intializing vector

This is a discussion on intializing vector within the C++ Programming forums, part of the General Programming Boards category; Code: # include<vector> .... main() { vector <int> k; k[0]=1; k[1]=2; } I receive failure notice during compiling. Do I ...

  1. #1
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    intializing vector

    Code:
    # include<vector>
    ....
    main()
     {
       vector <int> k;
        k[0]=1;
        k[1]=2;
     }
    I receive failure notice during compiling. Do I have to use pushback command or I can avoid it?

  2. #2
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    The vector name is a handle and only contains enough memory to allow the vector to manage the storage of it's elements. A vector internally allocates storage on the free store for the space to store the elements. So, if you don't tell a vector initially how many elements to create storage for, you have to use the vector class methods, like push_back() to create storage for a new element.

    Code:
    #include <iostream>
    #include <vector>
    using namespace std;
    
    int main()
    {
        int a[10];
        cout<<sizeof a<<endl; //int's are 4 bytes each, so size=40
    
        vector <int> v(10);  
        cout<<sizeof v<<endl;  //size is only 16
    
        v[0]=1;
        v[1]=2;
        
        cout<<v[0]<<endl<<v[1]<<endl;
    
        vector <int> k;  //doesn't create any memory for the elements
        k.push_back(10);
        k.push_back(20);
    
        cout<<k[0]<<endl<<k[1]<<endl;
        
    
        return 0;
    }
    Last edited by 7stud; 02-07-2005 at 11:01 AM.

  3. #3
    Cat without Hat CornedBee's Avatar
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    What 7stud says is correct, but it can't be the cause for your error, because that stuff is not detected until runtime.

    What is the error message you get? I see that you forgot to give main a return type.
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
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  4. #4
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    Vector size

    Does it mean that size of a vector with one element is 0?

  5. #5
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    Quote Originally Posted by strickey
    I receive failure notice during compiling.
    Maybe you forgot a using directive?
    Code:
    #include<vector>
    using std::vector;
    
    int main()
    {
       vector <int> k(2);
       k[0]=1;
       k[1]=2;
       return 0;
    }
    If I did your homework for you, then you might pass your class without learning how to write a program like this. Then you might graduate and get your degree without learning how to write a program like this. You might become a professional programmer without knowing how to write a program like this. Someday you might work on a project with me without knowing how to write a program like this. Then I would have to do you serious bodily harm. - Jack Klein

  6. #6
    Registered User hk_mp5kpdw's Avatar
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    Quote Originally Posted by strickey
    Does it mean that size of a vector with one element is 0?
    The size (as returned by the size member function) will be 1. Internally however, the size of a vector object (as returned by sizeof) is fixed. The vector object maintains what are essentially some pointers to the data area (which grows as needed), along with a couple extra data variables that it needs:

    Code:
    vector<int> v;
    
    cout << sizeof(v) << ',' << v.size() << endl;
    
    v.push_back(10);
    cout << sizeof(v) << ',' << v.size() << endl;
    
    v.push_back(20);
    cout << sizeof(v) << ',' << v.size() << endl;
    Output on my machine:
    Code:
    16,0
    16,1
    16,2
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