（8）STL算法之替换

在STL算法中可以使用替换函数replace()、replace_if()完成替换操作。先来看下replace函数的底层实现。

// replace()  声明与实现
template <class ForwardIterator, class T>
void replace (ForwardIterator first, ForwardIterator last,const T& old_value, const T& new_value);

template <class ForwardIterator, class T>
void replace (ForwardIterator first, ForwardIterator last,const T& old_value, const T& new_value)
{
    while (first!=last) 
    {
        if (*first == old_value) 
            *first=new_value;
        ++first;
    }
}

再来看看replace_if()的底层实现...

template <class ForwardIterator, class UnaryPredicate, class T>
void replace_if (ForwardIterator first, ForwardIterator last,UnaryPredicate pred, const T& new_value );

template < class ForwardIterator, class UnaryPredicate, class T >
void replace_if (ForwardIterator first, ForwardIterator last,UnaryPredicate pred, const T& new_value)
{
    while ( first != last ) 
    {
        if ( pred( *first ) ) 
            *first = new_value;
        ++first;
    }
}

越看越觉得C语言真牛逼

这两个函数都是有条件的替换，replace()是将容器[first, last]范围中与old_value相等的元素，替换成new_value；而replace_if()是将容器[first, last]范围中能使函数pred为真的元素，替换成new_value。下来看一个例子。

#include <iostream>
#include <vector>
#include <algorithm>
#include <functional>   //  bind2nd(less<int>(), 7);
using namespace std;

void main()
{
	vector<int> v;
	int arr[] = { 1,2,3,4,5,6,7,8,9 };
	v.assign(arr, arr + 9);
	for (int val : v)
		cout << val << " ";   // 1 2 3 4 5 6 7 8 9
	cout << endl;
	replace(v.begin(),v.end(),5,10); //将等于 5 的元素替换成 10
	for (int val : v)
		cout << val << " ";   // 1 2 3 4 10 6 7 8 9
	cout << endl;
	replace_if(v.begin(), v.end(), bind2nd(less<int>(), 7), 11);  //将小于 7 的元素换成 11
	for (int val : v)
		cout << val << " ";   // 11 11 11 11 10 11 7 8 9
	cout << endl;
}

===========================================================================================================================================

再加两个替换函数replace_copy()、replace_copy_if()

template <class InputIterator, class OutputIterator, class T>
OutputIterator replace_copy (InputIterator first, InputIterator last,OutputIterator result,const T& old_value, const T& new_value);

template <class InputIterator, class OutputIterator, class T>
OutputIterator replace_copy (InputIterator first, InputIterator last,OutputIterator result,const T& old_value, const T& new_value)
{
    while (first!=last) 
    {
        *result = (*first==old_value)? new_value: *first;
        ++first; 
        ++result;
    }
    return result;
}

template <class InputIterator, class OutputIterator, class UnaryPredicate, class T>
OutputIterator replace_copy_if (InputIterator first, InputIterator last,OutputIterator result, UnaryPredicate pred,const T& new_value);

template <class InputIterator, class OutputIterator, class UnaryPredicate, class T>
OutputIterator replace_copy_if (InputIterator first, InputIterator last,OutputIterator result, UnaryPredicate pred,const T& new_value)
{
    while (first!=last) 
    {
        *result = ( pred(*first)) ? new_value : *first;
        ++first; 
        ++result;
    }
    return result;
}

用一个例子来学会它

#include <iostream>    
#include <algorithm>    
#include <vector>
using namespace std;

bool fun(int elem)
{
    return ((elem % 2 == 1));
}

int main() {
    int arr[] = { 10, 11, 12, 13, 14, 15, 11, 11 };
    vector<int> v1(8),v2;
    replace_copy(arr, arr + 8, v1.begin(), 11, 21);
    for (auto it = v1.begin(); it != v1.end(); ++it)
        cout << *it <<" ";    //  10 21 12 13 14 15 21 21
    cout << endl;
    v2.resize(v1.size());
    replace_copy_if(v1.begin(), v1.end(), v2.begin(), fun, 0);  //复制容器并将单数全部置 0 
    for (auto it = v2.begin(); it != v2.end(); ++it)
        cout << *it << " ";   //  10 0  12  0 14 0  0  0
    cout << endl;
    return 0;
}