Creating unordered map in C++ for counting the pairs

November 26, 2021

So let’s say I have an array, 1 2 1 2 3 4 2 1 and I want to store all the (arr[i], arr[i-1)
such that arr[i] != arr[i-1] as a pair in unordered_map for counting these pairs.

For e.g.

(1, 2)  -> 2
(2, 3) -> 1
(3, 4) -> 1
(4, 2) -> 1
(2, 1) -> 1

So the syntax I tried,

unordered_map<pair<int, int>,  int> umap;
        int temp; 
        cin>>temp;
        arr[i]=temp;
        for (int i=1; i< n; i++){
            cin>>temp;
            arr[i]=temp;
            
            umap[(arr[i-1], arr[i])]++;
            
        }
        
    }

Next thing, I also tried with proper definition.

unordered_map<pair<int, int>,  int> umap;
        cin>>temp;
        arr[i]=temp;
        for (int i=1; i< n; i++){
            cin>>temp;
            arr[i]=temp;
            pair<int, int> p(arr[i-1], arr[i]);
            umap[p]++;
            
        }
        
    }

Can anybody please help me getting the right syntax? Thanks in advance

>Solution :

You can’t just use unordered_map with a pair because there is no default hash implemented.
You can however use map which should work fine for your purpose because pair does implement <.
See Why can't I compile an unordered_map with a pair as key? when you really want unordered_map.

You can construct pair with curly braces like this