#include <algorithm>
    bool binary_search( forward_iterator start, forward_iterator end, const TYPE& val );
    bool binary_search( forward_iterator start, forward_iterator end, const TYPE& val, Comp f );

The binary_search() function searches from start to end for val. The elements between start and end that are searched should be in ascending order as defined by the < operator. Note that a binary search will not work unless the elements being searched are in order.

Only the < operator should be defined for the values. Two values a and b are considered equal when !(a<b) && !(b<a) is true.

If val is found, binary_search() returns true, otherwise false. If the function f is specified, then it is used to compare elements instead of operator< .

binary_search() runs in logarithmic time.

For example, the following code uses binary_search() to determine if the integers 0-9 are in an array of integers (nums[] should be sorted in ascending order):

   int nums[] = { -242, -1, 0, 5, 8, 9, 11 };
   int start = 0;
   int end = 7;
   for( int i = 0; i < 10; i++ ) {
     if( binary_search( nums+start, nums+end, i ) ) {
       cout << "nums[] contains " << i << endl;
     } else {
       cout << "nums[] DOES NOT contain " << i << endl;

When run, this code displays the following output:

   nums[] contains 0
   nums[] DOES NOT contain 1
   nums[] DOES NOT contain 2
   nums[] DOES NOT contain 3
   nums[] DOES NOT contain 4
   nums[] contains 5
   nums[] DOES NOT contain 6
   nums[] DOES NOT contain 7
   nums[] contains 8
   nums[] contains 9

Related Topics: equal_range, lower_bound, partial_sort, partial_sort_copy, sort, stable_sort, upper_bound