remove_if等效于std :: map

问题描述

我试图根据特定条件从地图中删除一系列元素。如何使用STL算法？

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

最初我想到使用 remove_if ，但它不可能作为remove_if对于关联容器不起作用。

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remove_if等效算法适用于map？

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

作为一个简单的选项，我想到循环通过地图和擦除。但是循环遍历地图并擦除了一个安全选项（因为擦除后迭代器无效）

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)

我使用下面的例子：

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;
}

推荐答案

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

你最初会增加迭代器两次（如果您删除了一个元素）;您可以跳过需要删除的元素。

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.

这是我在很多地方使用和记录的常见算法。

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

你是正确的，迭代器在擦除后无效，但只有迭代器引用被擦除的元素，其他迭代器仍然有效。因此在erase（）调用中使用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.

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