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()
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
写你自己的:
template<class T>
bool checkElementIsInSet(const T& elem, const std::set<T>& container)
{
return container.find(elem) != container.end();
}
其他回答
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
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();
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。
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++…;)
我使用
if(!my_set.count(that_element)) //Element is present...
;
但它的效率不如
if(my_set.find(that_element)!=my_set.end()) ....;
我的版本只是节省了我写代码的时间。对于竞争性编码,我更喜欢这种方式。