The **one_of_equal()** function in **C++ boost library** is found under the header *‘boost/algorithm/cxx11/one_of.hpp’* which tests if exactly one of the elements of a sequence against the value passed is same. It takes a sequence and a value, and returns true if the exactly one of the elements are same in the sequence to the value passed.

**Syntax**:

bool one_of_equal ( InputIterator first, InputIterator last, const &value)

or

bool one_of_equal ( const Range &R, const &value)

**Parameters**: The function accepts parameters as described below:

**first**: It specifies the input iterators to the initial positions in a sequence.**second**: It specifies the input iterators to the final positions in a sequence.**value**: It specifies a value which is to be checked against for one the elements of the sequence.**R**: It is the complete sequence.

**Return Value**: The function returns true if exactly one of the elements of the sequence is equal to value, else it returns false.

Below is the implementation of the above approach:

**Program-1**:

`// C++ program to implement the` `// above mentioned function` ` ` `#include <bits/stdc++.h>` `#include <boost/algorithm/cxx11/one_of.hpp>` `using` `namespace` `std;` ` ` `// Drivers code` `int` `main()` `{` ` ` ` ` `// Declares the sequence with` ` ` `int` `c[] = { 1, 2, 3 };` ` ` ` ` `// Run the function` ` ` `bool` `ans = boost::algorithm::one_of_equal(c, 1);` ` ` ` ` `// Condition to check` ` ` `if` `(ans == 1)` ` ` `cout << ` `"exactly one element is 1"` `;` ` ` `else` ` ` `cout << ` `"exactly one element is not 1"` `;` ` ` `return` `0;` `}` |

**Output:**

exactly one element is 1

**Program-2**:

`// C++ program to implement the` `// above mentioned function` ` ` `#include <bits/stdc++.h>` `#include <boost/algorithm/cxx11/one_of.hpp>` `using` `namespace` `std;` ` ` `// Drivers code` `int` `main()` `{` ` ` ` ` `// Declares the sequence` ` ` `int` `a[] = { 1, 2, 3, 6 };` ` ` ` ` `// Run the function` ` ` `bool` `ans` ` ` `= boost::algorithm::one_of_equal(a, a + 4, 4);` ` ` ` ` `// Condition to check` ` ` `if` `(ans == 1)` ` ` `cout << ` `"exactly one element is 4"` `;` ` ` `else` ` ` `cout << ` `"exactly one element is not 4"` `;` ` ` `return` `0;` `}` |

**Output:**

exactly one element is not 4

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