STL General: How to remove elements of a particular value from a container ?
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Thread: STL General: How to remove elements of a particular value from a container ?

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  1. #1
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    STL General: How to remove elements of a particular value from a container ?

    Q: What is the best way to remove all elements of a particular value from a container?

    A: Use the so-called 'remove/erase' idiom:

    Code:
    // taken from Item 32 of Scott Myers' Effective STL
    vector<int> v;
    v.erase(remove(v.begin(), v.end(), 99), v.end());
    This is because of the simple fact that 'remove()' is a generic algorithm which takes iterators as its arguments. The iterators know nothing of their container; the STL was designed so as to grant the smallest possible amount of coupling. Thus, the iterators cannot call any container's 'erase()' or similar function.

    'remove()' DOES return the container's new 'end()' (which just
    happens to be the first element that was removed) and this value
    can be used in the container's 'erase()' call.


    Last edited by Andreas Masur; July 24th, 2005 at 01:59 PM.

  2. #2
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    Re: STL General: How to remove elements of a particular value from a container ?

    Here is an example of using std::remove_if with a predicate function. The principle is the same as the original post using std::remove. The example is lengthier since I included enough detail for someone to copy, paste, compile the program, and analyze each step with a debugger.

    Code:
    #include <iostream>
    #include <vector>
    #include <algorithm>
    
    // predicate function that determines if a value is an odd number.
    bool IsOdd (int i) { return ((i%2)==1); }
    
    void remove_if_example()
    {
        std::vector<int> myints;
        for(int i = 0; i < 20; ++i)
        {
    	myints.push_back(i);
        }
        
        // remove odd numbers:                       
        std::vector<int>::iterator newEnd =
    	std::remove_if (myints.begin(), myints.end(), IsOdd);
    
        // print the vector range.  It still contains 20 elements!  The even
        // numbers were copied to the front of the vector while the odd numbers
        // were simply overwritten until the entire container has been traversed.
        std::cout << "vector contains:";
        for (std::vector<int>::iterator p = myints.begin(); p != myints.end(); ++p)
        {
    	std::cout << " " << *p;
        }
        std::cout << std::endl;
    
        // now erase the elements past the newEnd that was determined
        // by std::remove_if
        myints.erase(newEnd, myints.end());
    
        // print the range again.  Now the vector will only contain the
        // even numbers 
        std::cout << "vector contains:";
        for (std::vector<int>::iterator p = myints.begin(); p != myints.end(); ++p)
        {
    	std::cout << " " << *p;
        }
        std::cout << std::endl;
    }
    
    int main ()
    {    
        remove_if_example();
        return 0;
    }
    Last edited by Andreas Masur; December 25th, 2010 at 01:26 PM.

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