C++ 如何检测 std::map 循环中的最后一次迭代？
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原文地址: http://stackoverflow.com/questions/151046/
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How can I detect the last iteration in a loop over std::map?
提问by cdleary
I'm trying to figure out the best way to determine whether I'm in the last iteration of a loop over a map in order to do something like the following:
我试图找出确定我是否处于地图循环的最后一次迭代中的最佳方法，以便执行以下操作：
for (iter = someMap.begin(); iter != someMap.end(); ++iter) {
bool last_iteration;
// do something for all iterations
if (!last_iteration) {
// do something for all but the last iteration
}
}
There seem to be several ways of doing this: random access iterators, the distancefunction, etc. What's the canonical method?
似乎有几种方法可以做到这一点：随机访问迭代器、distance函数等。规范方法是什么？
Edit: no random access iterators for maps!
编辑：地图没有随机访问迭代器！
采纳答案by Mark Ransom
回答by dutchdukes
Since C++11, you can also use std::next()
从 C++11 开始，您还可以使用 std::next()
for (auto iter = someMap.begin(); iter != someMap.end(); ++iter) {
// do something for all iterations
if (std::next(iter) != someMap.end()) {
// do something for all but the last iteration
}
}
Although the question was asked a while ago, I thought it would be worth sharing.
虽然这个问题是前一段时间提出的，但我认为值得分享。
回答by Torlack
This seems like the simplest:
这似乎是最简单的：
bool last_iteration = iter == (--someMap.end());
回答by camh
If you just want to use a ForwardIterator, this should work:
如果您只想使用 ForwardIterator，这应该可以工作：
for ( i = c.begin(); i != c.end(); ) {
iterator cur = i++;
// do something, using cur
if ( i != c.end() ) {
// do something using cur for all but the last iteration
}
}
回答by KTC
Modified Mark Ransom's so it actually work as intended.
修改了 Mark Ransom 使其真正按预期工作。
finalIter = someMap.end();
--finalIter;
if (iter != final_iter)
回答by Pieter
Surprised no one mentioned it yet, but of course boost has something ;)
令人惊讶的是还没有人提到它，但当然 boost 有一些东西;)
Boost.Next(and the equivalent Boost.Prior)
Boost.Next（以及等效的 Boost.Prior）
Your example would look like:
您的示例如下所示：
for (iter = someMap.begin(); iter != someMap.end(); ++iter) {
// do something for all iterations
if (boost::next(iter) != someMap.end()) {
// do something for all but the last iteration
}
}
回答by mpoleg
The following code would be optimized by a compiler so that to be the best solution for this task by performance as well as by OOP rules:
以下代码将由编译器优化，以便根据性能和 OOP 规则成为此任务的最佳解决方案：
if (&*it == &*someMap.rbegin()) {
//the last iteration
}
This is the best code by OOP rules because std::map has got a special member function rbegin for the code like:
这是 OOP 规则的最佳代码，因为 std::map 有一个特殊的成员函数 rbegin 用于如下代码：
final_iter = someMap.end();
--final_iter;
回答by Angelin Nadar
Why to work to find the EOF so that you dont give something to it.
为什么要努力找到EOF，这样你就不会给它一些东西。
Simply, exclude it;
简单地说，排除它；
for (iter = someMap.begin(); someMap.end() - 1; ++iter) {
//apply to all from begin to second last element
}
KISS (KEEP IT SIMPLY SIMPLE)
吻（保持简单）
回答by paxos1977
#include <boost/lambda/lambda.hpp>
#include <boost/lambda/bind.hpp>
#include <algorithm>
using namespace boost::lambda;
// call the function foo on each element but the last...
if( !someMap.empty() )
{
std::for_each( someMap.begin(), --someMap.end(), bind( &Foo, _1 ) );
}
Using std::for_each will ensure that the loop is tight and accurate... Note the introduction of the function foo() which takes a single argument (the type should match what is contained in someMap). This approach has the added addition of being 1 line. Of course, if Foo is really small, you can use a lambda function and get rid of the call to &Foo.
使用 std::for_each 将确保循环紧密而准确...注意函数 foo() 的引入，它接受一个参数（类型应该与 someMap 中包含的内容相匹配）。这种方法增加了 1 行。当然，如果 Foo 真的很小，您可以使用 lambda 函数并摆脱对 &Foo 的调用。
回答by alfred
For someone who likes C++11 range-based loop:
对于喜欢 C++11 基于范围循环的人：
for (const auto& pair : someMap) {
if (&pair != &*someMap.rbegin()) ...
}
Notice only reference type works here, not auto pair
注意这里只有引用类型有效，而不是 auto pair
