如何检查一个元素是否在集合中?

是否有与以下代码更简单的等效代码:

myset.find(x) != myset.end()

当前回答

检查许多STL容器是否存在的典型方法,如std::map, std::set,…是:

const bool is_in = container.find(element) != container.end();

其他回答

Just to clarify, the reason why there is no member like contains() in these container types is because it would open you up to writing inefficient code. Such a method would probably just do a this->find(key) != this->end() internally, but consider what you do when the key is indeed present; in most cases you'll then want to get the element and do something with it. This means you'd have to do a second find(), which is inefficient. It's better to use find directly, so you can cache your result, like so:

auto it = myContainer.find(key);
if (it != myContainer.end())
{
    // Do something with it, no more lookup needed.
}
else
{
    // Key was not present.
}

当然,如果你不关心效率,你总是可以自己滚动,但在这种情况下,你可能不应该使用c++…;)

就是这个,一英里。

bool once(uintptr_t val) {
    return visited.emplace(val).second;
}

怎么会不是这样呢?

https://godbolt.org/z/9zP77jqMc

func5(unsigned long):
        sub     rsp, 24
        mov     QWORD PTR [rsp+8], rdi
        lea     rsi, [rsp+8]
        mov     edi, OFFSET FLAT:visited2
        call    std::pair<std::_Rb_tree_iterator<unsigned long>, bool> std::_Rb_tree<unsigned long, unsigned long, std::_Identity<unsigned long>, std::less<unsigned long>, std::allocator<unsigned long> >::_M_emplace_unique<unsigned long&>(unsigned long&)
        add     rsp, 24
        mov     eax, edx
        ret

从c++ 20开始,就有了bool std::contains(const K&) https://en.cppreference.com/w/cpp/container/set/contains

我能够为std::list和std::vector编写一个通用的包含函数,

template<typename T>
bool contains( const list<T>& container, const T& elt )
{
  return find( container.begin(), container.end(), elt ) != container.end() ;
}

template<typename T>
bool contains( const vector<T>& container, const T& elt )
{
  return find( container.begin(), container.end(), elt ) != container.end() ;
}

// use:
if( contains( yourList, itemInList ) ) // then do something

这样可以稍微清理一下语法。

但是我不能使用模板模板参数魔术使此工作任意stl容器。

// NOT WORKING:
template<template<class> class STLContainer, class T>
bool contains( STLContainer<T> container, T elt )
{
  return find( container.begin(), container.end(), elt ) != container.end() ;
}

任何关于改进上一个答案的评论都是很好的。

您还可以在插入元素时检查元素是否在set中。 单元素版本返回一个pair,其成员pair::first set指向一个迭代器,该迭代器要么指向新插入的元素,要么指向集合中已经存在的等效元素。如果插入了新元素,pair中的第二个元素将被设置为true,如果已经存在等效元素则为false。

例如:假设集合中已经有20作为元素。

 std::set<int> myset;
 std::set<int>::iterator it;
 std::pair<std::set<int>::iterator,bool> ret;

 ret=myset.insert(20);
 if(ret.second==false)
 {
     //do nothing

 }
 else
 {
    //do something
 }

 it=ret.first //points to element 20 already in set.

如果元素是新插入的,则than pair::first将指向新元素在set中的位置。