CODEWITHSUNDEEP
c++arraysequality
John Humphreys
John Humphreys

Reputation: 39284

How to compare arrays for content equality

Is there a way to get the equality operators to work for comparing arrays of the same type?

For example:

int x[4] = {1,2,3,4};
int y[4] = {1,2,3,4};
int z[4] = {1,2,3,5};
if (x == y) cout << "It worked!"

I'm aware that as is, it's just comparing pointer values - but I was hoping there's some kind of typedef trick or something like that so it wouldn't need a loop or a memcmp call.

Upvotes: 5

Views: 5742

Answers (6)

Jan Schultke
Jan Schultke

Reputation: 39558

Solution A - Use std::array(C++11)
std::array<int> x = {1, 2, 3, 4};
std::array<int> y = {1, 2, 3, 4};
if (x == y) // true

std::array avoids a lot of C-style array problems in general, so you might want to replace all your uses of int[] with it.

Solution B - Use std::spans(C++20)
if (std::span{x} == std::span{y})

std::span is a lightweight non-owning view into an array. Wrapping a range in a std::span and using the == operator should be just as cheap as using standard library algorithms.

Solution C - Use std::equal(C++98) or std::ranges::equal(C++20)
if (std::ranges::equal(x, y))

// the last argument is optional, but may be helpful for optimizations
if (std::equal(std::begin(x), std::end(x), std::begin(y), std::end(y))

Upvotes: 0

Vladimir
Vladimir

Reputation: 170839

You can use the standard std::equal algorithm (or std::ranges::equal since C++20):

if (std::equal(x, std::end(x), y)) cout << "It worked!";
// or since C++20
if (std::ranges::equal(x, y)) cout << "It worked!";

Upvotes: 20

Thomas Matthews
Thomas Matthews

Reputation: 57678

Since the arrays are of the same type and length, you could use memcmp to test for equality, that is equality of value and position:

int array1[4] = {1, 2, 3, 4};
int array2[4] = {5, 6, 7, 8};

if (memcmp(array1, array2, sizeof(array1)) == 0)
{
    cout << "arrays are equal" << "\n";
}
else
{
    cout << "arrays are not equal" << "\n";
}

Upvotes: 1

Sarfaraz Nawaz
Sarfaraz Nawaz

Reputation: 361362

Use std::equal as:

if(std::equal(x, x+ xsize, y)) std::cout << "equal";

It checks equality of elements in the same order. That means, according to std::equal the following arrays are not equal.

int x[4] = {1,2,3,4};
int y[4] = {1,2,4,3}; //order changed!

Upvotes: 11

Chad La Guardia
Chad La Guardia

Reputation: 5164

I think its also useful to note here that C++ does not allow you to overload operator== without at least 1 user-defined type. That would be the "nice" solution, but even if you could, it would probably be a bad idea. Arrays in C++...are sort of evil woodland creatures.

std::equal is probably your best bet. Though you could probably do what you want with a *gasp* macro, i think that is gacky.

Upvotes: -1

Nicholas Wilson
Nicholas Wilson

Reputation: 9685

Another way would be to wrap your arrays in the std::array template, which will make a small wrapper class for the array. Everything works pretty much like normal, except that you get a default definition of operator=, so you can use == as normal to do the expected thing.

Upvotes: 8

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