C++ STL算法系列5---equal() , mismatch()

equal和mismatch算法的功能是比较容器中的两个区间内的元素。这两个算法各有3个参数first1,last1和first2.如果对 于区间[first1,last1)内所有的first1+i,first1+i和first2所在位置处的元素都相等，则equal算法返回真，否则返 回假。mismatch算法的返回值是由两个迭代器first1+i和first2+i组成的一个pair,表示第1对不相等的元素的位置。如果没有找到 不相等的元素，则返回last1和first2+(last1-first1)。因此，语句equal(first1,last1,first2)和mismatch(first1,last1,first2).first==last1是等价的.

// Illustrating the generic equal and mismatch algorithms
#include <iostream>
#include <cassert>
#include <algorithm>
#include <string>
#include <list>
#include <deque>
#include <vector>
using namespace std;

int main()
{
  cout << "Illustrating the generic equal "
       << "and mismatch algorithms." << endl;
  list<string> driver_list;
  vector<string> vec;
  deque<string> deq;

  driver_list.insert(driver_list.end(), "Clark");
  driver_list.insert(driver_list.end(), "Rindt");
  driver_list.insert(driver_list.end(), "Senna");

  vec.insert(vec.end(), "Clark");
  vec.insert(vec.end(), "Rindt");
  vec.insert(vec.end(), "Senna");
  vec.insert(vec.end(), "Berger");

  deq.insert(deq.end(), "Clark");
  deq.insert(deq.end(), "Berger");

  // Show that driver_list and the first 3 elements of
  // vec are equal in all corresponding positions:
  assert (equal(driver_list.begin(), driver_list.end(),
                vec.begin()));

  // Show that deq and the first 2 elements of driver_list
  // are not equal in all corresponding positions:
  assert (!equal(deq.begin(), deq.end(),
                 driver_list.begin()));

  // Find the corresponding positions in deq and driver_list
  // at which unequal elements first occur:
  pair<deque<string>::iterator, list<string>::iterator>
    pair1 = mismatch(deq.begin(), deq.end(),
                     driver_list.begin());

  if (pair1.first != deq.end())
    cout << "First disagreement in deq and driver_list:
  "
         << *(pair1.first) << " and " << *(pair1.second)
         << endl;
  return 0;
}

equal算法类似于mismatch，equal算法也是逐一比较两个序列的元素是否相等，只是equal函数的返回值为bool值 true/false，不是返回迭代器值。它有如下两个原型，如果迭代器区间[first1，last1)和迭代器区间[first2， first2+(last1 - first1))上的元素相等（或者满足二元谓词判断条件binary_pred） ，返回true，否则返回false。

  函数原型：

template<class InputIterator1, class InputIterator2>
   bool equal(
      InputIterator1 _First1, 
      InputIterator1 _Last1, 
      InputIterator2 _First2
      );
template<class InputIterator1, class InputIterator2, class BinaryPredicate>
   bool equal(
      InputIterator1 _First1, 
      InputIterator1 _Last1, 
      InputIterator2 _First2, 
      BinaryPredicate _Comp
      );
 

example:

利用二元谓词判断条件absEqual，判断出两个vector向量容器的元素均绝对值相等。

#include <algorithm>  
#include <vector>  
#include <iostream>  
  
using namespace std;  
  
bool absEqual(int a, int b)  
{  
    return (a == abs(b) || b == abs(a)) ? true : false;  
}  
  
int main()  
{  
    vector<int> ivect1(5);  
    vector<int> ivect2(5);  
  
    for (vector<int>::size_type i = 0; i < ivect1.size(); ++i)  
    {  
        ivect1[i] = i;  
        ivect2[i] = (-1) * i;  
    }  
    if ( equal( ivect1.begin(), ivect1.end(), ivect2.begin(), absEqual ) )  
    {  
        cout << "ivect1 和 ivect2 元素的绝对值完全相等" << endl;  
    }   
    else  
    {  
        cout << "ivect1 和 ivect2 元素的绝对值不完全相等" << endl;  
    }  
    return 0;  
}