c++ array any two terms are equal - c++

Question

In an array, how do I check if any two variables are equal in something like
total_milk[7] = { b_milk, e_milk, d_milk, g_milk, a_milk, m_milk, h_milk };
without using casework

Answer 1

Iterate over the elements in the array, adding each element to an unordered_set.
The return value from unordered_set::insert() will tell you whether the element already was in the set.

Answer 2

Use two for loops and compare each element with other elements:
bool anyTwo(total_milk a[], std::size_t n) {
for (std::size_t i = 0; i < n - 1; i++) {
for (std::size_t j = i + 1; j < n; j++) {
if (a[i] == a[j]) {
return true;
}
}
}
return false;
}
This assumes your overloaded the == operator in your class. The second for loop counter j starts from i + 1 instead of 0 or i as there is no need to compare the already compared values or compare the element with itself.

Answer 3

The naive approach, which is fine for small data sets, is to simply use a comparison loop where each element is compared with every other element following it - there's no point comparing with those at or before an element since either the comparison has already been done or you will be comparing an item with itself.
The following complete program illustrates this approach:
#include <iostream>
int milk[] = { 3, 1, 4, 1, 5, 9 };
int main() {
for (size_t one = 0; one < sizeof(milk) / sizeof(*milk) - 1; ++one) {
for (size_t two = first + 1; two < sizeof(milk) / sizeof(*milk); ++two) {
if (milk[one] == milk[two]) {
std::cout << "Duplicate item: " << milk[one] << '\n';
return 1;
}
}
}
std::cout << "No duplicates\n";
}
For larger data sets, you can turn to the more optimised collections provided by the C++ library, such as the sets. A set is able to hold one of each value and has the useful property that it will return the fact that you tried to insert a duplicate, by returning both an iterator to the inserted/original item and a boolean value indicating whether it was new or a duplicate.
Like the earlier program, this one shows how you can use this method:
#include <iostream>
#include <unordered_set>
int milk[] = { 3, 1, 4, 1, 5, 9 };
int main() {
std::unordered_set<int> checkSet;
for (auto val: milk) {
auto iterAndBool = checkSet.insert(val);
if (! iterAndBool.second) {
std::cout << "Duplicate item: " << val << '\n';
return 1;
}
}
std::cout << "No duplicates\n";
}
A substantial improvement could be made to that using templates. This would allow it to handle arrays of any data type (assuming it has equality operators, of course) without have to code up specialisations for each. The code for that would be along the following lines:
#include <iostream>
#include <unordered_set>
template<class T> T *CheckDupes(T *collection, size_t count) {
std::unordered_set<T> checkSet;
for (size_t idx = 0; idx < count; ++idx) {
auto iterAndBool = checkSet.insert(collection[idx]);
if (! iterAndBool.second) {
return &(collection[idx]);
}
}
return nullptr;
}
int milk[] = { 3, 1, 4, 1, 5, 9 };
int main() {
int *dupe;
if ((dupe = CheckDupes<int>(milk, sizeof(milk) / sizeof(*milk))) != nullptr) {
std::cout << "Duplicate item: " << *dupe << '\n';
return 1;
}
std::cout << "No duplicates\n";
}
The templated function above will either return nullptr (if there are no duplicates) or the address of one of the duplicates. It's a simple matter to check that return value and act appropriately.
I suspect further improvement could be made to handle other collection types (not just naked arrays) but I'll leave that as an exercise for when you've mastered simpler templates :-)