Copying one array to another using pointers

Question

I have to use pointers to copy values of one array to another. The problem is I'm not allowed to use'[ ]' operators, which makes this more difficult for me. Here is my attempt:

#include <iostream>
using namespace std;

void cpyia(int old_array[],int new_array[],int length){

int *p1 = old_array;

int *p2 = new_array;

int *x = p2;

for(int i=0 ; i<length ; i++){

    p2 = x;
    p2 = p2 + i;        
        p2 = p1 + i;
}

for(int i=0; i<5; ++i){
     cout << p2[i] << endl;
}

}

  int main() {


int a[5]={1,2,3,4,5};

int b[5];

cpyia(a, b, 5);


}

An easier way to do it would be to put p2[i] = p1[i] in the loop, but I cant do that. Any help is appreciated.

Your code has a number of flaws it looks. Also there's not any value copied, you're just doing some point arithmetic gymnastics. — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Aug 23, 2015 at 12:46
"An easier way to do it would be to put p2[i] = p1[i] in the loop, but I cant do that." Besides I can't see why you can't do that, I suppose you've been asked to use something like *p2 = *p1;. — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Aug 23, 2015 at 12:50
@Jarod42: I would love to see the teacher's face if the OP did this... :) — Christian Hackl
– Christian Hackl, Commented Aug 23, 2015 at 15:33

Paolo M · Accepted Answer · 2015-08-23 12:56:55Z

4

The standard way of implementing your function is as follow:

for(int i = 0; i < length; ++i)
    *new_array++ = *old_array++;

To be a bit more explicit, it's the same as:

void cpyia(int old_array[],int new_array[],int length){
    int *p1 = old_array;
    int *p2 = new_array;

    for(int i=0 ; i<length ; i++){
        *(p2+i) = *(p1+i);
        // another way: *(p2++) = *(p1++);
    }
}

edited Aug 23, 2015 at 12:56

answered Aug 23, 2015 at 12:48

Paolo M

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Comments

Christian Hackl · Accepted Answer · 2015-08-23 14:14:40Z

In real code, you would use std::copy before even thinking about rewriting such a simple thing yourself.

Here is a complete example:

#include <iostream>
#include <algorithm>

void cpyia(int old_array[],int new_array[],int length){
    std::copy(old_array, old_array + length, new_array);
}

int main() {
    int a[5]={1,2,3,4,5};
    int b[5];

    cpyia(a, b, 5);

    // test results:
    for (int index = 0; index < 5; ++index)
    {
        std::cout << a[index] << " <-> " << b[index] << "\n";
    }
}

However, your question says that you are "not allowed to use" something, which sounds a lot like a homework assignment. In that case, you could look at possible implementations of std::copy to get an idea of how to do it. Here is one way:

void cpyia(int old_array[],int new_array[],int length){
    int* first = old_array;
    int* last = old_array + length;
    int* d_first = new_array;

    while (first != last) {
        *d_first++ = *first++;
    }
}

Turan · Accepted Answer · 2018-01-23 11:34:04Z

0

#include<iostream>
using namespace std;

int main() {

const int size = 5;
int arr1[size] = { 4,21,43,9,77 };
int arr2[size];

int *ptr_a = arr1;
int *ptr_b = arr2;
for (int i = 0; i < size; i++)
{
*(ptr_b + i) = *(ptr_a + i);
cout << *(ptr_b + i) << " ";

}
}

edited Jan 23, 2018 at 11:34

answered Jan 22, 2018 at 13:15

Turan

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