C++语法基础--泛型算法（generic algorithm)--find(),find_if()

- 日理万妓 2022-01-10 13:49 255阅读 0赞

**1.使用泛型算法必须包含algorithm头文件  
  
  
2.大多数算法是通过遍历由两个迭代器标记的一段元素来实现其功能，典型情况下，算法在遍历一段元素范围时，操纵其中的每一个元素。  
  
  
3.find()  
只要找到与给定值相等的元素，find就会返回指向该元素的迭代器  
原型：  
 template <class InputIterator, class T>  
InputIterator find (InputIterator first, InputIterator last, const T& val)；  
等效原理：  
 template<class InputIterator, class T>  
InputIterator find (InputIterator first, InputIterator last, const T& val)  
\{  
while (first!=last) \{  
if (\*first==val) return first;  
\++first;  
\}  
return last;  
\}**  
  
 **eg：**   
 **int main(void)  
\{  
int myints\[\] = \{ 10, 20, 30 ,40 \};  
int \* p;**  
  
  **// pointer to array element:**   
 **p = find (myints,myints+4,30);  
 \++p;  
 cout << \*p << '\\n';//40**  
  
  **vector<int> myvector (myints,myints+4);**   
 **vector<int>::iterator it;**  
  
  **// iterator to vector element:**   
 **it = find (myvector.begin(), myvector.end(), 30);  
 \++it;  
 cout<< \*it << '\\n';//40**  
  
   **return 0;**   
**\}  
  
  
  
  
4.迭代算法和容器绑定起来  
要求：  
\*遍历容器。  
\*元素类型支持==操作符，否则需要额外提供比较函数  
  
 eg：如果class A 没有重载==操作符，则程序编译失败  
 class A  
\{   
public:  
A(int i):x(i)\{\}  
void f()\{cout<<x<<endl;\}  
 bool operator==(A b) const  
\{   
return x==b.x;  
 \}  
private:  
int x;  
\};**  
  
  
 **int main(void)**   
**\{   
 A a(1),b(2),c(3);  
 vector<A> myvector ;  
 myvector.push\_back(a);  
 myvector.push\_back(b);  
 myvector.push\_back(c);  
 vector<A>::iterator it;**  
  
   **// iterator to vector element:**   
 **it = find (myvector.begin(), myvector.end(), b);  
  
 it->f();//2  
  
 return 0;**

**\}**

**4.find\_if**

**Returns an iterator to the first element in the range \[first,last) for which pred returns true.  
If no such element is found, the function returns last.**

**原型：  
template <class InputIterator, class UnaryPredicate>  
InputIterator find\_if (InputIterator first, InputIterator last, UnaryPredicate pred);**

**该算法等效于：  
template<class InputIterator, class UnaryPredicate>  
InputIterator find\_if (InputIterator first, InputIterator last, UnaryPredicate pred)  
\{  
while (first!=last) \{  
if (pred(\*first)) return first;  
\++first;  
\}  
return last;  
\}  
  
  
解析：  
first, last：  
Input iterators to the initial and final positions in a sequence.  
pred ：  
Unary function that accepts an element in the range as argument and returns a value convertible to bool.  
  
  
 Example:  
  
  
bool IsEven (int i) \{   
 return ((i%2)==0);  
\}  
  
  
int main ()**

**\{   
int arr\[\]=\{1,2,3,2,1\};  
 vector<int> myvector(arr,arr+5);  
 vector<int>::iterator it ;  
 it=std::find\_if (myvector.begin(), myvector.end(), IsEven);  
 cout << "The first Even value is " << \*it << '\\n';  
  
  
 return 0;  
\}  
  
  
Output:The first Even value is 2**