std::transform

cppreference std::transform

Parameters

unary_op

unary operation function object that will be applied.

NOTE: function object的概念在C++\Language-reference\Functions\Function.md中进行了介绍。

The signature of the function should be equivalent to the following:

Ret fun(const Type &a);

The signature does not need to have const &.

NOTE: 上述函数原型有const &，但是又说实际可用不包含，如何实现？

Notes

std::transform does not guarantee in-order application of unary_op or binary_op. To apply a function to a sequence in-order or to apply a function that modifies the elements of a sequence, use std::for_each

NOTE: 哪些情况下是不保证in-order的？

Example

#include <algorithm>
#include <cctype>
#include <iostream>
#include <string>
#include <vector>

int main()
{
    std::string s("hello");
    // 转换为大写
    std::transform(s.begin(), s.end(), s.begin(),
            [](unsigned char c) -> unsigned char {  return std::toupper(c);});

    // 获得对应的ASCII值
    std::vector<std::size_t> ordinals;
    std::transform(s.begin(), s.end(), std::back_inserter(ordinals),
            [](unsigned char c) -> std::size_t {    return c;});

    std::cout << s << ':';
    for (auto ord : ordinals)
    {
        std::cout << ' ' << ord;
    }

    // 相加
    // C++11写法
    std::transform(ordinals.cbegin(), ordinals.cend(), ordinals.cbegin(),
            ordinals.begin(), std::plus<std::size_t> { });

    // C++17写法
    // 支持 Class template argument deduction (CTAD) (since C++17) ，参见 https://en.cppreference.com/w/cpp/language/class_template_argument_deduction
    // std::transform(ordinals.cbegin(), ordinals.cend(), ordinals.cbegin(),
    //      ordinals.begin(), std::plus<> { });

    std::cout << '\n';
    for (auto ord : ordinals)
    {
        std::cout << ord << ' ';
    }
    std::cout << '\n';
}
// g++ --std=c++11 test.cpp

fluentcpp std::transform, a central algorithm

std::transform on a range

std::transform on two ranges

#include <algorithm>
#include <cctype>
#include <iostream>
#include <string>
#include <vector>
using namespace std;
int main()
{
    vector<int> numbers1 = { 1, 5, 42, 7, 8 };
    vector<int> numbers2 = { 10, 7, 4, 2, 2 };
    vector<int> results;
    std::transform(numbers1.begin(), numbers1.end(),
            numbers2.begin(),
            std::back_inserter(results),
            [](int i, int j) {  return i+j;});
}
// g++ --std=c++11 test.cpp