Help with homework

[wubbits]

So currently my online class is learning about pointers. I dont quite understand them and their usage but for one of the assignments it tells us:

Create a function printArray to display the values from above (basically move your code to display the values to a function). printArray should accept the array and its size as parameters. Pass the array size as a pointer to an int.

What does it mean "Pass the array size as a pointer to an int"?

Heres my code on what I thought it meant:

#include<iostream>
#include<ctime>
#include<cstdlib>
using namespace std;

void printArray(int *num, int size);
int main()
{
	srand(time(NULL)); //seed
	int size = 10; //array size
	int numbers[size]; //array
	
	for(int i = 0; i < size; i++) //get random numbers into array
	{
		numbers[i] = rand();
	}
	for(int i = 0; i < size; i++) //display array using pointer
	{
		cout << *(numbers + i) << endl;
	}	
	cout << endl;
	
	printArray(numbers, size);
	return 0;

}
void printArray(int *num, int size)
{
	int *ptr = num;
	cout << "ptr: " << endl;
	for(int i = 0; i < size; i++)
	{
		cout << *(num + i) << endl;
	}
}

[fiji885]

#include<iostream>
#include<ctime>
#include<cstdlib>
using namespace std;

void printArray(int *num, int* size);
int main()
{
	srand(time(NULL)); //seed
	int size = 10; //array size
	int numbers[size]; //array
	
	for(int i = 0; i < size; i++) //get random numbers into array
	{
		numbers[i] = rand();
	}
	for(int i = 0; i < size; i++) //display array using pointer
	{
		cout << *(numbers + i) << endl;
	}	
	cout << endl;
	
	printArray(numbers, &size);
	return 0;

}

/* I am horrible with the English language, and explaining things, but
 * my best guess by "Pass the array size as a pointer to int" means that
 * int size needs to be a int* size, and you pass the address as an argument
 * into the second parameter field.
 *
 * Example:
 * printarray(numers, &size);
 * */

void printArray(int *num, int* size)
{
  /* If my guess is right, that means you also need to modify this a bit */

  int *ptr = num;
	cout << "ptr: " << endl;
  /* Since size is a pointer, you have to deference it with the dereference
   * operator *. You deference it because you want the value of the size
   * instead of the memory address */
	for(int i = 0; i < *size; i++)
	{
		cout << *(ptr + i) << endl;
	}
}

I don't quite understand them

A pointer can be used to point to a memory address.
We can also reassign the memory address they are pointing to.

For example, right now p (which is a pointer) is pointing to
the memory address that contains a value of 20.

[p]     [10]
  \   
   ---> [20]

If we wanted print it, we could do

std::cout << p << std::endl;

but that would print out a memory address. Instead, we have to dereference it
using the dereference operator *

std::cout << *p << std::endl; // prints a value now instead of an address 

The thing with pointers, as mentioned, as that we can repoint them to another
address (as long as its the same type). So we can say that p points to the
address of the value 10 now.

[p]---->[10]
      
        [20]

and when we print this (assuming we print it by dereferencing it as well),
we will get the value of 10.

[wubbits]

sorry for the late response @fiji885 , how do we make it repoint at something? Whats the real purpose of using them if all they do is point to memory addresses? I dont quite understand memory in cpp either.

[fiji885]

> how do we make it repoint at something?
You just assign it another memory location of the same type. Take a look at this code, for example

#include <iostream>

int main() {
  int a = 100;
  int b = 600;
  int *p = &a;
  std::cout << *p << std::endl;
  p = &b;
  std::cout << *p << std::endl;
  return 0;
}

> Whats the real purpose of using them if all they do is point to memory addresses?
This one has always been a little fuzzy for me. With pointers, you can dynamically allocate variables. There have been times where I had to use pointers, but that deals with manipulating a specific location in memory (for example in my data structures course, I had to use a pointer in order arrange items in a binary tree). I only use pointers if I really have to, otherwise I just reference. I'll link you think. I think it's a good read.

https://stackoverflow.com/questions/162941/why-use-pointers

[MikeyBoy]

Whats the real purpose of using them if all they do is point to memory addresses?

See my post in this thread for reasons why we we might want to use pointers:

http://www.cplusplus.com/forum/general/105593/

Note that modern C++ standards have introduced newer techniques for resolving these situations, so there's less need to use pointers than in older C++ and C code.