arrays and pointers

Question

As a beginner to c++ I'm struggling to understand pointers and array. I wrote the following program:

int main (void){
int p[3]={0};
int * iptr = new int [4];

iptr++;
*iptr=2;
iptr++;
*iptr=3;

for (int i=0;i<4;i++){
    cout << "iptr: " << *iptr << endl;
    iptr++;
}

return 0;
}

However I'm not getting the expected results. I would have expected elements [1] and [2] to have the values 2 and 3 respectively. However I get a result along the lines of:

iptr: 3
iptr: -842150451
iptr: -33686019
iptr: 0

Can someone please tell me where I'm going wrong in my understanding?

Thanks Dan

Answer 1

As a beginner to c++ I'm struggling to understand pointers and array

The best advice that anyone can ever give you: Don't. Pointers and arrays are just not worth the effort. Use a C++ tool like std::vector or std::array and your life will be infinitely easier. Come back and learn them later after mastering the rest of the language, if you need them.

Answer 2

The problem is here:

int * iptr = new int [4];

iptr++;
*iptr=2;
iptr++;
*iptr=3;

after you call iptr++, iptr will no longer point to the beginning of the array. After you set the values, you should reset iptr to the original position ( iptr -= 2 ) or use an auxiliary pointer.

So 2 possible solutions:

iptr++;
*iptr=2;
iptr++;
*iptr=3;

iptr -= 2;

for (int i=0;i<4;i++){
    cout << "iptr: " << *iptr << endl;
    iptr++;
}

or (cleaner):

int* aux = iptr;
aux++;
*aux=2;
aux++;
*aux=3;

for (int i=0;i<4;i++){
    cout << "iptr: " << *iptr << endl;
    iptr++;
}

Answer 3

You've run past the end of the array. What you're printing is probably what happens to be in memory there, but technically this is undefined behaviour.

iptr is a pointer to the first element of an array of 4 elements:

int * iptr = new int [4];

iptr points to the second element after this:

iptr++;
*iptr=2; // set second element to 2

iptr points to the third element after this:

iptr++;
*iptr=3; // set third element to 3

Nothing here has reset iptr, so it still points to the third element.

for (int i=0;i<4;i++){
    cout << "iptr: " << *iptr << endl;
    iptr++;
}

So you print:

Third element, set to three:

iptr: 3

Fourth element, not set to any specific value by your program

iptr: -842150451

Past the end here, undefined behaviour but probably your implementation is just printing nearby memory.

iptr: -33686019
iptr: 0

Answer 4

Pointers don't automatically go back to the place where you set them originally. After you used

iptr++;

it now points to the new address, i.e element[2]

For looping, use another temporary pointer. Don't increment the original one. Look at this:

int arr[4];

int* iptr = arr;

iptr++; 
iptr = 2;
iptr++; 
iptr = 3;

for (int i=0;i<4;i++){
    cout << "arr: " << arr[i] << endl;        
}