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array in c++

Posted on 2010-08-22
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Last Modified: 2013-11-23
Hi there,

I have 2dim array which have huge number I have no idea about number my question is How can I initialize this array if not are there any different way like array list in Java which is accept two dim and extend automatically
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Question by:obad62
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Expert Comment

by:phoffric
ID: 33497411
In c++, if you say:
     int arr2d[3][4] = {0};
then every element of the array will be initialized to 0.
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LVL 32

Expert Comment

by:phoffric
ID: 33497412
If you say:
    int arr2d[3][4] = {5};
then arr2d[0][0] will be set to 5, and all the other elements in the array will be set to 0.
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phoffric earned 2000 total points
ID: 33497423
If you want to be able to extend an array, then I recommend using vector.
    http://www.cplusplus.com/reference/stl/vector/

There are various ways to construct/initialize a vector.
    http://www.cplusplus.com/reference/stl/vector/vector/

For example:
    vector<int> second (4,100);      // four ints with value 100

Below is the sample code for the vector constructor link.

To extend the array (without worrying about whether it needs extending), you can just add elements to the vector using push_back.
      http://www.cplusplus.com/reference/stl/vector/push_back/
// constructing vectors
#include <iostream>
#include <vector>
using namespace std;

int main ()
{
  unsigned int i;

  // constructors used in the same order as described above:
  vector<int> first;                                // empty vector of ints
  vector<int> second (4,100);                       // four ints with value 100
  vector<int> third (second.begin(),second.end());  // iterating through second
  vector<int> fourth (third);                       // a copy of third

  // the iterator constructor can also be used to construct from arrays:
  int myints[] = {16,2,77,29};
  vector<int> fifth (myints, myints + sizeof(myints) / sizeof(int) );

  cout << "The contents of fifth are:";
  for (i=0; i < fifth.size(); i++)
    cout << " " << fifth[i];

  cout << endl;

  return 0;
}

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Expert Comment

by:phoffric
ID: 33497440
Above was a 1D discussion of vectors. Here's a thread needing a 2D vector:
     http://w.e-e.com/pJzEMB
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LVL 40

Expert Comment

by:evilrix
ID: 33498588
If you want a simple 2D array, Boost's ublas library offers a simple to use matrix.
http://www.boost.org/doc/libs/1_38_0/libs/numeric/ublas/doc/matrix.htm

If you matrix is huge it is worth considering the sparse version.
http://www.boost.org/doc/libs/1_38_0/libs/numeric/ublas/doc/matrix_sparse.htm
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