In this article we will discuss different ways to compare two vectors.

## Comparing two vectors using operator ==

std::vector provides an equality comparison operator==, it can be used to compare the contents of two vectors. For each element in the vector it will call operator == on the elements for comparisons.
Let’s see how to do that,

Suppose we have 2 vectors of int i.e.

```std::vector<int> vecOfNums1{ 1, 4, 5, 22, 33, 2, 11, 89, 49 };
std::vector<int> vecOfNums2{ 1, 4, 5, 22, 33, 2, 11, 89, 49 };
```

Let’s compare 2 vector using operator == i.e.
```/*
* Comparing vectors using operator ==
*/
if (vecOfNums1 == vecOfNums2)
{
std::cout << "matched" << std::endl;
}
```

If contents of vectors are equal than it will return true else false.

Using operator == for comparing vectors has two limitations i.e.

• It will compare all the elements in vector, we can not compare the sub sections of vectors using it.
• It will compare all the elements in vector, by calling operator == on each element. You can not use custom comparators with it.

As operator == provides limited functionality for comparing two vectors. Therefore, std::equals() comes to rescue.

## Comparing two vectors using STL Algorithm std::equal()

STL Algorithms provide two over loaded versions of equal()

```bool equal (InputIterator1 first1, InputIterator1 last1, InputIterator2 first2);
```

It accepts two ranges and compare all the elements in range 1 i.e. [first1, last1) with all the elements in range 2 starting at first2.
It returns true if all the range1 is equal to all the elements in range2.

Let’s use this,

Compare all elements in two vectors using std::equal()

```// Compare all the elements of two vectors
bool result = std::equal(vecOfNums1.begin(), vecOfNums1.end(), vecOfNums2.begin());

if (result)
std::cout << "Both vectors are equal" << std::endl;
```

Partial comparison of two vectors using equals()
```/*
* Partial comparison of two vectors using equals()
*/

std::vector<int> vec1{ 1, 4, 5, 22, 33, 2, 11, 89, 49 };
std::vector<int> vec2{ 5, 22, 33};

// Compare 3 elements of vec2 with 3 elements in vec1 atrting at index 2.
result = std::equal(vec2.begin(), vec2.end(), vec1.begin() + 2);

if (result)
std::cout << "vec1 contains vec2 at position 2 " << std::endl;
```

## Comparing two vectors with custom comparators

Other over loaded version of std::equals() is as follows,

```bool equal (InputIterator1 first1, InputIterator1 last1, InputIterator2 first2, BinaryPredicate pred);
```

It compares all the elements in range 1 and range 2 using given binary predicate i.e. comparator.
Let’s see how to use this,

Suppose we have two vectors of strings i.e.

```//Two vectors of strings
std::vector<std::string> vecOfStr1{ "Hi" , "hello" , "There" , "From" };
std::vector<std::string> vecOfStr2{ "HI" , "HELLO" , "THERE" , "FROM" };
```

Now let’s compare two vectors of strings in case insensitive manner by passing a custom comparator in equals() i.e.

Comparator :

```// Comparator to compare two strings in case insensitive manner
std::function< bool (const std::string & , const std::string &) > comparator = [](const std::string & left, const std::string & right){
// Lambda function to compare 2 strings in case insensitive manner
return std::equal(left.begin(), left.end(), right.begin(), [](const char & l, const char & r) {
return (::toupper(l) == ::toupper(r));
});
};
```

Comparing 2 vectors using std::equals() and comparator i.e.
```// Compare two vectors of strings in case insensitive manner
result = std::equal(vecOfStr1.begin(), vecOfStr1.end(), vecOfStr2.begin(), comparator);

if (result)
std::cout << "Both vectors are equal" << std::endl;
else
std::cout << "Both vectors are not equal" << std::endl;
```

Output:
`Both vectors are equal`

Complete example is as follows,
```#include <iostream>
#include <vector>
#include <algorithm>
#include <functional>
#include <string>

int main()
{
std::vector<int> vecOfNums1{ 1, 4, 5, 22, 33, 2, 11, 89, 49 };
std::vector<int> vecOfNums2{ 1, 4, 5, 22, 33, 2, 11, 89, 49 };

/*
* Comparing vectors using operator ==
*/
if (vecOfNums1 == vecOfNums2)
{
std::cout << "matched" << std::endl;
}

/*
Comparing two vectors using std::equal()
*/

// Compare all the elements of two vectors
bool result = std::equal(vecOfNums1.begin(), vecOfNums1.end(), vecOfNums2.begin());

if (result)
std::cout << "Both vectors are equal" << std::endl;

/*
* Partial comparison of two vectors using equals()
*/

std::vector<int> vec1{ 1, 4, 5, 22, 33, 2, 11, 89, 49 };
std::vector<int> vec2{ 5, 22, 33};

// Compare 3 elements of vec2 with 3 elements in vec1 atrting at index 2.
result = std::equal(vec2.begin(), vec2.end(), vec1.begin() + 2);

if (result)
std::cout << "vec1 contains vec2 at position 2 " << std::endl;

/*
* Comparing two vectors using custom comparators
*/

//Two vectors of strings
std::vector<std::string> vecOfStr1{ "Hi" , "hello" , "There" , "From" };
std::vector<std::string> vecOfStr2{ "HI" , "HELLO" , "THERE" , "FROM" };

// Comparator to compare two strings in case insensitive manner
std::function< bool (const std::string & , const std::string &) > comparator = [](const std::string & left, const std::string & right){
// Lambda function to compare 2 strings in case insensitive manner
return std::equal(left.begin(), left.end(), right.begin(), [](const char & l, const char & r) {
return (::toupper(l) == ::toupper(r));
});
};

// Compare two vectors of strings in case insensitive manner
result = std::equal(vecOfStr1.begin(), vecOfStr1.end(), vecOfStr2.begin(), comparator);

if (result)
std::cout << "Both vectors are equal" << std::endl;
else
std::cout << "Both vectors are not equal" << std::endl;

return 0;
}

```

Output
```matched
Both vectors are equal
vec1 contains vec2 at position 2
Both vectors are equal
```