How to compare two vectors in C++

In this article, we will discuss different ways to compare two vectors in C++.

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.

The complete example is as follows,

#include <iostream>
#include <vector>

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;
    }

    return 0;
}

Output:

matched

---

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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()

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

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 };

    // 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;
    }

    return 0;
}

Partial comparison of two vectors using equals()

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

int main()
{
    /*
    * 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.
    auto result = std::equal(
                        vec2.begin(),
                        vec2.end(),
                        vec1.begin() + 2);

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

    return 0;
}

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Related Tutorials of C++ Vector

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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
COMPARATOR comparatorObj =
                [](STRING_REF left, STRING_REF 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
auto result = std::equal(
                vecOfStr1.begin(),
                vecOfStr1.end(),
                vecOfStr2.begin(),
                comparatorObj);


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

The Complete example is as follows,

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

typedef const std::string& STRING_REF;
typedef std::function< bool (STRING_REF, STRING_REF) > COMPARATOR;

int main()
{
    //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
    COMPARATOR comparatorObj =
                    [](STRING_REF left, STRING_REF 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
    auto result = std::equal(
                    vecOfStr1.begin(),
                    vecOfStr1.end(),
                    vecOfStr2.begin(),
                    comparatorObj);


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

    return 0;
}

Output

Both vectors are equal

Summary

We learned how to compare two vectors in C++.