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C++ Unordered_map::clear() Function
The C++ unordered_map::clear() function is used to destroy or drop the unordered_map containers by removing all elements and sets the size of unordered_map to zero.
Even after removing all the elements from the unordered_map, we can add the new element to the same unordered_map using the [] operator. for example; unordered_map[1] = "tutorialspoint";
Syntax
Following is the syntax of unordered_map::clear() function.
unordered_map.clear();
Parameters
This function does not accepts any parameter.
Return value
This function does not returns anything because the size of unordered_map after using the clear() function is zero.
Example 1
In the following example, let's demonstrate the uses of the clear() function, as follows:
#include <iostream> #include <unordered_map> using namespace std; int main(void) { unordered_map<char, int> um = { {'a', 1}, {'b', 2}, {'c', 3}, {'d', 4}, {'e', 5} }; cout << "Initial size of unordered map = " << um.size() << endl; um.clear(); cout << "Size of unordered map after clear operation = " << um.size() << endl; return 0; }
Output
Output of the above code is as follows −
Initial size of unordered map = 5 Size of unordered map after clear operation = 0
Example 2
Following is the example, where we are using the clear() function to clear the unordered_map and later we are adding two elements to the same unordered_map, as follows:
#include <iostream> #include <string> #include <unordered_map> using namespace std; int main () { unordered_map<string,string> Umap = { {"Aman","Kumar"}, {"Vivek","Verma"}, {"Jhon","Satya"} }; cout << "mymap contains:"; for (auto& x: Umap)cout << " " << x.first << "=" << x.second; cout << endl; //removing all the element from the map. Umap.clear(); //adding two element from the map Umap["TutorialsPoint"]="Tutorix"; Umap["Location"]="Hyderabad"; cout << "mymap contains:"; for (auto& x: Umap) cout << " " << x.first << "=" << x.second; cout << endl; return 0; }
Output
On executing we get the following output −
mymap contains: Jhon=Satya Vivek=Verma Aman=Kumar mymap contains: Location=Hyderabad TutorialsPoint=Tutorix
Example 3
Let's look at the following example, where we are displaying the unordered_map element before the use of the clear() function, and after that, we are displaying the size of the unordered_map, as follows:
#include <iostream> #include <string> #include <unordered_map> using namespace std; int main () { unordered_map<int, string> Umap; Umap[1] = "tutorialspoint"; Umap[2] = "Hyderabad India"; Umap[3] = "Tutorix"; Umap[4] = "Noida India"; cout<<"Before use of clear() functon"; for (auto& x: Umap) cout << "\n" << x.first << "=" << x.second; cout << endl; Umap.clear(); cout<<"After use of clear() function \n"; cout <<Umap.size(); return 0; }
Output
Following is the output of the above code −
Before use of clear() functon 4=Noida India 3=Tutorix 2=Hyderabad India 1=tutorialspoint After use of clear() function 0