C++ Algorithm lower_bound()

C++ Algorithm lower_bound() function is the version of binary search. This function is used to return an iterator pointing to the first element in an ordered range [first, last) that is not less than (i.e. greater than or equal to) to the specified value val.

The first version uses operator < to compare the elements and the second version uses comp function.

Syntax

  default (1)       template <class ForwardIterator, class T>                             ForwardIterator lower_bound (ForwardIterator first, ForwardIterator last,                                 const T& val);     custom (2)      template <class ForwardIterator, class T, class Compare>                           ForwardIterator lower_bound (ForwardIterator first, ForwardIterator last,                                 const T& val, Compare comp);  

Parameter

first: A forward iterator pointing to the first element in the range to be searched.

last: A forward iterator pointing to the past last element in the range to be searched.

comp: A user-defined binary predicate function that accepts two arguments and returns true if the two arguments are in order and false otherwise. It follows the strict weak ordering to order the elements.

val: A value of the lower bound to compare the elements in the range.

Return value

It returns an iterator pointing to the first element of the range that is not less than val or last if no such element is found.

Complexity

On average, complexity is logarithmic in the distance between first and last: performs up to log₂ (N) + 1 element comparisons Where N = last – first.

Data races

The object in the range [first, last) are accessed.

Exceptions

This function throws an exception if either an element comparison or an operation on iterator throws an exception.

Note: The invalid parameters cause an undefined behavior.

Example 1

Let’s see the simple example to demonstrate the use of lower_bound():

  #include <iostream>  #include <vector>  #include <algorithm>    using namespace std;    int main()  {      vector<int> v = {3, 1, 4, 6, 5};        decltype(v)::iterator it = lower_bound(v.begin(), v.end(), 4);      cout << *it << “, pos = ” << (it – v.begin()) << endl;    return 0;      }  

Output:

4, pos = 2

Example 2

Let’s see another simple example:

  #include <iostream>     // std::cout  #include <algorithm>    // std::lower_bound, std::upper_bound, std::sort  #include <vector>       // std::vector    using namespace std;    int main () {    int myints[] = {10,20,30,30,20,10,10,20};    vector<int> v(myints,myints+8);           // 10 20 30 30 20 10 10 20      sort (v.begin(), v.end());                // 10 10 10 20 20 20 30 30      vector<int>::iterator low,up;    low=lower_bound (v.begin(), v.end(), 20); //          ^    up= upper_bound (v.begin(), v.end(), 20); //                   ^      cout << “lower_bound at position ” << (low- v.begin()) << ‘n’;    cout << “upper_bound at position ” << (up – v.begin()) << ‘n’;      return 0;  }  

Output:

lower_bound at position 3  upper_bound at position 6

Example 3

Let’s see another simple example:

  #include <algorithm>  #include <iostream>  #include <iterator>  #include <vector>     using namespace std;     template<class ForwardIt, class T, class Compare=less<>>  ForwardIt binary_find(ForwardIt first, ForwardIt last, const T& value, Compare comp={})  {      // Note: BOTH type T and the type after ForwardIt is dereferenced       // must be implicitly convertible to BOTH Type1 and Type2, used in Compare.       // This is stricter than lower_bound requirement (see above)         first = lower_bound(first, last, value, comp);      return first != last && !comp(value, *first) ? first: last;  }    int main()   {      vector<int> data = { 1, 1, 2, 3, 3, 3, 3, 4, 4, 4, 5, 5, 6 };         auto lower = lower_bound(data.begin(), data.end(), 4);      auto upper = upper_bound(data.begin(), data.end(), 4);         copy(lower, upper, ostream_iterator<int>(cout, ” “));         cout << ‘n’;         // classic binary search, returning a value only if it is present         data = { 1, 2, 4, 6, 9, 10 };          auto it = binary_find(data.cbegin(), data.cend(), 4); //< choosing ‘5’ will return end()         if(it != data.cend())        cout << *it << ” found at index “<< distance(data.cbegin(), it);         return 0;  }  

Output:

4 4 4   4 found at index 2

Example 4

Let’s see another simple example:

  #include <iostream>  #include <vector>  #include <algorithm>    using namespace std;    bool ignore_case(char a, char b) {     return(tolower(a) == tolower(b));  }    int main(void) {     vector<char> v = {‘A’, ‘b’, ‘C’, ‘d’, ‘E’};     auto it = lower_bound(v.begin(), v.end(), ‘C’);       cout << “First element which is greater than ‘C’ is ” << *it << endl;       it = lower_bound(v.begin(), v.end(), ‘C’, ignore_case);       cout << “First element which is greater than ‘C’ is ” << *it << endl;       it = lower_bound(v.begin(), v.end(), ‘z’, ignore_case);       cout << “All elements are less than ‘z’.” << endl;       return 0;  }  

Output:

First element which is greater than 'C' is b  First element which is greater than 'C' is d  All elements are less than 'z'.

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C++ algorithm lower_bound() function

C++ Algorithm lower_bound()

Syntax

Parameter

Return value

Complexity

Data races

Exceptions

Note: The invalid parameters cause an undefined behavior.

Example 1

Example 2

Example 3

Example 4

Underwriting of Shares

C++ Math abs() Functions

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