How to assign an element from a vector to another?

[hajimeml]

I have two pointers pointing to two different double vectors. Could anybody please let me know the proper way of assigning an element from a vector to the other? Here is a sample code. Please note that even I specify the size of DoubleArray (i.e. DoubleArray *ptr1 = new DoubleArray[3]; ), I still get a bus error. As I am trying to integrate my code to another program written by others, I have to use the pointer to DoubleArray data structure. In their program, they just use DoubleArray *ptr
as far as I can tell. Thanks.



// Assign the third element of the vector pointed to by ptr2 to the second
// element of the vector pointed to by ptr1

#include <iostream>
#include <vector>
#include <string>

using namespace std;

typedef vector<double> DoubleArray;

int main()
{

DoubleArray *ptr1 = new DoubleArray;
DoubleArray *ptr2 = new DoubleArray;

// Initialize the elements of the first vector (pointed to by ptr1)
(*ptr1)[0] = 10;
(*ptr1)[1] = 20;
(*ptr1)[2] = 30;

// Initialize the elements of the second vector (pointed to by ptr2)
(*ptr2)[0] = 123;
(*ptr2)[1] = 456;
(*ptr2)[2] = 789;

cout << "Second element of the vector pointed to by ptr1: " << (*ptr1)[1] << "\n";
cout << "Third element of the vector pointed to by ptr2: " << (*ptr2)[2] << "\n";

//Assign the third element of the vector pointed to by ptr2 to the second element of the
//vector pointed to by ptr1

(*ptr1)[1] = (*ptr2)[2];

cout << "Modified second element of the vector pointed to by ptr1: " << (*ptr1)[1] << "\n";

delete ptr1;
delete ptr2;

return 0;

}

[Philip Nicoletti]

1) if you use operator [] , the element must already exist in the vector.
In your example above, all the vectors are of size 0.

2) you mentioned doing:

DoubleArray *ptr1 = new DoubleArray[3];

That creates an array of DoubleArray of size 3 .. but each element
has a size of 0.

3) You probably want:

DoubleArray *ptr1 = new DoubleArray (3);

4) Why are you using pointers and new ?????

[Paul McKenzie]

I have two pointers pointing to two different double vectors.

Why are you making a simple C++ program look like Java code? Here is the same example, but without usage of "new".

#include <iostream>
#include <vector>
#include <string>

using namespace std;

typedef vector<double> DoubleArray;

int main()
{

  DoubleArray ptr1(3);
  DoubleArray ptr2(3);

  // Initialize the elements of the first vector (pointed to by ptr1)
  ptr1[0] = 10;
  ptr1[1] = 20;
  ptr1[2] = 30;

  // Initialize the elements of the second vector (pointed to by ptr2)
  ptr2[0] = 123;
  ptr2[1] = 456;
  ptr2[2] = 789;

  cout << "Second element of the vector pointed to by ptr1: " << ptr1[1] << "\n";
  cout << "Third element of the vector pointed to by ptr2: " <<  ptr2[2] << "\n";

  //Assign the third element of the vector pointed to by ptr2 to the second element of the
  //vector pointed to by ptr1

  ptr1[1] = ptr2[2];

  cout << "Modified second element of the vector pointed to by ptr1: " << ptr1[1] << "\n";

return 0;

}

Vectors are supposed to reduce or eliminate altogether usage of using "new" in a program. You don't win any kudos or thumbs up by overusing operator "new".

Regards,

Paul McKenzie

[hajimeml]

Thank you for your example. In the sample code, both ptr1 and ptr2 have three elements. The program I try to modify seems to read data from several data files. It seems that the previous programmers used DoubleArray *ptr1 because the number of data in the files not pre-determined. Could you please let me know how to modify your sample so that ptr1 and ptr2 points to DoubleArrays whose sizes are determined in run-time? Are there any good books on vector?

[Lindley]

Vectors hardly need entire books. They're too simple for that.

Just use this reference:
http://www.cplusplus.com/reference/stl/vector/

[hajimeml]

4) Why are you using pointers and new ?????

I used new because when I encountered a bug, somebody suggested that I need to initialize the pointers. So, I thought I could use new to retrieve the memory location. As for why I use pointers to type DoubleArray, it is because I have to integrate my code to an existing program written by others. They often use DoubleArray *ptr in the code.

[Lindley]

Their code sucks then.

But even so, you have to understand why they're using pointers. Are they handing these things around outside of any particular scope? Or are they just using the things as an alternative to pass-by-reference?

If the former, using "new" makes sense (although it is bad design). If the latter, don't bother---just pass the address of a vector on the stack.