Searching in a Map Using std::map Functions in C++

In C++, map container is defined as std::map class template that also contains member function to search for an element on the bases of the keys. In this article, we will learn different methods to search for an element with the given key in C++.

The recommended method to search for the given key in a map container is by using map find() function. Let’s take a look at a simple example:

#include <bits/stdc++.h>
using namespace std;

int main() {
    map<char, int> m = {{'a', 1}, {'c', 3}, 
                       {'b', 4}, {'d', 2}};
    char key = 'b';
    
    // Search the key
    auto it = m.find(key);

    // Checking if key present or not
    if (it != m.end())
        cout << it->second;
    else
        cout << key << " is NOT Present";
    return 0;
}

4

Explanation: The map find() function returns an iterator to the given key, if it presents otherwise return an map end() iterator. So we compare it with map end() to check if the element exists or not.

std::map class provides some more methods to efficiently search for an element with the given key. They are as follows:

Using Map lower_bound()

The map lower_bound() function returns an iterator to the key which is just greater or equal to the given key. So, if an element with the given key is present, this function will return an iterator to it.

#include <bits/stdc++.h>
using namespace std;

int main() {
    map<char, int> m = {{'a', 1}, {'c', 3},
                       {'b', 4}, {'d', 2}};
    char key = 'b';

    // Search the given key
    auto it = m.lower_bound(key);

    // Checking if key present or not
    if (it != m.end() && it->first == key)
        cout << it->second;
    else
        cout << "NOT Present";
    return 0;
}

4

Explanation: As lower_bound() function returns an iterator to key whose value is just greater or equal to the given key, so to check whether the given key is present or not, we have to compare the value of searched key and given key.

Using Map upper_bound()

The map upper_bound() method returns an iterator to the key which is just greater than given key. If the element with the given key exists, it will be present just before this element pointed by this iterator.

#include <bits/stdc++.h>
using namespace std;

int main() {
    map<char, int> m = {{'a', 1}, {'c', 3},
                       {'b', 4}, {'d', 2}};
    char key = 'b';

    // Search the given key
    auto it = m.upper_bound(key);
    it--;

    // Checking if key present or not
    if (it->first == key)
        cout << it->second;
    else
        cout << "NOT Present";
    return 0;
}

4

Explanation: The iterator returned by map upper_bound() is decremented to point to the element one place before. (possible matched element).

Using Map equal_range()

The map equal_range() method returns an iterator of pair, where pair first points to lower_bound() of the given key, and pair second points to upper_bound() of the given key. If key is not present, both pair first and pair second points to the next greater element.

#include <bits/stdc++.h>
using namespace std;

int main() {
    map<char, int> m = {{'a', 1}, {'c', 3},
                       {'b', 4}, {'d', 2}};
    char key = 'b';

    // Search the given key
    auto it = m.equal_range(key);

    // Checking given key is Present or not
    if (it.first != m.end() && it.first->first == key)
        cout << it.first->second;
    else
        cout << key << " is NOT Present";
    return 0;
}

4

Explanation: We compared the key of pair first with the given key to check whether the key is present or not.