How to sort std::pair first only? - c++

Question

I have task to make method that:
get numbers from string
every number is equal to its summed sub-numbers
now sort these sums
display the sorted sums and their original values
I made that function:
void orderWeight(const std::string &strng)
{
std::vector<pair<int, int>> result;
int r{}, rr{}, rrr{};
std::stringstream ss(strng);
while(ss>>r){
rrr = r;
while(r!=0){
rr += r%10;
r /= 10;
}
result.push_back( make_pair(rr, rrr));
rr = 0;
r = 0;
}
std::sort( result.begin(), result.end());
for( int i=0; i<result.size();i++){
cout<<result[i].first<<" "<<result[i].second<<endl;
}
}
This function for arguments - 56 65 74 100 99 68 86 180 90 returns :
1 100
9 90
9 180
11 56
11 65
11 74
14 68
14 86
18 99
But I don't want to sort it by first value THEN by second as You can see in
9 90
9 180
If two numbers are the same, don't sort them by second value, just how are they in a string.
How can I make it happen?

Answer 1

You can make it happen by providing a predicate to sort that tells it how to sort. For example, a simple lambda:
std::sort( result.begin(), result.end(),
[](const auto& lhs, const auto& rhs) {
return lhs.first < rhs.first;
});
The above will sort only by the first value.
See also: https://en.cppreference.com/w/cpp/algorithm/sort