map find() function in C++ STL

The map::find() is a built-in function in C++ STL that returns an iterator or a constant iterator that refers to the position where the key is present in the map. If the key is not present in the map container, it returns an iterator or a constant iterator which refers to map.end()

. 
Syntax: 

iterator=map_name.find(key)
        or 
constant iterator=map_name.find(key)

Parameters: The function accepts one mandatory parameter key, which specifies the key to be searched in the map container. 

Return Value: The function returns an iterator or a constant iterator which refers to the position where the key is present in the map. If the key is not present in the map container, it returns an iterator or a constant iterator which refers to map.end(). 

Time Complexity for Searching Element:
The time complexity for searching elements in std::map is O(log n). Even in the worst case, it will be O(log n) because elements are stored internally as a Balanced Binary Search tree (BST) whereas, in std::unordered_map best case and average case time complexity for searching is O(1) because elements are stored in a Hash table and therefore the key acts as an index while searching in unordered maps. But the worst-case time complexity for searching is O(N).

Below is the illustration of the above function:

C++

// C++ program for illustration
// of map::find() function
#include <bits/stdc++.h>
using namespace std;

int main()
{

    // Initialize container
    map<int, int> m;

    // Insert elements in random order
    m.insert({ 2, 30 });
    m.insert({ 1, 40 });
    m.insert({ 3, 20 });
    m.insert({ 4, 50 });

      int s1=2; //element1 to find (exist in the map)
      int s2=5; //element2 to find (does not exist in the map)

    cout << "Element "<<s1;
      if(m.find(s1)!=m.end()){
      //if the element is found before the end of the map
      cout<<" : found : Value : "<<m[s1]<<endl;
      //if the element is present then you can access it using the index
    }
      else cout<<" : Not found"<<endl;

      cout << "Element "<<s2;
    if(m.find(s2)!=m.end()){
      //if the element is found before the end of the map
      cout<<" : found : Value : "<<m[s2];
      //if the element is present then you can access it using the index
    }
     else cout<<" : Not found"<<endl;

    return 0;
}

Output

Element 2 : found : Value : 30
Element 5 : Not found

Time Complexity: O(log n)
Auxiliary Space: O(n)

Below code is a program to print all the elements after finding a element:

CPP

// C++ program for illustration
// of map::find() function
#include <bits/stdc++.h>
using namespace std;

int main()
{

    // Initialize container
    map<int, int> mp;

    // Insert elements in random order
    mp.insert({ 2, 30 });
    mp.insert({ 1, 40 });
    mp.insert({ 3, 20 });
    mp.insert({ 4, 50 });

    cout << "Elements from position of 3 in the map are : \n";
    cout << "KEY\tELEMENT\n";

    // find() function finds the position
    // at which 3 is present
    for (auto itr = mp.find(3); itr != mp.end(); itr++) {

        cout << itr->first << '\t' << itr->second << '\n';
    }

    return 0;
}

Output

Elements from position of 3 in the map are : 
KEY    ELEMENT
3    20
4    50

Time Complexity: O(log n)
Auxiliary Space: O(n)