# C++ Algorithm Library - lower_bound() Function

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## Description

The C++ function std::algorithm::lower_bound() finds the first element not less than the given value. This function excepts element in sorted order. It uses binary function for comparison.

## Declaration

Following is the declaration for std::algorithm::lower_bound() function form std::algorithm header.

### C++98

```template <class ForwardIterator, class T, class Compare>
ForwardIterator lower_bound(ForwardIterator first, ForwardIterator last,
const T& val, Compare comp);
```

## Parameters

• first − Forward iterator to the initial position.

• last − Forward iterator to the final position.

• val − Value of the lower bound to search for in the range.

• comp − A binary function which accepts two arguments and returns bool.

## Return value

Returns an iterator to the first element not less than the given value. If all the element in the range compare less than val, then function returns last.

## Exceptions

Throws exception if either the binary function or an operation on an iterator throws exception.

Please note that invalid parameters cause undefined behavior.

Linear.

## Example

The following example shows the usage of std::algorithm::lower_bound() function.

```#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

bool ignore_case(char a, char b) {
return(tolower(a) == tolower(b));
}

int main(void) {
vector<char> v = {'A', 'b', 'C', 'd', 'E'};
auto it = lower_bound(v.begin(), v.end(), 'C');

cout << "First element which is greater than \'C\' is " << *it << endl;

it = lower_bound(v.begin(), v.end(), 'C', ignore_case);

cout << "First element which is greater than \'C\' is " << *it << endl;

it = lower_bound(v.begin(), v.end(), 'z', ignore_case);

cout << "All elements are less than \'z\'." << endl;

return 0;
}
```

Let us compile and run the above program, this will produce the following result −

```First element which is greater than 'C' is b
First element which is greater than 'C' is d
All elements are less than 'z'.
```
algorithm.htm