C++ Algorithm partition_point()

C++ Algorithm partition_point() function is used to return the first element in the given range for which pred is not true. The elements are sorted in a way that the elements which satisfy the condition come before those who do not satisfy.

Syntax

  template <class ForwardIterator, class UnaryPredicate>  ForwardIterator partition_point (ForwardIterator first, ForwardIterator last,                                     UnaryPredicate pred);  

Parameter

first: An forward iterator pointing to the first element in the range to check for a condition.

last: A forward iterator pointing to the past last element of the range.

pred: A user defined unary predicate function that defines the condition to be tested.

Return value

This function returns a forward iterator to point to the first element that does not fulfill the condition tested by pred or returns last if one is not found.

Complexity

Complexity is logarithmic in the range [first, last).

Data races

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

Exceptions

This function throws an exception if either an element’s 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 partition_point():

  #include <algorithm>  #include <array>  #include <iostream>  #include <iterator>    using namespace std;     int main()  {      array<int, 9> v = { 1, 2, 3, 4, 5, 6, 7, 8, 9 };         auto is_even = [](int i)          { return i % 2 == 0; };      partition(v.begin(), v.end(), is_even);         auto p = std::partition_point(v.begin(), v.end(), is_even);         cout << “Before partition:n    “;      copy(v.begin(), p, ostream_iterator<int>(cout, ” “));      cout << “nAfter partition:n    “;      copy(p, v.end(), ostream_iterator<int>(cout, ” “));            return 0;  }  

Output:

Before partition:      8 2 6 4   After partition:      5 3 7 1 9

Example 2

Let’s see another simple example:

  #include <iostream>     // std::cout  #include <algorithm>    // std::partition, std::partition_point  #include <vector>       // std::vector    using namespace std;    bool IsOdd (int i) { return (i%2)==1; }    int main () {    vector<int> foo {1,2,3,4,5,6,7,8,9};    vector<int> odd;      partition (foo.begin(),foo.end(),IsOdd);      auto it = partition_point(foo.begin(),foo.end(),IsOdd);    odd.assign (foo.begin(),it);      // print contents of odd:    cout << “odd:”;    for (int& x:odd) cout << ‘ ‘ << x;    cout << ‘n’;      return 0;  }  

Output:

odd: 1 9 3 7 5

Example 3

Let’s see another simple example:

  #include<iostream>   #include<algorithm> // for partition algorithm   #include<vector> // for vector   using namespace std;   int main()   {       // Initializing vector       vector<int> vect = { 2, 1, 5, 6, 8, 7 };               // partitioning vector using stable_partition()       // in sorted order       stable_partition(vect.begin(), vect.end(), [](int x)       {           return x%2 == 0;               });               // Displaying partitioned Vector       cout << “The partitioned vector is : “;       for (int &x : vect) cout << x << ” “;       cout << endl;               // Declaring iterator       vector<int>::iterator it1;               // using partition_point() to get ending position of partition       auto it = partition_point(vect.begin(), vect.end(), [](int x)       {           return x%2==0;       });               // Displaying partitioned Vector       cout << “The vector elements returning true for condition are : “;       for ( it1= vect.begin(); it1!=it; it1++)       cout << *it1 << ” “;       cout << endl;               return 0;           }   

Output:

The partitioned vector is : 2 6 8 1 5 7   The vector elements returning true for condition are : 2 6 8

Example 4

Let’s see another simple example:

  #include <iostream> // std::cout   #include <algorithm> // std::partition, std::partition_point   #include <vector> // std::vector     using namespace std;      bool IsNegative(int i) { return (i < 0); }       int main()   {       vector<int> data{ 1, -1, 3, -4, 5, 2, -2, 4, -5, -3 };       vector<int> negative, positive;           // partition data on the basis of odd elements using        // pred IsNegative()       stable_partition(data.begin(), data.end(), IsNegative);           // gets the partition point based on odd elements       auto it = partition_point(data.begin(), data.end(),  IsNegative);           // assign elements to odd from beginning till       // partition point.       negative.assign(data.begin(), it);       positive.assign(it, data.end());           // print contents of odd:       cout << “Negative: “;       for (int& x : negative)           cout << ‘ ‘ << x;       cout << ‘n’;           // print contents of even:       cout << “Positive: “;       for (int& x : positive)           cout << ‘ ‘ << x;       cout << ‘n’;           return 0;   }  

Output:

Negative:  -1 -4 -2 -5 -3  Positive:  1 3 5 2 4

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C++ algorithm partition_point() function

C++ Algorithm partition_point()

Syntax

Parameter

Return value

Complexity

Data races

Exceptions

Note: The invalid parameters cause an undefined behavior.

Example 1

Example 2

Example 3

Example 4

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