Implementation of lower_bound on vector pairs - c++

Question

I know we need to include some compare function in order to achieve this.
But not able to write for this one.
For example:
Elements of vector={(2,4),(4,2),(5,1),(5,3)}
to find=5
lower_bound() should return 2
code->
#define pp pair<int,int>
bool cmp(const pp &l,const pp &r) {
return l.first < r.first;
}
int main() {
vector<pp> v;
sort(v.begin(), v.end(), cmp);
int id=(int)(lower_bound(v.begin(), v.end(), ??) - v.begin());
}

Answer 1

Pairs (just like tuples) compare lexicographically anyway. You don't need to define any special comparators for this.
And since you're using lower_bound you'll be searching for the first element that does not compare less than the val you're searching, so you should use a min value as the second pair element. To sum up, all can be done in "two" lines of code :
sort(v.begin(),v.end());
auto id = distance(v.begin(), lower_bound(v.begin(),v.end(),
make_pair(5, numeric_limits<int>::min())) );
Some Notes :
Use std::distance to calculate the number of elements between two iterators
The return type of std::distance is an unsigned type. Unless you need negative indexing (Python like syntax for "count from the end" indexes) it's a good practice to keep your indexes unsigned.

Answer 2

Since you don't care about the second value of pp, just construct a temporary pp object with any value as the second element.
int id = std::lower_bound(v.begin(), v.end(), pp(5, 0), cmp) - v.begin();

Answer 3

I think you should compare the pairs as per definition of lower_bound
So,
typedef pair<int,int> pp;
//...
int id=(int)(lower_bound(v.begin(),v.end(),
pp(5,std::numeric_limits<int>::min())), //Value to compare
[](const pp& lhs, const pp& rhs) // Lambda
{
return lhs < rhs ; // first argument < second
}
) - v.begin()
);

Answer 4

You can use lower_bound on vector of pairs with custom compare operator .
You need to pass four arguments in that case like this :-
it1 = iterator position from where to search
it2 = iterator position till where to search
lower_bound (it1 ,it2 , finding_element, your_comparator )
auto myComp = [&](pair<int,string> e1, pair<int,string> e2) {
if(e1.second!=e2.second)
return e1.second<e2.second;
else
return e1.first<e2.first;
};
void Show_sample_code()
{
vector<pair<int,string>> data={{1, "sahil"}, {2, "amin"}};
sort(data.begin(), data.end(), myComp);
pair<int, string> p={1,"sahil"};
auto it=lower_bound( data.begin(), data.end(), p, myComp ) ;
if(it!=data.end())
cout<<"found at index="<<distance(data.begin(), it)<<endl;
else
cout<<"notfound"<<endl;
return;
}