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I want to find the std::lower_bound of the a std::vector of std::pair's according to the second element with lambda.
std::vector < std::pair <int, double> > vec;
vec.resize(5);
auto it = std::lower_bound(vec.begin(), vec.end(), lambda);
// what is that lambda here?
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The lower_bound() method in C++ is used to return an iterator pointing to the first element in the range [first, last) which has a value not less than val. This means that the function returns an iterator pointing to the next smallest number just greater than or equal to that number.
Lower bound for vector pairs(a,b) will return an iterator whose first element will be greater or equal to a and second value b is greater than or equal to b. If the case is not fulfilled iterator will return a value whose pairs are not present in the pairs of vectors.
set::lower_bound() function in C++ STL If the key passed in the parameter exceeds the maximum value in the container, then the iterator returned points to the element beyond last element in the set container. Time Complexity of set::lower_bound() is O(logn), where n is the size of the set.
lower_bound in c++ stl returns an iterator even when the element is not present.
You're missing an argument here, std::lower_bound takes a begin and end iterator, a value (this is what you missed) and finally can take a lambda.
#include <algorithm>
#include <vector>
int main()
{
typedef std::pair<int, double> myPair; // typedef to shorten the type name
std::vector <myPair> vec(5);
myPair low_val; // reference value (set this up as you want)
auto it = std::lower_bound(vec.begin(), vec.end(), low_val,
[](myPair lhs, myPair rhs) -> bool { return lhs.second < rhs.second; });
}
Reference page for lower_bound is here.
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