Compare arrays for equality in C++

This post will discuss how to compare arrays for equality in C++.

1. Using `==` operator

We can easily compare two std::array using the == operator. It checks whether the contents of the two containers are equal or not. Consider the following code demonstrating this:

#include <iostream>

#include <array>

int main()

{

std::array<int, 5> first {1, 2, 3, 4, 5};

std::array<int, 5> second {1, 2, 3, 4, 5};

if (first == second) {

std::cout << "Both arrays are equal";

} else {

std::cout << "Both arrays are not equal";

}

return 0;

}

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Output:

Both arrays are equal

Note that you can’t compare two C-style arrays using this approach, since it decays to pointers to the first elements of the respective arrays, and we will end up comparing two addresses. The following C++ program demonstrates it:

#include <iostream>

int main()

{

int first[] = {1, 2, 3, 4, 5};

int second[] = {1, 2, 3, 4, 5};

if (first == second) {

std::cout << "Both arrays are equal";

} else {

std::cout << "Both arrays are not equal";

}

return 0;

}

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Output:

Both arrays are not equal

2. Using `std::equal`

Alternatively, we can use the std::equal function to determine if corresponding elements in the specified ranges are equal. The following example shows how to compare two arrays with the std::equal function.

Notice the length check before calling the std::equal function. This is done to avoid incorrect output when the second array contains more elements than in the first array.

#include <iostream>

#include <algorithm>

int main()

{

int first[] = {1, 2, 3, 4, 5};

int second[] = {1, 2, 3, 4, 5};

int m = sizeof(first) / sizeof(*first);

int n = sizeof(second) / sizeof(*second);

if (m == n && std::equal(first, first + m, second)) {

std::cout << "Both arrays are equal";

} else {

std::cout << "Both arrays are not equal";

}

return 0;

}

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Output:

Both arrays are equal

With C++11, we can pass an iterator to the beginning and end of the array to the std::equal function:

#include <iostream>

#include <iterator>

int main()

{

int first[] = {1, 2, 3, 4, 5};

int second[] = {1, 2, 3, 4, 5};

int m = sizeof(first) / sizeof(*first);

int n = sizeof(second) / sizeof(*second);

if (m == n && std::equal(std::begin(first), std::end(first), std::begin(second))) {

std::cout << "Both arrays are equal";

} else {

std::cout << "Both arrays are not equal";

}

return 0;

}

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Output:

Both arrays are equal

3. Using Custom Logic

Another option is to write custom logic for comparing two arrays in C++. The idea is to iterate with indices and compare the corresponding elements at the respective position in both arrays. Here’s what the code would look like:

#include <iostream>

template <typename T, int m, int n>

bool isEqual(T (&first)[m], T (&second)[n]) {

if (m != n) {

return false;

}

for (int i = 0; i < m; i++){

if (first[i] != second[i]) {

return false;

}

return true;

}

int main()

{

int first[] = {1, 2, 3, 4, 5};

int second[] = {1, 2, 3, 4, 5};

if (isEqual(first, second)) {

std::cout << "Both arrays are equal";

} else {

std::cout << "Both arrays are not equal";

}

return 0;

}

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Output:

Both arrays are equal

4. Using `memcmp()` function

Finally, we can compare two blocks of memory using the memcmp() function. It returns 0 if the contents of both memory blocks are equal, as shown below:

#include <iostream>

#include <cstring>

int main()

{

char first[] = "C";

char second[] = "C++";

int n = memcmp(first, second, sizeof(first));

if (n == 0) {

std::cout << "Both arrays are equal";

} else {

std::cout << "Both arrays are not equal";

}

return 0;

}

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Output:

Both arrays are not equal

That’s all about comparing arrays for equality in C++.

Compare arrays for equality in C++

1. Using == operator

2. Using std::equal

3. Using Custom Logic

4. Using memcmp() function

Share this:

1. Using `==` operator

2. Using `std::equal`

4. Using `memcmp()` function