Why Does Using the Erase-Remove_if Idiom to Remove Elements From a Vector Not Work As Expected?

Understanding the erase-remove_if Idiom for Vector Deletion

In C , the erase-remove_if idiom is widely used to efficiently remove elements from a vector based on a given criteria. Let's delve into a specific example where this idiom was implemented incorrectly and explore the potential pitfalls.

The Problem:

Consider a vector called stopPoints containing pairs of integers and direction values. The goal is to delete all pairs containing a specific integer (e.g., 4) from the vector using the erase-remove_if idiom. However, after executing the code, the result was unexpected.

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

After this operation, the vector still contained instances of pairs with the .first value set to 4.

The Solution:

The error lies in the incorrect usage of the erase function. In the erase-remove_if idiom, the std::erase function requires two iterators—the iterator to the first element to remove and the iterator to the end of the container.

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

In this corrected implementation, the range from the iterator returned by std::remove_if (pointing to the first element to remove) to the end of the vector is erased, effectively removing all elements that match the predicate (.first == 4 in this case).

Explanation:

• std::remove_if scans the vector, moving elements that match the predicate (elements with .first == 4) to the end.
• std::remove_if returns an iterator pointing to the first element not matching the predicate (i.e., the first element to keep).
• std::erase removes the range of elements from the returned iterator to the end of the vector, effectively deleting all elements with .first == 4.

Conclusion:

The erase-remove_if idiom is a powerful tool for efficiently removing elements from a vector meeting a specific criteria. Understanding how the functions std::remove_if and std::erase interact is crucial for its correct implementation.

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