how to erase entries in a map on-the-fly ?

[aryan1]

Hi All,

I think that it is not legitimate to erase entries in a map in the following way:

    MyMap temp;

    //fill in temp here

    MyMap::iterator it; //MyMap is a typedef of STL map

    for(it = temp.begin(); it != temp.end(); it++)
    {
        if( condition )
        {
             temp.erase(it->first); //erase by key
        }
    }

The purpose is to transform the above code into one so that entries meeting a certain condition can be erased from map on one pass.

Thanks.

[laserlight]

The purpose is to transform the above code into one so that entries meeting a certain condition can be erased from map on one pass.

I shall paraphrase the example that I often refer to, from Josuttis' The C++ Standard Library:

for (it = temp.begin(); it != temp.end();)
{
    if (condition)
    {
         temp.erase(it++);
    }
    else
    {
        ++it;
    }
}

[Paul McKenzie]

Hi All,

I think that it is not legitimate to erase entries in a map in the following way:

    MyMap temp;

    //fill in temp here

    MyMap::iterator it; //MyMap is a typedef of STL map

    for(it = temp.begin(); it != temp.end(); it++)
    {
        if( condition )
        {
             temp.erase(it->first); //erase by key
        }
    }

The purpose is to transform the above code into one so that entries meeting a certain condition can be erased from map on one pass.

Thanks.

#include <algorithm>
bool SomeCondition(const MyMap::value_type& v)
{
  //...return true if v is to be erased, false otherwise
}

temp.erase(std::remove_if(temp.begin(), temp.end(), SomeCondition),
                   temp.end());

Regards,

Paul McKenzie

[laserlight]

Paul McKenzie : it is a std::map, so std::remove_if is not appropriate.

[monarch_dodra]

Paul McKenzie : it is a std::map, so std::remove_if is not appropriate.

Yes, I was going to say the same thing. 95% sure it compiles, and 95% sure it messes up your map.

[monarch_dodra]

Some people will show you this form:

for (it = temp.begin(); it != temp.end();)
{
    if (condition)
    {
         it = temp.erase(it);
    }
    else
    {
        ++it;
    }
}

While this is more straight forward and easier to understand, it does not conform to the standard. erase does not actually return an iterator.

[Paul McKenzie]

Yes, I was going to say the same thing. 95% sure it compiles, and 95% sure it messes up your map.

Yes, it won't work. Only forward iterators should be used, so associative containers seem to be off-limits.

Regards,

Paul McKenzie

[JohnW@Wessex]

To quote "Effective STL"

Use this for std::vector, std::string & std:eque

for (it = temp.begin(); it != temp.end();)
{
    if (condition)
    {
         it = temp.erase(it);
    }
    else
    {
        ++it;
    }
}

For others use

for (it = temp.begin(); it != temp.end();)
{
    if (condition)
    {
         temp.erase(it++);
    }
    else
    {
        ++it;
    }
}

For std::list use either.