問題描述

我試圖根據特定條件從地圖中刪除一系列元素.我如何使用 STL 算法做到這一點?

I was trying to erase a range of elements from map based on particular condition. How do I do it using STL algorithms?

最初我想使用 remove_if 但這是不可能的，因為 remove_if 不適用于關聯容器.

Initially I thought of using remove_if but it is not possible as remove_if does not work for associative container.

是否有任何適用于地圖的remove_if"等效算法?

Is there any "remove_if" equivalent algorithm which works for map ?

作為一個簡單的選擇，我想到了遍歷地圖并擦除.但是循環遍歷地圖并擦除安全選項嗎?(因為擦除后迭代器無效)

As a simple option, I thought of looping through the map and erase. But is looping through the map and erasing a safe option?(as iterators get invalid after erase)

我使用了以下示例:

bool predicate(const std::pair<int,std::string>& x)
{
    return x.first > 2;
}

int main(void) 
{

    std::map<int, std::string> aMap;

    aMap[2] = "two";
    aMap[3] = "three";
    aMap[4] = "four";
    aMap[5] = "five";
    aMap[6] = "six";

//      does not work, an error
//  std::remove_if(aMap.begin(), aMap.end(), predicate);

    std::map<int, std::string>::iterator iter = aMap.begin();
    std::map<int, std::string>::iterator endIter = aMap.end();

    for(; iter != endIter; ++iter)
    {
            if(Some Condition)
            {
                            // is it safe ?
                aMap.erase(iter++);
            }
    }

    return 0;
}

推薦答案

差不多.

for(; iter != endIter; ) {
     if (Some Condition) {
          iter = aMap.erase(iter);
     } else {
          ++iter;
     }
}

如果您確實從中刪除了一個元素，那么您最初擁有的會增加迭代器兩次；您可能會跳過需要刪除的元素.

What you had originally would increment the iterator twice if you did erase an element from it; you could potentially skip over elements that needed to be erased.

這是我在很多地方看到使用和記錄的常用算法.

This is a common algorithm I've seen used and documented in many places.

您是正確的，迭代器在擦除后失效，但只有引用被擦除元素的迭代器，其他迭代器仍然有效.因此在 erase() 調用中使用 iter++.

You are correct that iterators are invalidated after an erase, but only iterators referencing the element that is erased, other iterators are still valid. Hence using iter++ in the erase() call.

這篇關于remove_if 等效于 std::map的文章就介紹到這了，希望我們推薦的答案對大家有所幫助，也希望大家多多支持html5模板網！