Is there a way to find duplicates effectively? - c++

Question

I'm trying to find duplicates even though they have been put in by user input. I've tried a few methods, for loop to do the checks but that doesnt work.
Anyone got any idea how I might be able to do this effectively?
#include <iostream>
#include <algorithm>
int main()
{
int size, * array;
std::cout << "Indtast arrayet's stoerrelse." << std::endl;
std::cin >> size; // optager input til størrelse af arrayet fra brugeren
array = new int[size]; // dynamisk allokeret array
while (size >= 100) { // while loop til at tjekke om arrayets størrelse er for stort
std::cout << "Forkert input af array." << std::endl;
break;
}
while (size <= 100) { // while loop til når arrayet er af rigtig størrelse
float sum = 0;
float gns = 0;
std::cout << "Tast op til " << size << " vaerdier ind." << std::endl;
for (int i = 0; i < size; i++) { // for loop til at optage input til hver element i arrayen
std::cout << "Indtast vaerdi for array-nr: " << i << std::endl;
std::cin >> array[i]; // optager input til hvert element
sum += array[i]; // finder summen af elementerne i arrayet
}
std::cout << "Min-vaerdi er: " << *std::min_element(array, array + size) << std::endl;
std::cout << "Max-vaerdi er: " << *std::max_element(array, array + size) << std::endl;
std::cout << "Summen er: " << sum << std::endl;
gns = sum / size; // udregner gennemsnittet
std::cout << "Gennemsnittet er: " << gns << std::endl;
return 0;
}
}

Answer 1

Here is a modern (C++20) approach using ranges. As suggested in the comments, you should sort your vector and then use adjacent_if. It is always recommended to use the algorithms from the standard library than to try to re-implement common stuff like this yourself using basic procedural tools like for and while loops.
This approach would work with an array as well, but I suggest using an std::vector.
#include <vector>
#include <algorithm>
#include <iostream>
void print(std::vector<int> const& v){
// print the vector
for (auto const& i: v){
std::cout << i << ", ";
}
std::cout<<"\n";
};
int main()
{
std::vector<int> v{1, 2, 1, 1, 3, 3, 3, 4, 5, 4};
std::cout << "Input vector = \t\t";
print(v);
// sort the vector
std::ranges::sort(v);
std::cout << "Sorted vector = \t";
print(v);
// find duplicates and print them
auto it = std::ranges::adjacent_find(v);
while (it != v.end()){
std::cout
<< "Duplicates start at v["
<< std::distance(v.begin(), it)
<< "] = "
<< *it
<< "\n";
it = std::ranges::adjacent_find(it, v.end(), std::ranges::not_equal_to());
it = std::ranges::adjacent_find(it, v.end());
}
for (; it != v.end(); ++it){
std::cout << *it << std::endl;
}
// remove duplicates
auto ret = std::ranges::unique(v);
v.erase(ret.begin(), ret.end());
std::cout << "No duplicates = \t";
print(v);
return 0;
}
Output:
Input vector = 1, 2, 1, 1, 3, 3, 3, 4, 5, 4,
Sorted vector = 1, 1, 1, 2, 3, 3, 3, 4, 4, 5,
Duplicates start at v[0] = 1
Duplicates start at v[4] = 3
Duplicates start at v[7] = 4
No duplicates = 1, 2, 3, 4, 5,
Try it on compiler explorer: https://godbolt.org/z/YP3zb7PYf