How Can I Efficiently Deduplicate and Sort a Large Vector in C ?

Optimizing Vector Deduplication and Sorting

Erasing duplicates and sorting a large vector can be a crucial task in programming. Let's examine different approaches and explore how to achieve efficiency in this process.

1. Combining Sort and Unique:

The provided code snippet attempts to erase duplicates and sort the vector using std::erase and std::sort. However, it doesn't work because std::unique requires a sorted input iterator.

To fix this, we can first sort the vector and then apply std::unique to remove duplicates:

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

2. Erasing Duplicates First or Sorting First:

Erasing duplicates before sorting is generally more efficient in terms of time complexity since the std::unique algorithm has a time complexity of O(n), where n is the size of the input vector. However, sorting after erasing duplicates is necessary to ensure that the vector remains sorted.

3. Using a Set:

As mentioned in the linked answer, using a std::set can be more efficient for handling large vectors with significant duplication. A set automatically removes duplicates when inserting elements. We can convert the vector to a set, insert the elements, and then convert it back to a vector, thereby achieving deduplication and sorting in a single step.

Performance Comparison:

Benchmarking different approaches (vector with sort unique, manual set conversion, and set constructor conversion) reveals that when the number of duplicates is significant, converting to a set and dumping data back into a vector is surprisingly faster than using vector-based techniques.

In conclusion, for large vectors with high duplication, using a set provides the most efficient way to erase duplicates and sort them. Additionally, manual set conversion tends to be faster than using the set constructor.

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