CODEWITHSUNDEEP
c++
user2758510
user2758510

Reputation: 159

how to copy an array into somewhere else in the memory by using the pointer

I am completely new in c++ programming. I want to copy the array called distances into where pointer is pointing to and then I want to print out the resul to see if it is worked or not.

this is what I have done:

int distances[4][6]={{1,0,0,0,1,0},{1,1,0,0,1,1},{1,0,0,0,0,0},{1,1,0,1,0,0}};

int *ptr;
ptr  = new int[sizeof(distances[0])];
for(int i=0; i<sizeof(distances[0]); i++){
   ptr=distances[i];
   ptr++;
}

I do not know how to print out the contents of the pointer to see how it works.

Upvotes: 0

Views: 115

Answers (2)

niko
niko

Reputation: 9393

use cout to print values like cout << *ptr;

int distances[4][6]={{1,0,0,0,1,0},{1,1,0,0,1,1},{1,0,0,0,0,0},{1,1,0,1,0,0}};
int *ptr;
ptr  = new int[sizeof(distances[0])]; 

// sizeof(distances[0]) => 6 elements * Size of int = 6 * 4 = 24 ;
// sizeof(distances) => 24 elements * Size of int =  24 * 4 = 96 ;   
// cout << sizeof(distances) << endl << sizeof(distances[0]);

for(int i=0; i<sizeof(distances[0]); i++){
     ptr = &distances[0][i];
     cout << *ptr <<  " ";
     ptr++; 
}

Explaination of your code

ptr = distances[i] => ptr = & distances[i][0];
// To put it simple, your assigning address of 1st column of each row
 // i = 0 , assigns ptr = distances[0][0] address so o/p 1
 // i = 1, assigns ptr = distances[1][0] address so o/p 1
 // but when i > 3, i.e i = 4 , your pointing to some memory address because
// your array has only 4 rows and you have exceeded it so resulting in garbage values

I agree with @juanchopanza solution, calculation of your pointer size is wrong 

 int distances[3][6]={{1,0,0,0,1,0},{1,1,0,0,1,1},{1,0,0,0,0,0}};
 // for this case it fails because 
 // 6 elements * 4 = 24 but total elements are just 18

Use sizeof(distances)/sizeof(int);

Upvotes: 0

juanchopanza
juanchopanza

Reputation: 227418

First of all, the size calculation of your ptr array is wrong. It only seems to work because the size of an int on your platform is 4, which is also one of the dimensions in your array. One way to get the right size is

 size_t len = sizeof(distances)/sizeof(int);

Then you can instantiate your array and use std::copy to copy the elements:

int* ptr  = new int[len];
int* start = &dist[0][0];
std::copy(start, start + len, ptr);

Finally, print them out:

for (size_t i = 0; i < len; ++i)
  std::cout << ptr[i] << " ";
std::cout << std::endl;

....
delete [] ptr; // don't forget to delete what you new

Note that for a real application you should favour using std::vector<int> over a manually managed dynamically allocated array:

// instantiates vector with copy of data
std::vector<int> data(start, start + len);

// print 
for (i : data)
  std::cout << i << " ";
std::cout << std::endl;

Upvotes: 2

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