# map lower_bound() function in C++ STL

• Difficulty Level : Easy
• Last Updated : 29 Oct, 2020

The map::lower_bound(k) is a built-in function in C++ STL which returns an iterator pointing to the key in the container which is equivalent to k passed in the parameter.
Syntax:

```map_name.lower_bound(key)
```

Parameters: This function accepts a single mandatory parameter key which specifies the element whose lower_bound is to be returned.
Return Value: The function returns an iterator pointing to the key in the map container which is equivalent to k passed in the parameter. In case k is not present in the map container, the function returns an iterator pointing to the immediate next element which is just greater than k. If the key passed in the parameter exceeds the maximum key in the container, then the returned iterator points to the number of elements in the map as key and element= any element.

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If the passed parameter exceeds the maximum key in the container, then returned iterator will point to the map::end() like in std::set.

## CPP

 `// C++ function for illustration``// map::lower_bound() function``#include ``using` `namespace` `std;` `int` `main()``{` `    ``// initialize container``    ``map<``int``, ``int``> mp;` `    ``// insert elements in random order``    ``mp.insert({ 2, 30 });``    ``mp.insert({ 1, 10 });``    ``mp.insert({ 5, 50 });``    ``mp.insert({ 4, 40 });``    ``for` `(``auto` `it = mp.begin(); it != mp.end(); it++)``    ``{``        ``cout << (*it).first << ``" "` `<<``                              ``(*it).second << endl;``    ``}` `    ``// when 2 is present``    ``auto` `it = mp.lower_bound(2);``    ``cout << ``"The lower bound of key 2 is "``;``    ``cout << (*it).first << ``" "` `<< (*it).second << endl;` `    ``// when 3 is not present``    ``// points to next greater after 3``    ``it = mp.lower_bound(3);``    ``cout << ``"The lower bound of key 3 is "``;``    ``cout << (*it).first << ``" "` `<< (*it).second;` `    ``// when 6 exceeds``    ``it = mp.lower_bound(6);``    ``cout << ``"\nThe lower bound of key 6 is "``;``    ``cout << (*it).first << ``" "` `<< (*it).second;``    ``return` `0;``}`

Output :

```1 10
2 30
4 40
5 50
The lower bound of key 2 is 2 30
The lower bound of key 3 is 4 40
The lower bound of key 6 is 4 0
```

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