Why Is My Erase-Remove_If Code Leaving Duplicates?

Erase-Remove_If Idiom: Correct Application

The erase-remove_if idiom is a versatile technique for efficiently removing elements from a container based on a predicate. However, improper implementation can lead to unexpected results.

In the given example, you aimed to remove pairs with .first value of 4 from a vector of pairs using erase-remove_if. However, your code left you with duplicates, indicating an error.

The correct code should be:

stopPoints.erase(std::remove_if(stopPoints.begin(),
                                stopPoints.end(),
                                [](const stopPointPair stopPoint)-> bool 
                                       { return stopPoint.first == 4; }), 
                 stopPoints.end());

The explanation lies in the mechanism of erase-remove_if.

How It Works

std::remove_if swaps elements within the vector to group elements that do not match the predicate towards the beginning. It then returns an iterator to the first element to be removed, marking the separation between elements to be kept and removed.

std::vector::erase starts at the returned iterator and erases all subsequent elements, effectively removing all elements that match the predicate.

In your initial code, you omitted the second parameter to std::erase, leading to only the element indicated by the returned iterator being removed. This resulted in the duplicates because subsequent elements matching the predicate were not erased.

By including the second parameter, stopPoints.end(), we instruct erase to remove the range starting from the returned iterator to the end of the vector, ensuring all matching elements are removed.

For more comprehensive information on the erase-remove idiom, refer to the Wikipedia entry: https://en.wikipedia.org/wiki/Erase–remove_idiom

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