Mystructure *ms2 = (Mystructure *)item2;

    return ms1->order - ms2->order;
}

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but it should be a binary predicate returning 'true' or 'false'. If you want the sort order to be ascending to 'order', you shouzld make that

bool __fastcall Compare(void *item1,void *item2)
{   // used for ordering the list according to the order member of the structure
    Mystructure *ms1 = (Mystructure *)item1;
    Mystructure *ms2 = (Mystructure *)item2;

    return ms1->order < ms2->order;
}

Open in new window

0
 
LVL 2

Author Closing Comment

by:alcindor
ID: 39639727
Excellent !
Thanks for that.
I was mis-led by the help on TList::sort which uses a compare function that returns an int.
0
 
LVL 86

Expert Comment

by:jkr
ID: 39639773
Yes, I thought so, since your comparator reminded me of something similar - yet seeing the 'int' return value rang an alarm bell ;o)
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