problem in traversing through unordered_map cpp

Question

only half of the test cases pass when i traverse through unordered_map, but when i use vector all of them pass, is there any mistake in my code?

#include<bits/stdc++.h>
using namespace std;
int main()
{
    int n;  cin>>n;
    unordered_map<int,int> mp;
    vector<int> v;  v.reserve(n);
    while(n--)
    {
        int x,y;  cin>>x>>y;
        int h=y+x;
        if(mp[h]==0){ v.push_back(h); }
        mp[h]++;
    }
    int s; cin>>s;  int sa=0;
    for(auto i:mp)
    {
        int j=i.first+s;
        long long u=mp[j]*(i.second);
        sa=sa+u;
    }
    cout<<sa;
}

And what are you trying to do? BTW, this looks like UB because of default inserted int if(mp[h]==0) — pptaszni
– pptaszni, Commented Aug 11, 2020 at 12:57
What the test cases are testing? What is your code expected to do? What is unexpected in your code? — armagedescu
– armagedescu, Commented Aug 11, 2020 at 13:07

cigien · Accepted Answer · 2020-08-11 13:25:07Z

1

In this loop:

for(auto i:mp)
{
    int j=i.first+s;
    long long u=mp[j]*(i.second);
    sa=sa+u;
}

you are potentially modifying mp when you do mp[j], in the case that j is not a key in the unordered_map. Modifying a range that you are iterating over in a range-for loop invokes undefined behavior.

You can use find to check if the key j exists and do something else in that case (but not modify the unordered_map).

answered Aug 11, 2020 at 13:25

cigien

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