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std::binary_search

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<td>
Defined in header <algorithm>
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<td >
template< class ForwardIt, class T >
bool binary_search( ForwardIt first, ForwardIt last, const T& value );
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<td > (1) </td> <td class="t-dcl-nopad"> </td> </tr> <tr class="t-dcl ">

<td >
template< class ForwardIt, class T, class Compare >
bool binary_search( ForwardIt first, ForwardIt last, const T& value, Compare comp );
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<td > (2) </td> <td class="t-dcl-nopad"> </td> </tr> Template:ddcl list end

Checks if the sorted range [first, last) contains an element equal to value. The first version uses operator< to compare the elements, the second version uses the given comparison function comp.

Contents

Parameters

first, last - the range of elements to examine
value - value to compare the elements to
comp - comparison function object (i.e. an object that satisfies the requirements of Compare) which returns ​true if the first argument is less than the second.

The signature of the comparison function should be equivalent to the following:

 bool cmp(const Type1 &a, const Type2 &b);

The signature does not need to have const &, but the function object must not modify the objects passed to it.
The type Type1 must be such that an object of type T can be implicitly converted to Type1. The type Type2 must be such that an object of type ForwardIt can be dereferenced and then implicitly converted to Type2. ​

Type requirements
-
ForwardIt must meet the requirements of ForwardIterator.

Return value

true if an element equal to value is found, false otherwise.

Complexity

Logarithmic in the distance between first and last

Possible implementation

First version
template<class ForwardIt, class T>
bool binary_search(ForwardIt first, ForwardIt last, const T& value)
{
    first = std::lower_bound(first, last, value);
    return (first != last && !(value < *first));
}
Second version
template<class ForwardIt, class T, class Compare>
bool binary_search(ForwardIt first, ForwardIt last, const T& value, Compare comp)
{
    first = std::lower_bound(first, last, value);
    return (first != last && !(comp(value, *first));
}

Example

#include <iostream>
#include <algorithm>
#include <vector>
 
int main()
{
    std::vector<int> haystack {1, 3, 4, 5, 9};
    std::vector<int> needles {1, 2, 3};
 
    for (auto needle : needles) {
        std::cout << "Searching for " << needle << '\n';
        if (std::binary_search(haystack.begin(), haystack.end(), needle)) {
            std::cout << "Found " << needle << '\n';
        } else {
            std::cout << "no dice!\n";
        }
    }
}

Output:

Searching for 1
Found 1
Searching for 2
no dice!
Searching for 3
Found 3

See also

Template:cpp/algorithm/dcl list equal range