CODEWITHSUNDEEP
c++stl
Riclea Andrei
Riclea Andrei

Reputation: 79

C++ std::set::erase doesn't work

I have these following variables:

vector< unordered_map< char, set<int> > > LNFA, NFA;
vector< set<int> > L_enclosure;
set<char> characters;
int nr_states;
set<int> acceptance_states, NFA_accept;

And I tried using this function to remove a certain number from std::set from NFA variable. But if i print the content in it it display even the items that i want to delete. These variables are globally declared.

This is the function:

void remove_state(int x) {
    for (int i = 0; i < nr_states; ++i) {
        for (auto jt : NFA[i]) {
            jt.second.erase(x);
        }
    }
}

What am i possibly doing wrong to not receive the desired output?

Upvotes: 1

Views: 1543

Answers (1)

Sam Varshavchik
Sam Varshavchik

Reputation: 118300

for (auto jt : NFA[i]) {
  jt.second.erase(x);

The range iteration here iterates by value. You need to iterate by reference:

for (auto &jt : NFA[i]) {
  jt.second.erase(x);

Your original range iteration was logically equivalent to the following (not quite, actually, but the minutiae differences are not relevant for the purpose of this question):

for (auto b=NFA[i].begin(); b != NFA[i].end(); ++b)
{
    auto jt=*b;

    jt.erase(x);
}

So you end up erasing the value from a copy of the actual std::set. Not a useful result.

You need to make your range iteration use a reference, so that your range iteration becomes logically equivalent to:

auto &jt=*b;

Upvotes: 5

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