Seg fault in unordered_map >

Question

(Segmentation fault) Error occurs while accessing the value associated with the key in the unordered map. Any pointers a where I am making the mistake?

Segmentation fault in the following snippet of code:

void read_mdl(const std::string& fname) {
        std::ifstream f(fname);

        std::unordered_map<int, std::vector<double>> pt_;

        while(f) {
            int xi = -1;
            int pa = 0;
            double score = -1;
            f >> xi >> pa >> score;
            if(xi == -1) { break; }

            std::unordered_map<int, std::vector<double>>::iterator it = pt_.find(pa);

            std::cout << xi << " " << pa << " " << score << std::endl;

            if (it != pt_.end()) {
                // Update from @Matthias247
                 auto a = pt_.insert(std::make_pair(pa, std::vector<double>(n_, 0)));
                 it = a.first;
            }

            (it->second)[xi] = score; // causing segmentation fault
        }
    }

Your way to check for no more data from the stream is flawed. See e.g. [this old answer of mine](http://stackoverflow.com/a/13379073/440558) as for why. Also read ["Why is iostream::eof inside a loop condition considered wrong?"](http://stackoverflow.com/questions/5605125/why-is-iostreameof-inside-a-loop-condition-considered-wrong). Use e.g. `while (f >> xi >> pa >> score)` instead. — Some programmer dude, Jan 28 '17 at 12:32

Matthias247 · Accepted Answer · 2017-01-28T12:38:59.427

0

Your iterator is broken in the case where it doesn't find the element. You insert a new element in this case with pt_.insert but don't change the iterator. Accessing it afterwards leads to the segfault. Either do a new find or lookup if insert returns an iterator and assign that to it.

Update:

If I understand the insert API correctly it should work like

if (it != pt_.end()) {
    auto a = pt_.insert(std::make_pair(pa, std::vector<double>(n_, 0)));
    it = a.first;
}

(it->second)[xi] = score;

edited Jan 28 '17 at 12:38

answered Jan 28 '17 at 12:32

Matthias247

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Error still persisted – letsBeePolite Jan 28 '17 at 13:03
1

It was nevertheless one of the reasons why it crashed. – Matthias247 Jan 28 '17 at 13:14

score 0 · Answer 2 · answered Jan 28 '17 at 13:11

0

Found the bug.

It was in the incorrect checking of whether the key exists or not.

With respect to my snippet:

if (it != pt_.end()) // incorrect way

if (it == pt_.end()) // right way

answered Jan 28 '17 at 13:11

letsBeePolite

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Seg fault in unordered_map >

2 Answers2