To check whether a key is present in a map or not, we can use the [] operator. However, if the key is not found in the map, the [] operator will automatically insert that key, causing the size of the map to increase. Therefore, it is preferable to use the find function instead.
unordered_map<int, bool> m; m[1] = true; m[2] = true; //just want to check whether 3 in map or not ? if (m[3] == true) // insert to map (3,false) cout << "3 in map" << endl; if (m.find(3) != m.end()) { //so we found 3 in map. cout << " 3 already in map" << endl; } else { cout << "not in map" << endl; } 
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