How to traverse through all values for a given key in multimap?

Given a multimap and a key of the multimap, our task is to simply display the (key – value) pairs of the given key. In multimap we can have multiple (key – value) pair for the same key. Suppose our multimap contains

key     value
1         10
2         20
2         30
2         40
3         50
4         60
4         70

key : 2
key     value
2         20
2         30
2         40

Like in unordered_map in C++ STL we cant fetch values like

int key = 2;
multimap  map;

// insert values in map
cout << "Key : " << key;
cout << "Value : " < second;

Output :

Key : 2
Value : 20

Because the above method will only return the first occurrence of the key present, This method fails if there are multiple (key – value) pairs for the same key.

There are two ways by which we can achieve the expected results :
Method 1 (Simple Traversal) Traverse through whole map and whenever the key is equal to given key we display the key-value pair.

C++

// CPP program to find all values for a  
// given key. 
#include <bits/stdc++.h> 
using namespace std; 
  
int main() 
{ 
    multimap <int, int> map; 
  
    // insert the values in multimap 
    map.insert(make_pair(1, 10)); 
    map.insert(make_pair(2, 20)); 
    map.insert(make_pair(2, 30)); 
    map.insert(make_pair(2, 40)); 
    map.insert(make_pair(3, 50)); 
    map.insert(make_pair(4, 60)); 
    map.insert(make_pair(4, 70)); 
      
    int key = 2;     
    for (auto itr = map.begin(); itr != map.end(); itr++)     
        if (itr -> first == key)         
            cout << itr -> first << "  " 
                 << itr -> second << endl; 
  
    return 0; 
}  

Java

// JAVA program to find all values for a  
// given key. 
import java.util.*; 
  
class GFG 
{ 
  
static class pair 
{  
    int first, second;  
    public pair(int first, int second)  
    {  
        this.first = first;  
        this.second = second;  
    }  
}  
  
public static void main(String[] args) 
{ 
    HashSet <pair> map = new LinkedHashSet<>(); 
  
    // add the values in multimap 
    map.add(new pair(1, 10)); 
    map.add(new pair(2, 20)); 
    map.add(new pair(2, 30)); 
    map.add(new pair(2, 40)); 
    map.add(new pair(3, 50)); 
    map.add(new pair(4, 60)); 
    map.add(new pair(4, 70)); 
      
    int key = 2;  
    for (pair itr : map)  
        if (itr.first == key)      
            System.out.println(itr.first+ " "
                + itr.second); 
} 
} 
  
// This code is contributed by 29AjayKumar 

C#

// C# program to find all values for a  
// given key. 
using System; 
using System.Collections.Generic; 
  
class GFG 
{ 
  
class pair 
{  
    public int first, second;  
    public pair(int first, int second)  
    {  
        this.first = first;  
        this.second = second;  
    }  
}  
  
// Driver code 
public static void Main(String[] args) 
{ 
    HashSet<pair> map = new HashSet<pair>(); 
  
    //.Add the values in multimap 
    map.Add(new pair(1, 10)); 
    map.Add(new pair(2, 20)); 
    map.Add(new pair(2, 30)); 
    map.Add(new pair(2, 40)); 
    map.Add(new pair(3, 50)); 
    map.Add(new pair(4, 60)); 
    map.Add(new pair(4, 70)); 
      
    int key = 2;  
    foreach (pair itr in map)  
        if (itr.first == key)      
            Console.WriteLine(itr.first+ " "
                + itr.second); 
} 
} 
  
// This code is contributed by Rajput-Ji 

Output:

2  20
2  30
2  40

Method 2(Using Binary Search)Find the lower_bound and upper_bound for the given key and traverse between them.
lower_bound(key) : returns the iterator pointing to the first element which is greater than or
equal to key.
upper_bound(key) : returns the iterator pointing to the first element which is greater than key.

key     value
1         10
2         20 <-- lower_bound(20)
2         30
2         40
3         50 <-- upper_bound(20)
4         60
4         70

#include <bits/stdc++.h> 
using namespace std; 
  
int main() 
{ 
    multimap <int, int> map; 
  
    // insert the values in multimap 
    map.insert(make_pair(1, 10)); 
    map.insert(make_pair(2, 20)); 
    map.insert(make_pair(2, 30)); 
    map.insert(make_pair(2, 40)); 
    map.insert(make_pair(3, 50)); 
    map.insert(make_pair(4, 60)); 
    map.insert(make_pair(4, 70)); 
      
    int key = 2;     
    auto itr1 = map.lower_bound(key); 
    auto itr2 = map.upper_bound(key); 
      
    while (itr1 != itr2)     
    { 
        if (itr1 -> first == key) 
           cout << itr1 -> first << "  "  
                << itr1 -> second << endl;         
        itr1++; 
    }     
    return 0; 
} 

Output:

2  20
2  30
2  40

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My Personal Notes

C++

Java

C#

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