A technique to initialize array data when accessing it at the first time. This is presented in Programming Pearls' exercises. Here's a very nice explanation of what the following code does: https://research.swtch.com/sparse

initialize_array.cpp

template<unsigned int N>
struct Array
{
    Array() : top(0) {}
    
    // Invariant: if a data has been initialized, it will have the following properties:
    //    1. index_key[i] < top
    //    2. indexes[index_key[i]] == i
    int Get(unsigned int i) {
        if (index_key[i] < top && indexes[index_key[i]] == i) {
            // the key has been initialized, we just return it
            return data[i];
        }
        // otherwise, we initialize the data.
        // Here for simplicity, we set the data to 0.
        index_key[i] = top;
        indexes[top] = i;
        ++top;
        data[i] = 0;
        return data[i];
    }
    
    int data[N];
    unsigned int index_key[N];
    unsigned int indexes[N];
    unsigned int top;
};