intializing vector

Code:

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# include<vector>
....
main()
 {
  vector <int> k;
    k[0]=1;
    k[1]=2;
 }

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I receive failure notice during compiling. Do I have to use pushback command or I can avoid it?

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:

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#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;
}

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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.

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

Quote:

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Originally Posted by strickey

I receive failure notice during compiling.

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Maybe you forgot a using directive?

Code:

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#include<vector>
using std::vector;

int main()
{
  vector <int> k(2);
  k[0]=1;
  k[1]=2;
  return 0;
}

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Quote:

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Originally Posted by strickey

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

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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:

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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;

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Output on my machine:

Code:

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16,0
16,1
16,2

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