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
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。
其他回答
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();
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++…;)
另一种简单地判断元素是否存在的方法是检查count()
if (myset.count(x)) {
// x is in the set, count is 1
} else {
// count zero, i.e. x not in the set
}
然而,大多数时候,我发现自己需要访问元素,无论我在哪里检查它的存在。
所以我还是要找到迭代器。当然,最好也把它和end进行比较。
set< X >::iterator it = myset.find(x);
if (it != myset.end()) {
// do something with *it
}
C + + 20
在c++ 20中,set获得一个contains函数,因此如下所述成为可能:https://stackoverflow.com/a/54197839/895245
if (myset.contains(x)) {
// x is in the set
} else {
// no x
}
在c++ 20中,我们最终得到std::set::contains方法。
#include <iostream>
#include <string>
#include <set>
int main()
{
std::set<std::string> example = {"Do", "not", "panic", "!!!"};
if(example.contains("panic")) {
std::cout << "Found\n";
} else {
std::cout << "Not found\n";
}
}
从c++ 20开始,就有了bool std::contains(const K&) https://en.cppreference.com/w/cpp/container/set/contains