How to Correctly Use the Erase-Remove Idiom for Removing Pairs in a Vector Based on a Specific First Element?

Using the Erase-Remove Idiom with Pair Containers

Suppose we have a vector of pairs>, where the first element denotes a value and the second an enum direction. To remove specific pairs based on their first element, the erase-remove idiom is a suitable approach.

However, as demonstrated in the question, removing pairs with a first value of 4 results in unexpected behavior. To rectify this, we need to provide the correct range to the std::erase function.

The improved code looks like this:

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

Here's why this works correctly:

• std::remove_if: This function takes a range (from the beginning to the end of the vector) and a predicate lambda that checks if the pair's first element is equal to 4. It rearranges the elements in the vector such that all matching pairs are at the end.
• Returned Iterator: The result of std::remove_if is an iterator pointing to the first element that matches the predicate (i.e., the first element to be removed).
• std::erase: The erase function deletes a range starting from the iterator returned by std::remove_if to the end of the vector. Consequently, all matching pairs are eliminated.

By specifying the correct range in the std::erase call, we ensure that all pairs with the specified first value are removed effectively.

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