Can't understand the problem with iterator?

Question

The problem states that we have to find the min no of students to remove so that the ith student can pass the exam. So I am basically adding the students in a multiset as it stores sorted values and while the sorted sum is greater than required marks we subtract it out and move to the next one.

The problem comes with the input:

3 4 3 9 1 1 9 8 9

with m : required marks to pass being 14

Here at the 6th index of input which is 9 which has not been added to the multiset is being deleted somehow.

The output that i am getiing when running the troubled input:

0 0 0 ;4--;3-- 2 ;9-- 1 ;9-- 1 ;9--;4--;9-- 3 ;9--;9--;9-- 3 ;9--;9--;9--;9-- 4

The values in :""-- contain the *x which being subtracted from sum there is an extra 9 but i don't know how?

multiset<int> st;
    int setsum =0;
    for(int i=0;i<n;i++)
    {
        int sum = setsum+ar[i];
        if((sum)<=m)
        {
            cout<<"0 ";
        }
        else
        {
            //cout<<sum<<"-*";
            int cnt = 0;
            auto x = st.rbegin();
            while(sum>m)
            {
                sum -= *x;
                //cout<<";"<<*x<<"--";
                x--;
                //if(i==3)
                    //cout<<*x<<"++";
                cnt++;
            }
            cout<<" "<<cnt<<" ";
        }
        st.emplace(ar[i]);
        setsum += ar[i];
    }

***ll stands for long long int by the way*** My advice is next time you post at `StackOverflow` to do a search and replace on your code to remove that before adding your code to your question. — drescherjm, Jun 20 '19 at 18:12
This seems like a very good time to [learn how to debug your programs](https://ericlippert.com/2014/03/05/how-to-debug-small-programs/), — Some programmer dude, Jun 20 '19 at 18:17
@HardikMehta: I haven't downvoted, but one of the more important things in a "Why isn't my code working?" question is to provide a [MCVE]. You haven't done so, as the code in the question isn't sufficient to reproduce the problem (I can't copy it into a source file, compile it and run it to reproduce your behavior), nor have you provided the expected output, making it much harder for people to help you. — ShadowRanger, Jun 20 '19 at 18:21
Sorry for not providing the expected output, the problem in my code though is as you stated.Will do it from next time onwards. Thanks, @ShadowRanger — Hardik Mehta, Jun 20 '19 at 18:24
The true beauty of making a [mcve] is the making often makes the question redundant. The request for a minimal-but-complete example is an attempt to force the asker to employ a very effective debugging technique that likely would have allowed the asker to find the bug without asking for help. As you shave off more and more bits of your code that are not required to produce the bug, said bug has fewer and fewer places to hide. Eventually it has nowhere left to hide, and if haven't solved the problem at that point, you likely have a *really* interesting question. — user4581301, Jun 20 '19 at 18:53

score 1 · Accepted Answer · answered Jun 20 '19 at 18:18

Probably not the only problems, but I can't help but notice two major bugs in your use of reverse iterators:

You decrement the iterator (--x) instead of incrementing it (++x); the whole point of reverse iterators is that the direction is reversed, so you should be incrementing the iterator to move backwards through st. The only reason you'd use --x is if you had bidirectional iterator and wanted to move opposite the "natural" iteration order (so forward iterators would run backward, and reverse iterators would run forward).
You never check if you've reached the end of st; if you run off the end of st before sum > m (we have no definition of m, and thus no way to tell if this condition is necessarily true prior to running off the end of st), you hit undefined behavior. The simplest fix is to simply update the test to while (sum > m && x != st.rend()), though that may affect your code logic later on (since now exiting the loop isn't a guarantee that sum is less than or equal to m), necessitating further tests.

Can't understand the problem with iterator?

1 Answers1