C++ : How to compare two vectors | std::equal() & Comparators
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
Does this method cover two dimensional vectors? a vector of vectors?
vector < vector > vector1;
vector < vector > vector2;
equal(vector1.begin(), vector1.end(), vector2.begin());