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/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;
- }