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

Hi All,

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

Code:

---

    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.

Quote:

---

Originally Posted by aryan1

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:

Code:

---

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

---

Quote:

---

Originally Posted by aryan1

Hi All,

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

Code:

---

    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.

---

Code:

---

#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

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

Quote:

---

Originally Posted by laserlight

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.

Some people will show you this form:

Code:

---

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.

Quote:

---

Originally Posted by monarch_dodra

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

To quote "Effective STL"

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

Code:

---

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

---

For others use

Code:

---

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

---

For std::list use either.