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

Python3

# Python program to find all values for a 
# given key.
map = []

# insert the values in multimap
map.append((1, 10));
map.append((2, 20));
map.append((2, 30));
map.append((2, 40));
map.append((3, 50));
map.append((4, 60));
map.append((4, 70));

key = 2;    
for i in map:
    if i[0] == key:
        print(i[0],i[1])

# This code is contributed by shubhamsingh10

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

Javascript

<script>

class pair
{
    constructor(first,second)
    {
        this.first=first;
        this.second=second;
    }
}

let map = new Set();
// 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));

let key = 2; 
for (let itr of map.values()) 
    if (itr.first == key)     
        document.write(itr.first+ " "
                   + itr.second+"<br>");


// This code is contributed by rag2127
</script>

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(2)
2         30
2         40
3         50 <-- upper_bound(2)
4         60
4         70
 

CPP

#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;
}

Java

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Map<Integer, List<Integer>> map = new HashMap<>();

        // Insert the values in the map
        map.put(1, Arrays.asList(10));
        map.put(2, Arrays.asList(20, 30, 40));
        map.put(3, Arrays.asList(50));
        map.put(4, Arrays.asList(60, 70));

        int key = 2;

        // Print the values for the specific key
        if (map.containsKey(key)) {
            for (int value : map.get(key)) {
                System.out.println(key + "  " + value);
            }
        }
    }
}

Python3

from collections import defaultdict

# Create a defaultdict with int as the default factory
map = defaultdict(list)

# insert the values in the multimap
map[1].append(10)
map[2].extend([20, 30, 40])
map[3].append(50)
map[4].extend([60, 70])

key = 2
itr1 = map[key]

for val in itr1:
    print(key, val)

C#

using System;
using System.Collections.Generic;

class MainClass {
    public static void Main (string[] args) {
        Dictionary<int, List<int>> map = new Dictionary<int, List<int>>();

        // Insert the values in the map
        map.Add(1, new List<int> { 10 });
        map.Add(2, new List<int> { 20, 30, 40 });
        map.Add(3, new List<int> { 50 });
        map.Add(4, new List<int> { 60, 70 });

        int key = 2;

        // Print the values for the specific key
        if (map.ContainsKey(key)) {
            foreach (int value in map[key]) {
                Console.WriteLine(key + "  " + value);
            }
        }
    }
}

JavaScript

// Create a JavaScript object to store the values
const map = {
    1: [10],
    2: [20, 30, 40],
    3: [50],
    4: [60, 70]
};

const key = 2;

// Check if the key exists in the map
if (map.hasOwnProperty(key)) {
    // Print the values for the specific key
    map[key].forEach(value => {
        console.log(key + "  " + value);
    });
}

Output: 

2  20
2  30
2  40

Time complexity: O(logn) where n is size of given multimap

Auxiliary Space: O(n)