aJ. aJ. - 29 days ago 11
C++ Question

remove_if equivalent for std::map

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

Initially I thought of using

remove_if
but it is not possible as remove_if does not work for associative container.

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

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)

I used following example:

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;
}

Answer

Almost.

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

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.

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

[EDIT] 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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