Array Sum with Pointers i have a problem in the sum - c++

Question

I have this code. and I have a problem with it. When I run it for these numbers "1,1,1,1,1" it answers me right but when I use these numbers "2,1,3,2,2" or any other numbers it answers me wrong. What is the problem?
#include <iostream>
using namespace std;
int main() {
int size = 5;
int array1[size];
int i, j, *p;
int sum = 0;
p = &array1[j];
for (int i = 0; i < size; i++) {
cout << "give next number";
cin >> array1[i];
cout << "\n";
}
cout << "the array is:"
<< "\n";
for (int j = 0; j < size; j++) {
cout << array1[j] << "\n";
sum = sum + *p;
}
cout << "the sum of array elements is: " << sum;
return 0;
}

Answer 1

So you have one problem
p = &array1[j];
What you are doing is taking the address of jth element of an array. In you case j is uninitialized which leads to UB since j might contain any variable.
To fix this you can initialize j to 0 (j = 0). Or to just get an address of first element in array you can do following:
p = array;
Than comes your loop, which is summit value at address of arr[j] which is UB as I stated above.
cout << "the array is:" << "\n";
for (j = 0; j < size; j++) {
cout << array1[j] << "\n";
sum = sum + *(p + j);
}
Your problem here was that you were adding array1[0] all the time. (That is if you initialized j to 0).
Other things to note is that you are re declaring i and j
int i, j, *p;
...
for (int i = 0; ...)
...
for (int j = 0; ...)
You could do just
for (i = 0; ...)
...
for (j = 0; ...)
to set already declared variables to 0.
Here is entire program:
#include <iostream>
int main() {
int size = 5;
int array1[size];
int i, j, *p;
int sum = 0;
// p = &array1[j]; // UB j not initialized but used
/* solution 1
j = 0;
p = &array1[j]
*/
// solution 2 which is same as solution 1
p = array1; // gets address of array[0]
for (i = 0; i < size; i++) { // no need for `int` in front of i
// i is already declared above
// my preference is to declare i here
// and remove declaration above
std::cout << "give next number";
std::cin >> array1[i];
std::cout << "\n";
}
std::cout << "the array is:"
<< "\n";
for (j = 0; j < size; j++) { // same as above
std::cout << array1[j] << "\n";
sum = sum + *(p + j);
}
std::cout << "the sum of array elements is: " << sum;
return 0;
}
input:
give next number5
give next number4
give next number3
give next number2
give next number1
output
the array is:
5
4
3
2
1
the sum of array elements is: 15