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

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++;
}