How to remove redundant output - c++

Question

My problem is this:
"Given a list of numbers and a number k, return whether any two numbers from the list add up to k.
For example, given [10, 15, 3, 7] and k of 17, return true since 10 + 7 is 17."
And wrote this code:
#include<iostream>
#include<iterator>
template<int N>
bool adding(int (&list)[N], int k) {
//get size of array
int length = (sizeof(list) / sizeof(list[0]));
std::cout << k << std::endl;
bool a = false;
//init 2 pointer on array
int *p1;
p1 = &list[0];
int *p2;
p2 = &list[0];
int sum = 0;
int lengthNew = length;
//check if p1+p2 = k
for (int i = 0; i < length; i++) {
for (int j = 0; j < lengthNew; j++) {
sum = *p1 + *p2;
if (p1 == p2) {
p2++;
j++;
}
else if (sum == k) {
std::cout << sum << " = " << k << "\t*p1= " <<*p1<<"\t*p2= "<< *p2 << std::endl;
a = true;
}
p2++;
}
p2 = p2 - length;
p1++;
}
return a;
}
int main() {
int myInts[] = { 19,1,2,18,13,4,10,5,5,12,7,10,8,16 };
int k = 21;
adding(myInts, k);
int w;
std::cin >> w;
return 0;
}
I get this output:
21 = 21 *p1= 19 *p2= 2
21 = 21 *p1= 2 *p2= 19
21 = 21 *p1= 13 *p2= 8
21 = 21 *p1= 5 *p2= 16
21 = 21 *p1= 5 *p2= 16
21 = 21 *p1= 8 *p2= 13
21 = 21 *p1= 16 *p2= 5
21 = 21 *p1= 16 *p2= 5
As you can see there are always two equal outputs. I know that it's because I am settin p2 back to the beginning, but I don't know how to do it otherwise. Could someone show me how to bypass the redundant information? I thought that i could save the addresses of the outputs and compare if this combination is already used. But I think it's not a good solution.

Answer 1

Because addition is commutative, you don't have to go through the whole array for every element.
It's enough to check all the elements after the element you're currently checking.
bool adding(const std::vector<int>& list, int k) {
bool result = false;
for (int i = 0; i < list.size(); i++) {
for (int j = i + 1; j < list.size(); j++) {
int sum = list[i] + list[j];
if (sum == k) {
std::cout << sum << " = " << k << "\tn1= " <<list[i]<<"\tn2= "<< list[j] << std::endl;
result = true;
}
}
}
return result;
}
Each pair of elements will be only compared once now.