Find unique numbers in array

Question

Well, I have to find how many different numbers are in an array.

For example if array is: 1 9 4 5 8 3 1 3 5

The output should be 6, because 1,9,4,5,8,3 are uniqu

Answer 1

Let me join the party ;)

You could also use a hash-table:

#include 
#include 

int main() {

    int a[] = { 1, 9, 4, 5, 8, 3, 1, 3, 5 };
    const size_t len = sizeof(a) / sizeof(a[0]);

    std::unordered_set s(a, a + len);

    std::cout << s.size() << std::endl;
    return EXIT_SUCCESS;

}

Not that it matters here, but this will likely have the best performance for large arrays.

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If the difference between smallest and greatest element is reasonably small, then you could do something even faster:

• Create a vector that spans the range between min and max element (if you knew the array elements at compile-time, I'd suggest the std::bitset instead, but then you could just compute everything in the compile-time using template meta-programming anyway).
• For each element of the input array, set the corresponding flag in vector.
• Once you are done, simply count the number of trues in the vector.