How to copy values from one array to another in C with pointer arithmetic

Question

I am trying to copy the values from one array to another using strictly pointer arithmetic. This is the code I have right now:

   int *p;
   int arraySize = 20;

   int array[arraySize];     

   for (p = array; p< array+(sizeof(array)/sizeof(int)); p++){
        int random = rand() % 200;
        *p = random;
   }

   for (p = array; p< array+(sizeof(array)/sizeof(int)); p++){
        printf("%d\t%x\n", *p, p);
   }

   //the code above works fine

   printf("\n");

   //grow the new array by one to insert value at end later
   int array2[(arraySize+1)];

   int *p2;

   for(p2 = array2; p2< array2+(sizeof(array2)/sizeof(int)); p2++){
       *(p2) = *(p);
   }

   for(p2 = array2; p2< array2+(sizeof(array2)/sizeof(int)); p2++){
       printf("%d\t%x\n", *p2, p2);
   }

But when i run the code all that is outputted is 0 at each of the memory locations. what am i doing wrong that is preventing the values from being copied over?

The problem with your code is that it changes p2, but it never changes p. Voting to close as a typo. — Sergey Kalinichenko
– Sergey Kalinichenko, Commented May 8, 2017 at 17:15
doing p++ gives random numbers mixed with some zeros as output in the memory locations — theplaid
– theplaid, Commented May 8, 2017 at 17:17
Eugene and das have both identified the problem; I'm just here to ask---why do it this way? When you use the [x] syntax, *(ptr + (x * sizeof(int))) is exactly what the compiler is doing (where int is the array type, as this example)... — Brian A. Henning
– Brian A. Henning, Commented May 8, 2017 at 17:18
because it is a requirement in our assignment. but when incrementing p with p++ it is not giving the correct output... — theplaid
– theplaid, Commented May 8, 2017 at 17:22

Rob K · Accepted Answer · 2017-05-08 17:38:34Z

1

Your problem is in this loop where you're copying from *p to *p2:

for(p2 = array2; p2< array2+(sizeof(array2)/sizeof(int)); p2++){
    *(p2) = *(p);
}

You increment p2 but never increment p and you never reset p to point back to the beginning of array. At the beginning of this loop, p is pointing at the location just off the end of array. Change your loop to this:

for(p2 = array2, p = array; p2 < array2 + (sizeof(array2)/sizeof(int)) - 1; p2++, ++p){
    *(p2) = *(p);
}

edited May 8, 2017 at 17:38

answered May 8, 2017 at 17:20

Rob K

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