C++语法基础--泛型算法（generic algorithm)--只读算法accumulate(),find_first_of(),count(),count_if()

分手后的思念是犯贱 2022-01-11 08:07 154阅读 0赞

***注：泛型算法实在太多，太繁杂，全都掌握也不现实，也没有必要。所以暂时只研究一些常用的算法。***

**\*accumulate(在numeric头文件中定义）  
Computes the sum of the given value init and the elements in the range \[first, last).  
The first version uses operator+ to sum up the elements, the second version uses the given binary function op.  
原型：  
 template <class InputIterator, class T>  
T accumulate ( InputIterator first, InputIterator last, T init );  
template <class InputIterator, class T, class BinaryOperation>  
T accumulate ( InputIterator first, InputIterator last, T init,  
BinaryOperation binary\_op )  
解析：  
 first,last:指定要累加的范围  
init:指定初始值  
binary\_op:累加的方法  
  
该算法等效于：  
template <class InputIterator, class T>  
T accumulate ( InputIterator first, InputIterator last, T init )  
\{  
while ( first!=last )  
init = init + \*first++; // or: init=binary\_op(init,\*first++) for the binary\_op version  
return init;  
\}  
  
  
Examples：  
 int main () \{  
int init = 10;  
int numbers\[\] = \{1,2,3\};  
  
  
//"using default accumulate:  
cout << accumulate(numbers,numbers+3,init);//16  
 cout << endl;  
  
  
 //using custom function:  
 cout << accumulate (numbers, numbers+3, init, myfunction );//13  
 cout << endl;  
  
  
  
 return 0;  
\}  
  
  
  
\*find\_first\_of()：  
\*元素可以存储在不同类型的容器中，只要求元素之间可以比较即可  
Returns an iterator to the first element in the range \[first1,last1) that matches any of the elements in \[first2,last2).If no such element is found, the function returns last1.  
原型：  
 equality ：  
template <class ForwardIterator1, class ForwardIterator2>  
ForwardIterator1 find\_first\_of (ForwardIterator1 first1, ForwardIterator1 last1,  
ForwardIterator2 first2, ForwardIterator2 last2);  
predicate ：  
template <class ForwardIterator1, class ForwardIterator2, class BinaryPredicate>  
ForwardIterator1 find\_first\_of (ForwardIterator1 first1, ForwardIterator1 last1,  
ForwardIterator2 first2, ForwardIterator2 last2,  
BinaryPredicate pred)  
该算法等效于  
 template<class InputIterator, class ForwardIterator>  
InputIterator find\_first\_of ( InputIterator first1, InputIterator last1,  
ForwardIterator first2, ForwardIterator last2)  
\{  
while (first1!=last1) \{  
for (ForwardIterator it=first2; it!=last2; ++it) \{  
if (\*it==\*first1) // or: if (pred(\*it,\*first1)) for version (2)  
return first1;  
\}  
\++first1;  
\}  
return last1;  
\}  
  
  
  
  
Examples:  
 //判断元素是否相等  
 bool equal(int a, int b)  
 \{   
 return a==(b-1);  
 \}  
  
  
int main ()   
\{   
 int arr1\[\] = \{1,2,3,4,5\};  
 vector<int> vec1 (arr1,arr1+5);  
 vector<int>::iterator it;  
  
  
 int arr2\[\] = \{2,4,6\};  
  
  
 // using default comparison:  
 it = find\_first\_of (vec1.begin(), vec1.end(), arr2, arr2+3);  
  
  
 if (it!=vec1.end())  
 cout << "The first match is: " << \*it << '\\n';//The first match is:2  
  
  
 // using predicate comparison:  
 it = find\_first\_of (vec1.begin(), vec1.end(),  
 arr2, arr2+3, equal);//比较时，equal函数令arr2的每一个元素都减1  
  
  
 if (it!=vec1.end())  
 cout << "The first match is: " << \*it << '\\n';//The first match is:1  
  
  
 return 0;  
\}  
  
  
  
  
\*count()  
Returns the number of elements in the range \[first,last) that compare equal to val.  
The function uses operator== to compare the individual elements to val.  
  
原型：  
template <class InputIterator, class T>  
typename iterator\_traits<InputIterator>::difference\_type  
count (InputIterator first, InputIterator last, const T& val)  
  
该算法等效于：  
template <class InputIterator, class T>  
typename iterator\_traits<InputIterator>::difference\_type  
count (InputIterator first, InputIterator last, const T& val)  
\{  
typename iterator\_traits<InputIterator>::difference\_type ret = 0;  
while (first!=last) \{  
if (\*first == val) ++ret;  
\++first;  
\}  
return ret;  
\}  
  
  
Examples:**

**int main ()**

**\{**

**// counting elements in array:  
 int myints\[\] = \{1,1,3,2,2,3,1,3\};  // 8 elements  
 int mycount = count (myints, myints+8, 1);  
 cout << "1 appears " << mycount << " times.\\n";//1 appears 3 times.  
  
  
 // counting elements in container:  
 std::vector<int> myvector (myints, myints+8);  
 mycount = count (myvector.begin(), myvector.end(), 2);  
 cout << "2 appears " << mycount << " times.\\n";//2 appears 2 times.  
  
  
 return 0;  
\}**

**\*count\_if  
 Return number of elements in range satisfying condition  
 Returns the number of elements in the range \[first,last) for which pred is true.**

**原型：  
template <class InputIterator, class Predicate>  
 typename iterator\_traits<InputIterator>::difference\_type  
 count\_if (InputIterator first, InputIterator last, UnaryPredicate pred)  
 该算法等效于：  
  
template <class InputIterator, class UnaryPredicate>  
 typename iterator\_traits<InputIterator>::difference\_type  
 count (InputIterator first, InputIterator last, UnaryPredicate pred)  
\{   
 typename iterator\_traits<InputIterator>::difference\_type ret = 0;  
 while (first!=last) \{   
 if (pred(\*first)) ++ret;  
 \++first;  
 \}  
 return ret;  
\}  
 解析：  
  
first, last：  
 InputIterator to the initial and final positions of the sequence of elements.  
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 ()**  
**\{   
 vector<int> myvector;  
 for (int i=1; i<10; i++)   
 \{   
 myvector.push\_back(i);// myvector: 1 2 3 4 5 6 7 8 9  
 \}**  
  
  
**int mycount = count\_if (myvector.begin(), myvector.end(), IsEven);**  
 **cout << "myvector contains " << mycount << " Even values.\\n";  
 //myvector contains 4 Even values.**  
  
  
**return 0;**  
**\}**