C ++中的循环（如python）（基于范围） [英] Cycles in c++ like in python (range-based for)

问题描述

在c ++中像在python中那样在c ++中执行循环的最简单方法是什么？

What is the easiest way to do cycles in c ++ like in python?

for i in range(10): #or range(4, 10, 2) etc
    foo(i)

I意思是简单的单行代码

I mean something simple and one-line like this

for(auto i: range(10)) //or range(4, 10, 2) or range(0.5, 1.0, 0.1) etc
    foo(i);

但不是这样：

std::vector<int> v(10);
std::iota(begin(v), end(v), 0);
for(auto i: v) {
    foo(i);
}

或者这个

for(auto i: []{vector<size_t> v(10); return iota(begin(v), end(v), 0), v;}() ) {
    foo(i);         
}

当然，使用这些示例并不困难，或者仅使用 for（;;），但我希望有一种方法可以在python中简短地进行。

Of course, it is not difficult to use these examples or just for(;;) but I hope there is a way to do it briefly and succinctly in python.

推荐答案

不提供现成的类似Python的 range 概念，但是您可以滚动自己的 Range 类，具有一个简单的迭代器，例如：

A Python-like range notion is not provided out-of-the-box, but you could roll your own Range class with a simple iterator, like this:

#include <iostream>

template <typename T>
class Range
{
public:
  class iterator
  {
  public:
    explicit iterator(T val, T stop, T step) : m_val(val), m_stop(stop), m_step(step) { }
    iterator& operator ++ ()
    {
      m_val += m_step;
      if ((m_step > 0 && m_val >= m_stop) ||
          (m_step < 0 && m_val <= m_stop))
      {
        m_val = m_stop;
      }
      return *this;
    }
    iterator operator ++ (int) { iterator retval = *this; ++(*this); return retval; }
    bool operator == (iterator other) const {return m_val == other.m_val;}
    bool operator != (iterator other) const {return !(*this == other);}
    T operator * () const { return m_val; }
  private:
    T m_val, m_stop, m_step;
  };

  explicit Range(T stop)
    : m_start(0), m_stop(stop), m_step(1)
  { }

  explicit Range(T start, T stop, T step = 1)
    : m_start(start), m_stop(stop), m_step(step)
  { }

  iterator begin() const { return iterator(m_start, m_stop, m_step); }
  iterator end() const { return iterator(m_stop, m_stop, m_step); }

private:
  T m_start, m_stop, m_step;
};

template <typename T>
Range<T> range(T stop) { return Range<T>(stop); }

template <typename T>
Range<T> range(T start, T stop, T step = 1) { return Range<T>(start, stop, step); }

int main()
{
  for (auto i : range(10)) { std::cout << " " << i; }
  std::cout << std::endl;
  for (auto i : range(4, 10, 2)) { std::cout << " " << i; }
  std::cout << std::endl;
  for (auto i : range(0.5, 1.0, 0.1)) { std::cout << " " << i; }
  std::cout << std::endl;
}

为了支持基于范围的 ，迭代器类型和 begin（） / end（）函数将完成此工作。 （当然，我上面的实现是快速而肮脏的，并且可能会得到改进。）

In order to support range-based for, an iterator type and begin()/end() functions will do the job. (Of course my implementation above is quick and dirty, and could probably be improved.)

您不会像那样滚动自己的类，但是一旦有了它，其用法与Python方法非常相似：

You will not get around rolling your own class like that, but once you have it, the usage is very much akin to the Python approach:

for (auto i : range(stop)) { ... }
for (auto i : range(start, stop, step)) { ... }

示例输出（在此处查看实时版本）：

$ g++ -std=c++11 -o test test.cpp && ./test
 0 1 2 3 4 5 6 7 8 9
 4 6 8
 0.5 0.6 0.7 0.8 0.9 1

如果只需要整数范围，还可以使用 boost :: irange （感谢Yakk的提醒）。

If you only need integer ranges, you can also use boost::irange (thanks to Yakk for the reminder).

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