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# std::equal

From cppreference.com
< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class InputIt1, class InputIt2 > bool equal( InputIt1 first1, InputIt1 last1, InputIt2 first2 ); | (1) |
template< class InputIt1, class InputIt2, class BinaryPredicate > bool equal( InputIt1 first1, InputIt1 last1, InputIt2 first2, BinaryPredicate p ); | (2) |
template< class InputIt1, class InputIt2 > bool equal( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2 ); | (3) | (since C++14)
template< class InputIt1, class InputIt2, class BinaryPredicate > bool equal( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2, BinaryPredicate p ); | (4) | (since C++14)

Returns true if the ranges `[first1, last1)` and `[first2, last2)` are considered equal, where `last2` denotes first2 + (last1 - first1) if it was not given. The overloads (1) and (3) use operator== to compare the elements, whereas overloads (2) and (4) use the given binary predicate `p`.

### [edit] Parameters

first1, last1 | - | the first range of the elements to compare
first2, last2 | - | the second range of the elements to compare
p | - | binary predicate which returns ​true if the elements should be treated as equal. The signature of the predicate function should be equivalent to the following: bool pred(const Type1 &a, const Type2 &b); The signature does not need to have const &, but the function must not modify the objects passed to it. The types Type1 and Type2 must be such that objects of types InputIt1 and InputIt2 can be dereferenced and then implicitly converted to Type1 and Type2 respectively. ​
Type requirements
- `InputIt1, InputIt2` must meet the requirements of `InputIterator`.

### [edit] Return value

If last1 - first1 != last2 - first2, returns false

Otherwise, if the elements in the two ranges are equal, returns true.

Otherwise returns false.

### [edit] Notes

`std::equal` may not be used to compare the ranges formed by the iterators from std::unordered_set, std::unordered_multiset, std::unordered_map, or std::unordered_multimap because the order in which the elements are stored in those containers may be different even if the two containers store the same elements.

When comparing entire containers for equality, operator== is usually preferred.

### [edit] Complexity

At most min(`last1` - `first1`, `last2` - `first2`) applications of the predicate `p`.

However, if `InputIt1` and `InputIt2` meet the requirements of `RandomAccessIterator` and last1 - first1 != last2 - first2 then no applications of the predicate `p` are made.

### [edit] Possible implementation

First version
    template<class InputIt1, class InputIt2>
    bool equal(InputIt1 first1, InputIt1 last1,
               InputIt2 first2)
    {
        for (; first1 != last1; ++first1, ++first2) {
            if (!(*first1 == *first2)) {
                return false;
            }
        }
        return true;
    }
Second version
    template<class InputIt1, class InputIt2, class BinaryPredicate>
    bool equal(InputIt1 first1, InputIt1 last1,
               InputIt2 first2, BinaryPredicate p)
    {
        for (; first1 != last1; ++first1, ++first2) {
            if (!p(*first1, *first2)) {
                return false;
            }
        }
        return true;
    }

### [edit] Example

The following code uses equal() to test if a string is a palindrome

Run this code

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

    void test(const std::string& s)
    {
        if(std::equal(s.begin(), s.begin() + s.size()/2, s.rbegin())) {
            std::cout << "\"" << s << "\" is a palindrome\n";
        } else {
            std::cout << "\"" << s << "\" is not palindrome\n";
        }
    }
    int main()
    {
        test("radar");
        test("hello");
    }

Output:

    "radar" is a palindrome
    "hello" is not palindrome

find find_if find_if_not (C++11) | finds the first element satisfying specific criteria (function template) [edit]
lexicographical_compare | returns true if one range is lexicographically less than another (function template) [edit]
mismatch | finds the first position where two ranges differ (function template) [edit]
search | searches for a range of elements (function template) [edit]

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