CODEWITHSUNDEEP
c++
Lu J.
Lu J.

Reputation: 51

How can I create a constant array with some calculation

I have code

int a[5];
for (int i = 0; i < 5; i++){
    a[i] = i * i;
}

Is there a way to make this array constant so that other code can use it but not change it.

Upvotes: 0

Views: 538

Answers (2)

Robert Andrzejuk
Robert Andrzejuk

Reputation: 5222

Using modern C++ (array and an Immediately Invoked Lambda (IIL)), this can be achieved:

const auto a = []() 
{
    std::array< int, 5 > x;

    for (int i = 0; i < 5; i++)
    {
        x[i] = i * i;
    }

    return x;
}(); // IIL

Using a lambda has benefits over a function call, that you can capture the local variables into the lambda and use them without the values passing through parameters ([&] or [=] instead of []).

Just like a function the lambda can easily be inlined into your code, so there can be no overhead.

Upvotes: 0

SergeyA
SergeyA

Reputation: 62563

Easiest way is to use constexpr function and the std::array:

constexpr std::array<int, 5> make_array() {
    std::array<int, 5> a{};
    for (int i = 0; i < 5; i++){
        a[i] = i * i;
    }
    return a;
}
//...
const std::array<int, 5> a = make_array();

NB. As noted by @M.M, this code is only valid for C++17, as pre-C++17 the operator[] on array wasn't constexpr.

Upvotes: 1

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