C++/C++11中std::transform的使用

std::transform函数是将某操作应用于指定范围的每个元素。要使用std::transform函数需要包含<algorithm>头文件。

以下是对std::transform的解释：

/*// reference: http://en.cppreference.com/w/cpp/algorithm/transformtemplate< 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++14int 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; // HELLOstd::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_eachfor (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 50bar.resize(foo.size()); // allocate spacestd::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 101std::cout << "foo contains:";for (std::vector<int>::iterator it = foo.begin(); it != foo.end(); ++it)std::cout << ' ' << *it; // 21 41 61 81 101std::cout << '\n';return 0;}

GitHub：https://github.com/fengbingchun/Messy_Test