How Can I Efficiently Remove Duplicates and Sort a C Vector?

Optimizing Duplicate Erasure and Sorting in a Vector

In C , vectors are a common data structure for storing elements. However, efficiently managing large vectors with duplicates and sorting requirements can be challenging.

Inefficient Approach

The code snippet provided attempts to erase duplicates and sort a vector using std::unique and std::sort:

vec.erase(
  std::unique(vec.begin(), vec.end()),
  vec.end());
std::sort(vec.begin(), vec.end());

However, this approach fails to remove duplicates correctly.

Preferred Approach

There are several alternative approaches that offer better performance:

1. Using a std::set

A std::set is a container that automatically maintains a sorted and unique set of elements. Converting the vector to a set can remove duplicates efficiently:

std::set<int> s(vec.begin(), vec.end());

The sorted data can then be transferred back to the vector:

vec.assign(s.begin(), s.end());

2. Erasing Duplicates Manually

Duplicates can also be erased manually by iterating through the vector and checking for consecutive duplicates:

for (auto it = vec.begin(); it != vec.end(); ) {
  if (*it == *(it+1)) {
    it = vec.erase(it);
  } else {
    ++it;
  }
}

Sorting Considerations

Sorting after duplicate removal is necessary to maintain a sorted order. However, the order may not be guaranteed in all cases:

Case 1: Sort First, Erase After

If the vector is sorted before erasing duplicates, std::unique will likely preserve the sorted order.

Case 2: Erase First, Sort After

If duplicates are erased before sorting, the order may not be guaranteed. This is because the order of removal may affect the indices of subsequent elements.

Performance

The performance of these approaches varies depending on the number of duplicates. For a large number of duplicates, converting to a set and back to a vector can be faster than erasing duplicates manually. However, for a small number of duplicates, manual erasure may be more efficient.

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