C++/C++11中std::transform的使用

std::transform在指定的范围内应用于给定的操作，并将结果存储在指定的另一个范围内。要使用std::transform函数需要包含<algorithm>头文件。

以下是std::transform的两个声明，一个是对应于一元操作，一个是对应于二元操作：

template <class InputIterator,class OutputIterator,1)">class UnaryOperation>
  OutputIterator transform (InputIterator first1,InputIterator last1,OutputIterator result,UnaryOperation op);
    
template <class InputIterator1,1)">class InputIterator2,class BinaryOperation>
  OutputIterator transform (InputIterator1 first1,InputIterator1 last1,InputIterator2 first2,BinaryOperation binary_op);

　　对于一元操作，将op应用于[first1,last1)范围内的每个元素，并将每个操作返回的值存储在以result开头的范围内。给定的op将被连续调用last1-first1次。op可以是函数指针或函数对象或lambda表达式。

　　如op的一个实现即将[first1,last1)范围内的每个元素加5，然后依次存储到result中。

int op_increase(int i) {return (i + 5)};

　　调用std::transform的方式如下：

std::transform(first1,last1,result,op_increase);

　　对于二元操作，使用[first1,last1)范围内的每个元素作为第一个参数调用binary_op,并以first2开头的范围内的每个元素作为第二个参数调用binary_op,每次调用返回的值都存储在以result开头的范围内。给定的binary_op将被连续调用last1-first1次。binary_op可以是函数指针或函数对象或lambda表达式。

　　如binary_op的一个实现即将first1和first2开头的范围内的每个元素相加，然后依次存储到result中。

 int op_add(int,a,1)">int b) {return (a + b)};

　　std::transform支持in place，即result和first1指向的位置可以是相同的。std::transform的主要作用应该就是省去了我们自己写for循环实现。

　　以下是摘自对std::transform的英文解释：

/*
// reference: http://en.cppreference.com/w/cpp/algorithm/transform
template< class InputIt,class OutputIt,class UnaryOperation >
OutputIt transform( InputIt first1,InputIt last1,OutputIt d_first,UnaryOperation unary_op )
{
  while (first1 != last1) {
  *d_first++ = binary_op(*first1++,*first2++);
  }
  return d_first;
}
template< class InputIt1,class InputIt2,class BinaryOperation >
OutputIt transform( InputIt1 first1,InputIt1 last1,InputIt2 first2,BinaryOperation binary_op );
std::transform applies the given function to a range and stores the result in another range,beginning at d_first.
(1): The unary operation unary_op is applied to the range defined by [first1,last1).
(2): The binary operation binary_op is applied to pairs of elements from two ranges:
     one defined by [first1,last1) and the other beginning at first2.
Parameters:
  first1,last1: the first range of elements to transform
  first2: the beginning of the second range of elements to transform
  d_first:the beginning of the destination range,may be equal to first1 or first2
  unary_op: unary operation function object that will be applied.
  binary_op: binary operation function object that will be applied.
Return value: Output iterator to the element past the last element transformed.
std::for_each: ignores the return value of the function and guarantees order of execution.
std::transform: assigns the return value to the iterator,and does not guarantee the order of execution.
*/

　　以下是std::transform用法举例：

#include "transform.hpp"
#include <algorithm> // std::transform
#include <string>
#include <cctype>  std::toupper
#include <iostream>
#include <vector>
#include <functional>  std::plus c++14
 
int test_transform1()
{
    std::string s(Hello);
    std::transform(s.begin(),s.end(),s.begin(),[](unsigned char c) { return std::toupper(c); });
    std::cout << s << std::endl;  HELLO
 
    std::transform(s.begin(),::tolower);
    std::cout << s << std::endl;  hello
 
    ////////////////////////////////
    std::vector<int> arr{ 1,3,1)">5 };
    std::vector<int> arr2{ int> arr3{  };
 
    std::transform(arr.begin(),arr.end(),arr.begin(),[](int d) -> int {return d * 5; });  for_each
    for (auto value : arr) {
        std::cout << value << "    ";  5 15 25
    }
    std::cout<<std::endl;
 
    std::for_each(arr2.begin(),arr2.end(),1)">int& a) {a *= ; });
     (auto value : arr2) {
        std::cout << value <<     }
    std::cout << std::endl;
 
    for (auto& value : arr3) {
        value *= ;
    }
     (auto value : arr3) {
        std::cout << value <<  std::endl;
 
    std::vector<std::string> names = { hi",1)">testfoo };
    std::vector<std::size_t> name_sizes;
 
    ///////////////////////////
    std::transform(names.begin(),names.end(),std::back_inserter(name_sizes),[](std::string name) {  name.size(); });
     (auto value : name_sizes) {
        std::cout << value <<  2 4 3
 std::endl;
 
    std::for_each(name_sizes.begin(),name_sizes.end(),[](std::size_t name_size) {
        std::cout << name_size <<     });
    std::cout <<return 0;
}
 
/////////////////////////////////////////////////////////
 reference: http://www.cplusplus.com/reference/algorithm/transform/
static int i) { return ++i; }
 
 test_transform2()
{
    std::vector<int> foo;
    std::vector< bar;
 
     set some values:
    for (int i = 1; i<6; i++)
        foo.push_back(i * 10);  foo: 10 20 30 40 50
 
    bar.resize(foo.size());  allocate space
 
    std::transform(foo.begin(),foo.end(),bar.begin(),op_increase);
     bar: 11 21 31 41 51
 
     std::plus adds together its two arguments:
    std::transform(foo.begin(),foo.begin(),std::plus<());
     foo: 21 41 61 81 101
 
    std::cout << foo contains:;
    for (std::vector<int>::iterator it = foo.begin(); it != foo.end(); ++it)
        std::cout << ' ' << *it;  21 41 61 81 101
    std::cout << '\n';
 
    ;
}

C++/C++11中std::transform的使用

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