C++ operations on values given by multiset::equal_range

Question

I'm trying to write a program that is taking 1000 random numbers from 0 to 9 and then counting how many times each number appeared:

    srand(time(NULL));
    multiset<int> M;//multiset that contains 1000 random numbers from 0 to 9
    for (int i = 0; i < 1000; i++){
        r.insert(rand() % 10);
        s.insert(rand() % 10);
    }

    vector <int> R(10);//vector that stores how many times each number appeared 
    //(so R[0] equals how many times 0 appeared and so on)

    pair<multiset<int>::iterator, multiset<int>::iterator> zero = M.equal_range(0);
    R[0] = zero.second - zero.first;

The problem is in the last line (in which I'm trying to count the number of times 0 appeared), it underlines minus and says that no operator matches these operands. But why? Aren't zero.first and zero.second ends of compartments? And how to fix it?

Edit: I have to use a multiset, vector and equal_range, these are the things my teacher picked, not me.

@Incomputable because that's what my teacher picked for this task and I don't really have any word in it. — Ania, Dec 18 '17 at 21:08
@Incomputable thank you very much, this is what I've been looking for. — Ania, Dec 18 '17 at 21:16
Ania please consider accepting @drbombe's answer, which will indicate that it indeed solved the issue. Also - powodzenia na UW — Fureeish, Dec 18 '17 at 21:17
@Ania, I would also advise on the code quality. When taking modulo it is not going to be uniform anymore. One could also make it much more prettier by using standard algorithms and iterators. — Incomputable, Dec 18 '17 at 21:18
@drbombe, it would be `++occurences[value]`, not inserts. It also automatically zero initializes them, sparing some lines of code and CPU cycles. — Incomputable, Dec 18 '17 at 21:20
@drbombe we don't really need any duplicates in any container if our only task is to count occurrences of numbers — Fureeish, Dec 18 '17 at 21:20

score 4 · Accepted Answer · edited Dec 18 '17 at 21:15

4

Use distance(zero.first, zero.second). The - operator cannot be applied to multiset iterators.

edited Dec 18 '17 at 21:15

David G

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answered Dec 18 '17 at 21:12

drbombe

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Thank you a lot, I tested it and it works, I will use it. – Ania Dec 18 '17 at 21:16
Superb! can you upvote my answer? I need some scores : ) – drbombe Dec 18 '17 at 21:17

score -1 · Answer 2 · answered Dec 18 '17 at 21:05

-1

This would be far easier using a map:

std::map<int, int> counts;
...
count[num]++; // map::operator[] will insert the value if it's not already in the sequence.

answered Dec 18 '17 at 21:05

SoronelHaetir

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"this would be easier using [...]" does not answer the questions provided – Fureeish Dec 18 '17 at 21:06
alternatives to a poor or suboptimal approach can complement an already complete answer. unfortunately this is not complete – Ankur S Jun 21 '18 at 12:48

C++ operations on values given by multiset::equal_range

2 Answers2