How to remove an item from a list of tuples in c ?-CodePudding

I'm iterating over my list of tuples : list<tuple<int,int>> edges, and want to remove some elements in it. This is necessary for me to reduce the total overhead as I am working with huge data.

std::list<tuple<int, int>>::iterator it;
for (it = edges.begin(); it != edges.end();   it)
{
  if (get<0>(*it) == 0 || get<1>(*it) == 0){
      edges.remove(*it);
  }

}

As I know, remove(element) works, but here edges.remove(*it) does not. How can I do this correctly?

CodePudding user response：

You can use erase() to specify an element to remove by an iterator.

It returns an iterator for the next element, so don't forget to catch that.

std::list<tuple<int, int>>::iterator it;
for (it = edges.begin(); it != edges.end(); ) // don't increment it here
{
  if (get<0>(*it) == 0 || get<1>(*it) == 0){
      it = edges.erase(it);
  } else {
        it;
  }
}

CodePudding user response：

In C 20, you can simply use a specialization of std::erase_if for std::list to do this.

#include <list>
#include <tuple>

int main() {
  std::list<std::tuple<int, int>> l;
  std::erase_if(l, [](const auto& elem) {
    auto& [first, second] = elem;
    return first == 0 || second == 0; });
}

Demo

However, since std::list itself has a remove_if member function, it is more appropriate to use it directly, since it applies to any C standard.

CodePudding user response：

In my opionion remove_if which is a dedicated and optimized function for a std::list should be used. This will avoid unnecessary indirections.

Please read here about it.

The result will be an efficient one liner.

Please see one of many potential solutions:

#include <iostream>
#include <list>
#include <tuple>

using MyTuple = std::tuple<int,int>;
using MyList = std::list<MyTuple>;

int main() {
    // Define some demo data
    MyList myList{{0,1},{2,3},{4,5},{6,0},{7,8},{9,10},{0,0}};
    
    
    // Predicate function. Define whatever you want
    auto unwanted = [](const MyTuple& t) {return std::get<0>(t)==0 or std::get<1>(t)==0;};
    
    // Remove all unwanted stuff
    myList.remove_if(unwanted);
    
    // Some debug output
    for (const auto&[l,r] : myList) 
        std::cout << l << ' ' << r << '\n';
}