C++ Algorithm Library - equal() Function


The C++ function std::algorithm::equal() tests whether two sets of element are equal or not. Size of the both set need not to be equal. It uses binary predicate for comparison.


Following is the declaration for std::algorithm::equal() function form std::algorithm header.


template <class InputIterator1, class InputIterator2, class BinaryPredicate>
bool equal(InputIterator1 first1, InputIterator1 last1,
   InputIterator2 first2, BinaryPredicate pred);


  • first1 − Input iterator to the initial position of the first sequence.

  • last1 − Input iterator to the final position of the first sequence.

  • first2 − Input iterator to the initial position of the second sequence.

  • pred − A binary predicate which accepts two arguments and returns a bool.

Return value

Returns true if all the element in the range of first1 to last1 are equal to those of the range starting at first2 otherwise it returns false.


Throws exception if either element comparison (or predicate) or an operation on an iterator throws exception.

Please note that invalid parameters cause undefined behavior.

Time complexity

Linear in the distance between first to last.


The following example shows the usage of std::algorithm::equal() function.

#include <iostream>
#include <vector>
#include <algorithm>
#include <string>

using namespace std;

/* Binary predicate which always returns true */
bool binary_pred(string s1, string s2) {
   return true;

int main(void) {
   vector<string> v1 = {"one", "two", "three"};
   vector<string> v2 = {"ONE", "THREE", "THREE"};
   bool result;

   result = equal(v1.begin(), v1.end(), v2.begin(), binary_pred);

   if (result == true)
      cout << "Vector range is equal." << endl;

   return 0;

Let us compile and run the above program, this will produce the following result −

Vector range is equal.