Pointers as iterators - Help needed

[DragonOsman]

A "Try this" in Chapter 20 has the following specs:

Write a function void copy(int* f1, int* e1, int* f2) that copies the elements
of an array of int s defined by [ f1 : e1 ) into another [ f2 : f2+(e1–f1) ). Use only
the iterator operations mentioned above (not subscripting).

The iterator operations mentioned above that are of course these ones:

p==q, p != q, *p, *p==val, val==*p, ++p

.

I'm having a hard time with this, though. Here's my code:

// Osman Zakir
// 11 / 8 / 2017
// Bjarne Stroustrup: Programming: Principles and Practice Using C++ 2nd Edition
// Chapter 20 Try This:
// Program to define a function void copy(int* f1, int* e1, int* f2) which copies the contents of one array into another.

#include "../../cust_std_lib_facilities.h"
#include <iostream>

void copy(int *f1, int *e1, int *f2);

int main()
{
	int *arr1 = new int[10];
	for (int i = 0; i < 10; ++i)
	{
		*(arr1 + i) = i + 1;
	}
	for (int i = 0; i < 10; ++i)
	{
		std::cout << *(arr1 + i) << '\n';
	}
	int *arr2 = new int[10];
	copy(arr1, arr1 + 10, arr2);
	for (int i = 0; i < 10; ++i)
	{
		std::cout << *(arr2 + i) << '\n';
	}
	delete[] arr1;
	delete[] arr2;
	keep_window_open();
}

void copy(int *f1, int *e1, int *f2)
{
	for (int i = *f1; i != (*f2 + (*e1 - *f1)); ++i)
	{
		*(f2 + i) = *(f1 + i);
		if (i >= 10)
		{
			break;
		}
	}
}

The output:

1
2
3
4
5
6
7
8
9
10
-842150451
2
3
4
5
6
7
8
9
10

The very fact that it seems like the array being copied into is bigger than the one being copied is already a problem for me. I don't understand how to make this work without making the index for the original array (the one being copied) go out of bounds. If I change the for loop to this (and if this is more how it should be):

for (int *i = f1; i != (f2 + (e1 - f1)); ++i)
{
		
}

, then how do I do the rest of it?

Thanks in advance.

[Peter87]

The second loop is closer to how it should be done, but you need to use two iterators, one for each array. Then you can use the dereference operator (*) to access the array elements that the iterators refer to which is necessary when doing the copying of values from one array to the other.

[DragonOsman]

Now I'm getting an error saying that some memory couldn't be read. This is how that function looks now, by the way:

void copy(int *f1, int *e1, int *f2)
{
	for (int *i = f1, *j = f2; i != (f2 + (e1 - f1)); ++i, ++j)
	{
		*j = *i;
	}
}

[Thomas1965]

Maybe like this:

#include <iostream>

void copy(int *f1, int *e1, int *f2);

int main()
{
  int org_numbers[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
  int new_numbers[10] = {0};

  copy(org_numbers, org_numbers + 10, new_numbers);

  for (int i : new_numbers)
    std::cout << i << "\n";
}

void copy(int *f1, int *e1, int *f2)
{
  while (f1 != e1)
  {
    *f2 = *f1;
    ++f1;
    ++f2;
  }
}

[Peter87]

i is used to iterate the [ f1 : e1 ) range so to check if the end has been reached you should use i != e1. If you really want to check against (f2 + (e1 - f1)) you should instead use the j iterator, j != (f2 + (e1 - f1)).

[DragonOsman]

Yeah, that seems to have worked.

void copy(int *f1, int *e1, int *f2)
{
	for (int *i = f1, *j = f2; j != (f2 + (e1 - f1)) && i != e1; ++i, ++j)
	{
		*j = *i;
	}
}

output:

1
2
3
4
5
6
7
8
9
10
1
2
3
4
5
6
7
8
9
10