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STL vector - deleting the last element

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STL vector - deleting the last element

 
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  #1
May 3rd, 2007
Hello! I'm working on an assignment that requires me to store custom classes in an STL vector and do things like search, display, and delete them, then write them all to file.

This is going well, but my code to delete an object (adapted from this thread) works on any object EXCEPT the last object; it crashes whenever I try to delete the last one in the chain.

Here is my code for the deletion:

C++ Syntax (Toggle Plain Text) 
  1. vector<course> cache; //holds objects in memory
  2. vector<course>::iterator point; //points at the vector
  3.  
  4.  
  5. //fill vector with data, etc etc
  6.  
  7.  
  8. cout << "Input course number to delete: ";
  9. cin >> target;
  10. point = cache.begin();
  11.  
  12. for(int n=0; point != cache.end(); point++, n++) {
  13.    if(point->get_number() == target)
  14.       { cache.erase(point); } //erases object
  15. }
Any idea what I'm doing wrong? Thanks!
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Re: STL vector - deleting the last element

 
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  #2
May 3rd, 2007
Eh, a general rule of thumb is not to loop with iterators if you're going to modify the container. The good news is that the algorithm header has a function template that you can use to remove all occurrences of an item:
#include <algorithm>

cout << "Input course number to delete: ";
cin >> target;

cache.erase ( remove ( cache.begin(), cache.end(), target ), cache.end() );
Last edited by Narue; May 3rd, 2007 at 2:16 pm.
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Re: STL vector - deleting the last element

 
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  #3
May 3rd, 2007
However, my course class is composed as follows:

C++ Syntax (Toggle Plain Text) 
  1. class course {
  2.    private:
  3.       string prefix;
  4.       int number;
  5.       string title;
  6.       int credits;
  7.       char grade;
  8.    public:
  9.       //Constructors
  10.       course();
  11.       course(string, int, string, int, char);
  12.       //Accessors
  13.       void set();
  14.       void print() const;
  15.       string get_prefix() const;
  16.       int get_number() const;
  17.       string get_title() const;
  18.       int get_credits() const;
  19.       char get_grade() const; 
  20.       //Operators
  21.       bool operator < (course) const;
  22.       bool operator > (course) const;
  23.       bool operator == (course) const;
  24.       bool operator != (course) const;
  25. };

Wouldn't that code erase an object any time it found target in the course number or credits, since both are ints?

(sorry for the dumb questions XD)
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Re: STL vector - deleting the last element

 
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  #4
May 3rd, 2007
Also, I got a ton of errors when I slap-dashed that into my code.
(and yes, I have included the algorithm library)
Attached Thumbnails
errors.jpg  
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Re: STL vector - deleting the last element

 
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  #5
May 3rd, 2007
Sorry, I didn't notice you were using custom objects. The easiest solution would be to do some research on remove_if. That function allows you to pass a predicate for custom comparisons. It would probably be easier than modifying your class to remove those errors.
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Re: STL vector - deleting the last element

 
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  #6
May 3rd, 2007
C++ Syntax (Toggle Plain Text) 
  1. struct eq_course_number
  2. {
  3.   explicit eq_course_number( int n ) : course_num(n) {}
  4.   bool operator() ( const course& c ) const
  5.   { return c.get_number() == course_num ; }
  6.   int course_num ;
  7. };
  8.  
  9. //
  10. cin >> target;
  11. if( remove_if ( cache.begin(), cache.end(),
  12.                       eq_course_number(target) ) != cache.end() )
  13.       cache.pop_back() ;

an alternative is to overload operator== to test for equivalence between course and an integer course_number. then remove could be used (as in Narue's post)
Last edited by vijayan121; May 3rd, 2007 at 2:56 pm.
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Re: STL vector - deleting the last element

 
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  #7
May 3rd, 2007
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Originally Posted by vijayan121 View Post
C++ Syntax (Toggle Plain Text) 
  1. if( remove_if ( cache.begin(), cache.end(),
  2.                       eq_course_number(target) ) != cache.end() )
  3.       cache.pop_back() ;
this will remove only one object; i'd assumed that courses in the cache would be unique. if you want to remove many elements which have the same course number, it needs to be written as Narue had suggested.
C++ Syntax (Toggle Plain Text) 
  1. cache.erase( remove_if ( cache.begin(), cache.end(),
  2.                       eq_course_number(target) ), cache.end() ) ;
this would also work if there is just one element or even none at all.
Last edited by vijayan121; May 3rd, 2007 at 3:24 pm.
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Re: STL vector - deleting the last element

 
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  #8
May 3rd, 2007
The object (to hold college course data) would probably only contain one object of a given course number, since it would be unlikely to be enrolled in two identical college courses in the same semester.

Could someone explain to me how the
C++ Syntax (Toggle Plain Text) 
  1. explicit eq_course_number( int n ) : course_num(n) {}
works? I'm farmiliar with using class objects and the () operator to get stuff done, but I'm having trouble passing extra parameters to functions like he does.

EDIT: And, I thought pop_back() will always remove the LAST element in a vector.
The code still works, I'm just a tad confused on how it does so
Last edited by yesm; May 3rd, 2007 at 3:37 pm. Reason: Added another question
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Re: STL vector - deleting the last element

 
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  #9
May 3rd, 2007
Note: I also threw in an overloaded == operator for use with integers
C++ Syntax (Toggle Plain Text) 
  1. bool course::operator == (int n) const //returns based on class #
  2. {//overloaded for use with an integer
  3.    if (number == n) {
  4.       return true;
  5.    } else {
  6.       return false;
  7.    }
  8. }
And now Narue's method works!
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Re: STL vector - deleting the last element

 
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  #10
May 3rd, 2007
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Originally Posted by yesm View Post
Could someone explain to me how the
C++ Syntax (Toggle Plain Text) 
  1. explicit eq_course_number( int n ) : course_num(n) {}
works?
eq_course_number is a class and the line you are referring to is the constructor. logically, the == operation is a binary one; the remove_if requires a unary predicate. so we have an object which stores the extra information in a member variable and uses it as the second parameter when the function ( operator() ) is called with just one parameter. in c++ jargon, this is called binding an argument.

the explicit keyword disables an implicit conversion from an int to an object of type eq_course_number. it does not affect anything else.
Last edited by vijayan121; May 3rd, 2007 at 3:42 pm.
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