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Searches for two adjacent elements that are equal (by some criterion)

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Definition

The adjacent_find() algorithm is defined in the standard header <algorithm> and in the nonstandard backward-compatibility header <algo.h>.

Interface

#include <algorithm>
template < class ForwardIterator >
   ForwardIterator adjacent_find(
      ForwardIterator first, 
      ForwardIterator last
   );

template < class ForwardIterator , class BinaryPredicate >
   ForwardIterator adjacent_find(
      ForwardIterator first, 
      ForwardIterator last, 
      BinaryPredicate comp
   );

Parameters:

Parameter	Description
first	A forward iterator addressing the position of the first element in the range to be searched
last	A forward iterator addressing the position one past the final element in the range to be searched
comp	The binary predicate giving the condition to be satisfied by the values of the adjacent elements in the range being searched

Description

The adjacent_find algorithm searches for two adjacent elements that are either equal or satisfy a specified condition.

Return Value

The first version of adjacent_find returns the first iterator i such that i and i+1 are both valid iterators in [first, last), and such that *i == *(i+1). It returns last if no such iterator exists.

The second version of adjacent_find returns the first iterator i such that i and i+1 are both valid iterators in [first, last) and such that binary_pred (*i, *(i+1)) is true. It returns last if no such iterator exists.

Complexity

The complexity is linear. If first == last then no comparison are performed; otherwise, at most (last - first) - 1 comparisons.

References

http://www.sgi.com/

Example 1

Problem

This program illustrates the use of the STL adjacent_find() algorithm (default version) to find adjacent equal values in a vector of integers.

Workings

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

using namespace std;

int main()
{
  int a[] = {1, 2, 3, 3, 3, 4, 5, 6, 7, 7, 8, 9, 10};
  vector<int> v(a, a+12);
  cout <<"\nHere are the values in the vector:\n";
  for (vector<int>::size_type i=0; i<v.size(); i++)
    cout <<v.at(i)<<" ";
 
  vector<int>::iterator p = adjacent_find(v.begin(), v.end());
  if (p == v.end())
    cout <<"\nThere are no, or no more, matching pairs.";
  else
    cout <<"\nFirst value of first matching pair is "<<*p<<".";
 
  p = adjacent_find(p+1, v.end());
  if (p == v.end())
    cout <<"\nThere are no, or no more, matching pairs.";
  else
    cout <<"\nFirst value of next matching pair is "<<*p<<".";

  p = adjacent_find(p+1, v.end());
  if (p == v.end())
    cout <<"\nThere are no, or no more, matching pairs.";
  else
    cout <<"\nFirst value of next matching pair is "<<*p<<".";

  p = adjacent_find(p+1, v.end());
  if (p == v.end())
    cout <<"\nThere are no, or no more, matching pairs.";
  else
    cout <<"\nFirst value of next matching pair is "<<*p<<".";

  return 0;
}

Solution

Output:

Here are the values in the vector:
1 2 3 3 3 4 5 6 7 7 8 9

First value of first matching pair is 3.

First value of next matching pair is 3.

First value of next matching pair is 7.

There are no, or no more, matching pairs.

References

http://www.smu.ca/academic/science/compsci/

Example 2

Problem

This program illustrates the use of the STL adjacent_find() algorithm (extended version) to find adjacent pairs of integers that are both divisible by 5 in a vector of integers.

Workings

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

using namespace std;

bool bothDivisibleByFive
( 
  int m, //in
  int n //in
)
{
  return (m%5 == 0) && (n%5 == 0);
}

int main()
{
  int a[] = {10, 21, 33, 40, 50, 60, 72, 77, 80, 95, 101};
  vector<int> v(a, a+11);
  cout <<"\nHere are the values in the vector:\n";
  for (vector<int>::size_type i=0; i<v.size(); i++)
   cout <<v.at(i)<<" ";

  vector<int>::iterator p = 
  adjacent_find(v.begin(), v.end(), bothDivisibleByFive);
  if (p == v.end())
    cout <<"\nThere are no, or no more, matching pairs.";
  else
    cout <<"\nThe first value of the first matching pair is "<<*p<<".";

  p = adjacent_find(p+1, v.end(), bothDivisibleByFive);
  if (p == v.end())
    cout <<"\nThere are no, or no more, matching pairs.";
  else
    cout <<"\nThe first value of the next matching pair is "<<*p<<".";

  p = adjacent_find(p+1, v.end(), bothDivisibleByFive);
  if (p == v.end())
    cout <<"\nThere are no, or no more, matching pairs.";
  else
    cout <<"\nThe first value of the next matching pair is "<<*p<<".";

  p = adjacent_find(p+1, v.end(), bothDivisibleByFive);
  if (p == v.end())
    cout <<"\nThere are no, or no more, matching pairs.";
  else
    cout <<"\nThe first value of the next matching pair is "<<*p<<".";

  return 0;
}

Solution

Output:

Here are the values in the vector:
10 21 33 40 50 60 72 77 80 95 101

The first value of the first matching pair is 40.

The first value of the next matching pair is 50.

The first value of the next matching pair is 80.

There are no, or no more, matching pairs.

References

http://www.smu.ca/academic/science/compsci/

adjacent_find

Definition

Interface

Description

Return Value

Complexity

References

Example 1

Example 2

See also