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, class UnaryOperation>
  OutputIterator transform (InputIterator first1, InputIterator last1,
                            OutputIterator result, UnaryOperation op);
    
template <class InputIterator1, class InputIterator2,
          class OutputIterator, class BinaryOperation>
  OutputIterator transform (InputIterator1 first1, InputIterator1 last1,
                            InputIterator2 first2, OutputIterator result,
                            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, 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 OutputIt, class BinaryOperation >
OutputIt transform( InputIt1 first1, InputIt1 last1, InputIt2 first2, OutputIt d_first, 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(), s.end(), s.begin(), ::tolower);
    std::cout << s << std::endl; // hello
 
    ////////////////////////////////
    std::vector<int> arr{ 1, 3, 5 };
    std::vector<int> arr2{ 1, 3, 5 };
    std::vector<int> arr3{ 1, 3, 5 };
 
    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(), [](int& a) {a *= 5; });
    for (auto value : arr2) {
        std::cout << value << "    "; // 5 15 25
    }
    std::cout << std::endl;
 
    for (auto& value : arr3) {
        value *= 5;
    }
    for (auto value : arr3) {
        std::cout << value << "    "; // 5 15 25
    }
    std::cout << std::endl;
 
    std::vector<std::string> names = { "hi", "test", "foo" };
    std::vector<std::size_t> name_sizes;
 
    ///////////////////////////
    std::transform(names.begin(), names.end(), std::back_inserter(name_sizes),
        [](std::string name) { return name.size(); });
    for (auto value : name_sizes) {
        std::cout << value << "    "; // 2 4 3
    }
    std::cout << std::endl;
 
    std::for_each(name_sizes.begin(), name_sizes.end(), [](std::size_t name_size) {
        std::cout << name_size << "    "; // 2 4 3
    });
    std::cout << std::endl;
 
    return 0;
}
 
/////////////////////////////////////////////////////////
// reference: http://www.cplusplus.com/reference/algorithm/transform/
static int op_increase(int i) { return ++i; }
 
int test_transform2()
{
    std::vector<int> foo;
    std::vector<int> 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.end(), bar.begin(), foo.begin(), std::plus<int>());
    // 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';
 
    return 0;
}